Cool Stars 23
15 – 19 June 2026, TOC Ariake, Tokyo Bay Area, Japan
The "Cambridge Workshops on Cool Stars, Stellar Systems and the Sun" are held biennially and have evolved to be the premier conference series for cool star research.
The meeting will cover the following five science themes:
This theme targets key processes including dynamos, mixing, asteroseismology, (and more) in the interors of cool objects.
This theme targets similarity and difference of various atmospheres from Sun-like stars and red dwarfs/giants/supergiants to brown dwarfs.
This theme covers gyrochronology using XUV+chromospheric emissions, stellar rotation and magnetic fields in connection with (super)flares and mass ejections/winds through the evolution of cool stars.
This theme focuses on the early stage of stellar evolution; some specific topics are accretion driven activity and star-planet interaction.
This theme focuses on the formation and evolution of cool objects with various metallicities and their roles in the galaxy evolution.
science theme: Fundamental properties and processes of Cool Star Interiors
schedule: Mon, 09:00 (invited)
Hideyuki Hotta
Nagoya University
The magnetic field in low-mass stars is generated by dynamo action in their convection zones. Turbulent thermal convection, in combination with stellar rotation, plays a central role in shaping this process. Because direct observations of solar and stellar interiors are extremely limited, numerical simulations are essential for advancing our understanding. In this talk, we summarize recent progress in numerical simulations of solar and stellar convection zones, with particular emphasis on the generation of differential rotation. On the solar side, there remains a significant discrepancy between helioseismic constraints and numerical models, known as the convective conundrum. On the stellar side, growing observational evidence for differential rotation and magnetic activity across a wide range of stellar parameters provides new opportunities to test and refine simulation-based models.
science theme: Fundamental properties and processes of Cool Star Interiors
schedule: Mon, 09:30 (contributed)
Vindya Vashishth
IIT (BHU) Varanasi, India
Authors: Vindya Vashishth; Bidya Binay Karak
Affiliations: IIT (BHU) Varanasi, India
The Babcock–Leighton (BL) dynamo, in which the poloidal field is generated through the decay and dispersal of tilted bipolar magnetic regions (BMRs), provides a promising paradigm for explaining the features of the solar magnetic cycle. A key ingredient of this mechanism is meridional circulation, which transports magnetic flux from decaying BMRs toward the poles. Since meridional flow properties are expected to vary with stellar rotation rate, they play an important role in shaping magnetic cycles in Sun-like stars. To investigate this, we use the STABLE (Surface Flux Transport and Babcock–Leighton) 3D kinematic dynamo model, where large-scale flows are obtained from mean-field hydrodynamic models for stars rotating at different rates. In rapid rotators, BMRs are expected to emerge at higher latitudes, where poloidal field generation is less efficient due to poor cross-equatorial cancellation, raising questions about the operation of the BL dynamo in such stars. To address this, we conduct a series of dynamo simulations exploring four spot-deposition scenarios based on different assumptions about toroidal flux-tube rise: (i) radial rise, (ii) parallel rise to the rotation axis, (iii) parallel rise with an increased Joy’s law tilt with rotation rate, and (iv) increased time delay and spot size. Our results show how changes in meridional flow influence the strength, duration, and parity of magnetic cycles. The field strength initially increases with stellar rotation rate but declines in rapid rotators, offering an explanation for the observed rotation–activity relation. Cyclic magnetic fields are obtained in all cases except case (iv) for a 1-day rotation period, which exhibits irregular behavior. Parallel-rise cases produce quadrupolar magnetic fields, whereas the remaining cases yield dipolar parity. These results demonstrate that the Babcock–Leighton dynamo operates even in rapidly rotating stars despite the emergence of high-latitude starspots.
science theme: Fundamental properties and processes of Cool Star Interiors
schedule: Mon, 09:45 (contributed)
Yoshiki Hatta
Nagoya University
Authors: Yoshiki Hatta(1); Hideyuki Hotta (1); Laurent Gizon (2,3,4)
Affiliations: (1) Nagoya University; (2) Max Planck Institute for Solar System Research; (3) The University of Göttingen; (4) New York University Abu Dhabi
The solar MC reflects the internal dynamics (angular momentum transport, thermal convection, etc.), as well as plays an essential role in the solar dynamo by transporting magnetic fields. It is therefore of great importance for us to reveal the solar MC profile. We can infer the solar MC profile by inverting acoustic travel times, which are times it takes acoustic wave packets to propagate through the solar interior. Still, mainly due to tiny velocity amplitudes (predicted to be, e.g., a few m/s around the bottom of the convection zone), the MC inferences have not converged yet; whether the solar MC is single- or double-cell is not completely clear. A promising alternative way is the forward modeling where a model MC profile is used to theoretically compute travel times, which are then compared with observed ones. Since current 3-D MHD numerical simulations of the solar convection predict either cylindrical or double-cell MC profiles, the forward modeling may allow us to distinguish these two profiles. In this talk, I will share results of forward modeling based on a state-of-the-art 3-D MHD numerical simulation conducted by Hotta (H25), which exhibits a double-cell MC profile. The comparison between the modeled travel times and those measured by Gizon et al. (G20) shows a reasonable agreement. This contrasts with previous attempts by Stejko et al. where cylindrical MC profiles in their 3-D MHD models are prone to leading to significantly smaller travel times than G20’s measurements. Moreover, differences between our modeled travel times and those computed based on the previous helioseismic inversion results can be explained by numerical artefacts in H25's MC around the subsurface equatorial region. Thus, this study gives us a hint for improving 3-D MHD numerical simulations as well, which clearly highlights the relevance of the forward modeling approach to investigate the solar MC from the both theoretical and observational perspective.
science theme: Fundamental properties and processes of Cool Star Interiors
schedule: Mon, 10:00 (contributed)
Travis Metcalfe
Center for Solar-Stellar Connections, WDRC
Authors: T.S. Metcalfe (1); J.L. van Saders (2); M.H. Pinsonneault (3); T.R. Ayres (4); O. Kochukhov (5); K.G. Stassun (6); A.J. Finley (7); V. See (8); I.V. Ilyin (9); K.G. Strassmeier (9)
Affiliations: (1) WDRC; (2) Univ. Hawaii; (3) Ohio State Univ.; (4) Univ. Colorado Boulder; (5) Uppsala Univ.; (6) Vanderbilt Univ.; (7) Univ. Paris-Saclay, CEA; (8) Univ. Birmingham; (9) Leibniz-Institut für Astrophysik Potsdam
Standard models of stellar angular momentum evolution fail to describe the rotation of stars older than the Sun, which exhibit anomalously rapid rotation in Kepler and TESS field samples. This phenomenon, known as weakened magnetic braking (WMB), is hypothesized to stem from a fundamental shift in magnetic morphology. We present a uniform reanalysis of a broad sample of stars (late F to early K) using direct spectropolarimetric magnetic field measurements and X-ray inferred mass-loss rates to bypass traditional, indirect activity proxies. Our results demonstrate an abrupt, two order of magnitude decrease in wind braking torque as the Rossby number approaches a critical point slightly above the solar value ($Ro \sim 1.01 Ro_\odot$). Notably, our results indicate that the Sun is already well within this transition, with its current wind braking torque significantly suppressed relative to the predictions of traditional spin-down models. The transition is clearly reflected in both large-scale magnetic field strengths and X-ray luminosities, signaling a disruption of global organizing flows and weakened coronal heating. We interpret these findings as evidence of a rotational threshold for the influence of Coriolis forces on global convective patterns, a transition that can be modeled as a supercritical Hopf bifurcation where global dynamo excitation ceases. This work provides a physical basis for the observed "flat-activity" state in old stars and establishes a new paradigm for the limitations of gyrochronology beyond the middle of main-sequence lifetimes.
science theme: Fundamental properties and processes of Cool Star Interiors
schedule: Mon, 10:15 (contributed)
Antoine Strugarek
CEA Paris-Saclay
Authors: Antoine Strugarek (1), Quentin Noraz (2), Sacha Brun (1), Adam Finely (3)
Affiliations: (1) CEA Paris-Saclay; (2) KU Leuven; (3) ESA, ESTEC
Magnetic activity in cool stars manifests itself through surface structures such as plages and starspots, which dominate photometric and spectroscopic variability and are key diagnostics of stellar magnetism. These structures arise from the concentration of magnetic flux at the stellar surface, yet the physical mechanisms on how they form and where they are rooted remain poorly understood, even in the solar case. In particular, spots form at intermediate spatial scales and follow the large-scale magnetic cycle, indicating an interplay between global and local dynamical processes.
We present a series of global convective dynamo simulations that approach photospheric conditions and explore increasingly turbulent regimes relevant to cool-star envelopes. In these models, magnetic flux concentrations form spontaneously near the surface, without imposed flux tubes or surface forcing. The resulting structures are strongly magnetised (local ME/KE > 100), and develop on spatial scales larger than the local convective scale. They preferentially arise in the low-density near-surface layers, where magnetic back-reaction is able to suppress convective heat transport. This suppression produces localized cool regions with reduced convection, resembling spot-like features and providing a plausible pathway toward sunspot and starspot formation in global dynamo models.
science theme: Fundamental properties and processes of Cool Star Interiors
schedule: Mon, 11:00 (invited)
Jamie Tayar
University of Florida
We now regularly make models of low-mass stars, predicting their interior structure and evolution as a function of mass and metallicity. While these predictions have been extremely useful, the ever-improving landscape of data has made it clear that there are still many things left to be learned about evolution, mixing, rotation, and magnetism in all phases from the pre-main-sequence to white dwarfs. I will describe recent improvements in our understanding of cool stars, as well as some of the open questions. Finally, I will highlight some of the opportunities for continued improvement in our understanding of cool star interiors, including ongoing work and upcoming missions.
science theme: Fundamental properties and processes of Cool Star Interiors
schedule: Mon, 11:30 (contributed)
Alexander Kosovichev
New Jersey Institute of Technology
Authors: A.G. Kosovichev (1); K. Mandal (1); K. Villamayor (1); A.V. Getling (2); V.V. Pipin (3)
Affiliations: (1) New Jersey Inst. Tech.; (2) Moscow Univ.; (3) Inst.Solar-Terr. Phys.
Magnetic cycles in solar-type stars reflect the physics of stellar dynamos, yet the basic mechanisms of magnetic field generation and transport remain debated. Using uninterrupted helioseismic observations of the Sun’s internal differential rotation since 1996, we detect migrating bands of zonal acceleration (torsional oscillations) that originate near the base of the convection zone and propagate in both radius and latitude, forming extended 22-year cycles. The inferred propagation is consistent with dynamo-wave behavior and points to the tachocline as the primary seat of cycle initiation. Helioseismic observations also show that these migrating wave patterns are accompanied by coherent, cycle-dependent changes in the meridional circulation and the internal sound-speed structure, consistent with thermal perturbations induced by magnetic modulation of convective heat transport. We discuss how the combined constraints - variations of the internal differential rotation and meridional circulation, coupled with thermodynamic signatures - can discriminate among flux-transport and distributed dynamo models and provide a helioseismically calibrated benchmark for interpreting magnetic activity in solar-type stars.
science theme: Fundamental properties and processes of Cool Star Interiors
schedule: Mon, 11:45 (contributed)
Anna Guseva
Polytechnic University of Catalonia (UPC)
Authors: A. Guseva (1,2), L. Manchon (2); L. Petitdemange (2); C. Pinçon (3, 2)
Affiliations: (1) Polytechnic Univ. of Catalonia (UPC); (2) The Paris Observatory; (3) Institut d'Astrophysique Spatiale (IAS), France
Despite significant progress in observing stellar magnetic fields, the physical processes that determine their strength and structure - likely shaped by their formation history - remain poorly understood. During the pre-main-sequence (PMS) phase, a star’s inner layers contract, gradually forming a radiative core, while its convective envelope slows due to magnetic interactions with the accretion disk and winds. This creates internal differential rotation, which can disrupt the dynamo processes generating strong dipolar fields observed in protostars. Such disruption could explain the diverse magnetic properties observed in main-sequence stars.
In this talk, I’ll share our recent work on the stability of dipolar magnetic fields inherited from the protostellar phase, focusing on how large-scale radial differential rotation, driven by stellar contraction and interactions with the surrounding medium, affects them. To this aim, we developed 3D convective dynamo simulations of rotating spherical shells, where we imposed differential rotation between the inner and outer boundaries, and density and gravity profiles close to those in PMS low-mass stars, based on predictions from the 1D stellar evolution code Cesam2k20.
Our results show that radial differential rotation can indeed trigger dipole collapse, leading to weaker, oscillatory magnetic fields, when it becomes stronger than the vigor of convection. We derived a collapse criterion from our 3D dynamo simulations and applied it to 1D PMS stellar evolution models, qualitatively reproducing the observed trends in the magnetic topology of low-mass stars when assuming efficient angular momentum transport in stellar radiative cores. This suggests a strong connection between stellar magnetic properties and PMS angular momentum evolution.
science theme: Fundamental properties and processes of Cool Star Interiors
schedule: Mon, 12:00 (contributed)
Henrique Reggiani
NOIRLab
Authors: H. Reggiani(1); J. Yana Galarza(2,3); D. Lorenzo-Oliveira(4); S. Covarrubias(2,5); M. Oyague(6); R. Valle(6); J. Chanamé(7)
Affiliations: (1) Gemini South/NOIRLab; (2) Carnegie Observatories; (3) Univ. de Concépcion; (4)Laboratório Nacional de Astrofísica; (5) Caltech; (6) U. Nacional Mayor de Saan Marcos; (7) Pontificia Univ. Católica de Chile
Standard Evolution Models are incapable of explaining the observed lithium depletion as a function of stellar age in solar-like stars. To do so, one needs to include extra-mixing theories into the models. Proposed mechanisms range from stellar rotation all the way to gravity waves. From an observational perspective, lithium alone seems to be insufficient to constrain which extra-mixing mechanism is responsible for the observed depletion. Along with lithium Beryllium it is one of the lightest elements that can be observed in stellar photospheres and that can be burned in relatively low effective temperatures (3.5 and 2.5 Million Kelvin for Be and Li, respectively). Its measurement in the photosphere of solar-like stars can thus help constrain stellar mixing models because the level of the combined lithium and beryllium depletion as a function of time will indicate how deep the photospheric material must be dredged to explain the observed abundances. In Reggiani et al. (2025), we studied Be in a sample of 20 solar-twins of different ages, which show the typical lithium depletion, to provide an extra constraint to models proposing different extra-mixing mechanisms. Based on our data, models that invoke convective overshoot and convective settling are preferred over typical rotationally induced mixing models, as the latter burn Be in excess while the former do not. Previous works also proposed mixing due to gravity waves as a possible explanation for observed abundances, which can fit our data as well. Furthermore, based on our solar twins, Be depletion likely happens within the first ∼1 Gyr. We also confirm previous findings of an increase in Be abundance as a function of metallicity, indicative of galactic production via cosmic-ray spallation solar twins.
science theme: Fundamental properties and processes of Cool Star Interiors
schedule: Mon, 12:15 (contributed)
Maria Camisassa
Universitat Politècnica de Catalunya
Authors: Camisassa, M. (1); Castro-Tapia, M. (2); Fuentes, J. R. (3); Zhang, S. (2); Schreiber, M. R. (4); Rebassa-Mansergas, A. (1); Torres, S. (1); Raddi, R. (1); Dominguez, I. (5)
Affiliations: (1) Universitat Politècnica de Catalunya; (2) McGill University; (3) University of Colorado Boulder; (4) Universidad Técnica Federico Santa María; (5) Universidad de Granada
White dwarf stars represent the final evolutionary stage of stars with initial masses lower than $\sim 10 M_\odot$. As such, their study provides valuable information on the properties of main-sequence, red giant, and asymptotic giant branch (AGB) stars. Recent analyses of volume-limited samples of magnetic white dwarfs indicate that the incidence of magnetism increases with both stellar age and mass. In this presentation, I show that these observational trends, as well as the inferred magnetic field strengths, are consistent with the emergence of a long-lived, deep-seated magnetic field. We propose that this field is originated by dynamo action within the convective cores of main-sequence stars. Under specific assumptions, we track the evolution of this magnetic field through the red giant and AGB phases and model its subsequent emergence during the white dwarf phase. We demonstrate that a magnetic field of order 50 kG, generated in the convective cores of main sequence stars and preserved within the stellar interior throughout later evolutionary stages, can naturally emerge at the white dwarf surface with strengths and timescales consistent with current observational constraints.
science theme: Advances in Models and Observations of Atmospheres in Cool Objects
schedule: Tue, 09:00 (contributed)
Richard Hoppe + Maria Bergemann
Max Planck Institute for Astronomy
TBA
science theme: Advances in Models and Observations of Atmospheres in Cool Objects
schedule: Tue, 09:15 (contributed)
Veronika Witzke
University of Graz
Authors: V. Witzke (1), A. I. Shapiro (1), C. Breu (1), R. Cameron (2), S. K. Solanki (2)
Affiliations: (1) University of Graz, (2) Max Planck Institute for Solar System Research
Cool M dwarfs have become prime targets in modern stellar and exoplanet astrophysics, as their magnetic activity strongly influence both planet detectability and the interpretation of observed signals. At the same time, their cool atmospheres are dominated by complex molecular opacities, making them among the most challenging stars to model reliably.
We present a new set of fully 3D radiative magnetohydrodynamic (MHD) simulations of cool M dwarfs spanning spectral types from M0 to M8, computed with the MURaM code. These models resolve surface convection and magnetic structuring self-consistently, allowing us to investigate how convective dynamics and magnetic fields shape the photosphere and the manifestation of faculae across the M-dwarf sequence.
Using the MPS-ATLAS code for detailed radiative transfer, we compute synthetic spectra and wavelength-dependent limb darkening (LD) from the 3D simulation cubes.
We examine how stellar parameters and magnetic activity imprint on spectra and LD, and compare these results to 1D models, providing physical insight into model limitations and uncertainties and guiding future improvements in stellar characterisation. We also present preliminary quantitative comparisons with observations of selected benchmark targets.
science theme: Advances in Models and Observations of Atmospheres in Cool Objects
schedule: Tue, 09:30 (contributed)
Aishwarya Iyer
NASA Goddard Space Flight Center
Authors: Aishwarya Iyer (1), Michael R. Line (2), Philip S. Muirhead (3), Jonathan J. Fortney (4), Jacqueline K. Faherty (5)
Affiliations: (1) NASA Goddard Space Flight Center, (2) Arizona State University, (3) Boston University (4) University of California Santa Cruz (5) American Museum of Natural History
M-dwarfs dominate the stellar population of the Galaxy, yet their interiors and atmospheres (especially for cooler M-dwarfs) host tightly coupled physical processes—dust condensation, convective feedback, and magnetically driven heterogeneity—that challenge standard stellar characterization methods. In SPHINX I, we introduced a validated grid of self-consistent radiative–convective model atmospheres and spectra suitable for early-to-mid M-dwarfs. Here, we present SPHINX II, a new-generation model grid extending to mid-to-late type M-dwarfs, explicitly incorporating both gray and physically motivated condensate cloud treatments alongside reduced convective mixing lengths appropriate for low-mass stars. SPHINX II is validated using 39 benchmark FGK+M binary systems observed with SpeX/IRTF and applied to 32 mid-to-late M-dwarfs from the SpeX Prism Library. The models yield statistically improved fits consistent with empirical benchmarks, achieving typical precisions of 0.078 dex in metallicity and 0.13 dex in C/O. Across the grid, condensate cloud mass peaks between 2100–2400 K, declines sharply toward both higher and lower temperatures, and reveals a transition to a cloud-free regime near 2900 K, with deep, buried clouds emerging below 2100 K. As a case study, modeling TRAPPIST-1 demonstrates that even mass-limited silicate clouds subtly but measurably suppress near-infrared flux and redden the mid-infrared continuum through shallow cloud formation near 0.01 bar.
Together, these results establish SPHINX II as a physically motivated framework for interpreting the spectra and fundamental properties of mid-to-late M-dwarfs, with direct implications for stellar physics, population studies, and exoplanet host characterization.
science theme: Advances in Models and Observations of Atmospheres in Cool Objects
schedule: Tue, 09:45 (contributed)
Niamh K. O'Sullivan
Flatiron Institute/University of Oxford
As our understanding of stellar physics increases, supergranulation remains one of the biggest mysteries in the study of stellar variability. We have a very limited understanding of how it affects our spectral observations of stars other than the Sun, and as such we do not fully understand the complex physics that underpins this phenomenon. In this talk I will outline how we can use high signal-to-noise solar observations to start to untangle the spectral impact of this signal. Using a novel time-domain Gaussian Process model, as opposed to the traditional frequency domain modelling techniques previously used, I will show how we can probe supergranulation flows, specifically timescale and amplitude, at different physical depths in the stellar atmosphere. We will investigate how modelled formation temperatures can be used in the context of supergranulation, and look at how the signal changes between spectral lines, the first step in the hunt for a ‘spectroscopic supergranulation indicator’. I will show preliminary results of applying this to other stellar types for the first time, in order to test the stellar scaling laws that we assume, but have not proven, govern the supergranulation signal. From this, I will demonstrate the need for a dedicated supergranulation observational campaign and talk about the Terra Hunting Experiment’s potential SHE survey, one of the first of these campaigns, and the preparations taken for it, as well as multi-instrument campaigns to characterise the signal.
science theme: Advances in Models and Observations of Atmospheres in Cool Objects
schedule: Tue, 10:00 (contributed)
Barbara Perri
AIM - CEA
Authors: B. Perri (1), M. Ausseresse (1), A. S. Brun (1), A. Strugarek (1), V. Réville (2)
Affiliations: (1) CEA-AIM; (2) IRAP
Stellar wind models are crucial in order to understand the environment around a star, and its possible impact on surrounding exoplanets. However, most models suffer from the same limitation: they compute quasi-static relaxed states that are not evolving. In order to take into account active stars that can have rapid changes of structure in their magnetic field that propagate up to the corona, we need to have time-dependent wind models that have assimilative boundary conditions.
We present such a model, improving the Wind Predict model (Réville+2015, Perri+2018) based on the PLUTO code (Mignonne+2007). We start with the simple polytropic version (no Alfvén waves included). Focusing first on the solar case where we have more data, we have modified the boundary conditions of the code in order to assimilate magnetic observations at regular times. We compare first to a standard test case from Lionello+2005, and quantify the impact of various interpolation technics on the quality of the final solution. We finally show a physical case, where the time-dependent model and quantify the addition of the time-dependency for the solar case. We then discuss the impact for stellar winds and stellar models.
science theme: Advances in Models and Observations of Atmospheres in Cool Objects
schedule: Tue, 10:15 (contributed)
Meng Jin
Lockheed Martin Solar & Astrophysics Lab
Authors: Meng Jin (1); Tong Shi (2); Parke Loyd (3); James Mason (4); Karin Dissauer (5); Nariaki Nitta (1); Marc DeRosa (1); Mark Cheung (6); Kevin France (7); Allison Youngblood (8); Xianyu Liu (9); Chip Manchester (9)
Affiliations: (1) Lockheed Martin Solar & Astrophysics Lab; (2) SETI Institute; (3) Eureka Scientific Inc; (4) JHU/APL; (5) University of Graz; (6) CSIRO; (7) University of Colorado, Boulder; (8) NASA/GFSC; (9) University of Michigan, Ann Arbor
Candidate events on stars, supported by clear events on the Sun, suggest “coronal dimming” signals may be a powerful way to study coronal mass ejections (CMEs) across a range of stars. However, magnetically confined events can also produce signals that resemble CME-induced dimming, complicating identification. Here, we present modeling and observational analysis of both confined and eruptive events on the Sun to quantitatively compare dimming morphology and evolution. We show that eruptive events preferentially produce deeper, longer-lived, and more spatially extended dimmings, whereas confined flares tend to yield weaker, shorter-lived, and more localized signatures. Using the newly developed SPECTRUM code, we synthesize EUV/UV spectra to identify features that distinguish eruptive from confined cases. We then extend these results to stellar scenarios, identifying challenges and recommending solutions for using coronal dimming to detect stellar CMEs. With insights from models validated with detailed solar observations, we argue that it is possible to use coronal dimming as a reliable indicator of stellar CMEs.
science theme: Advances in Models and Observations of Atmospheres in Cool Objects
schedule: Tue, 11:00 (invited)
Keiichi Ohnaka
Universidad Andrés Bello
TBA
science theme: Advances in Models and Observations of Atmospheres in Cool Objects
schedule: Tue, 11:30 (contributed)
Behzad Bojnordi Arbab
Chalmers University of Technology
Authors: Behzad Bojnordi Arbab; Miora Andriantsaralaza; Lea Planquart; Theo Khouri; Wouter Vlemmings
Affiliations: Chalmers University of Technology
The significant mass loss of cool Asymptotic Giant Branch (AGB) stars enriching the galactic environment originates in a dynamic extended atmosphere, a turbulent region where giant convective cells and pulsations create the necessary conditions for dust formation. Historically, observational constraints on this critical zone have been fragmented: radio interferometry (e.g., VLA, cm-wave ALMA) resolves the classical thermal surface, while infrared interferometry (e.g., VLTI) probes the outer molecular and dusty layers. Consequently, the continuous physical transition between the hydrostatic stellar surface and the molecular envelope has never been directly observed. In this presentation, I bridge this gap using high-resolution ALMA observations that reveal the evolution of the stellar atmosphere from the radio photosphere to the molecular pseudo-continuum.
We utilize high-resolution ALMA continuum observations of the AGB star R Leo, spanning a wide frequency range to probe the stellar atmosphere layer-by-layer and constrain changing opacity sources. Crucially, this dataset includes multi-epoch monitoring in Bands 4 and 7 to capture intrinsic temporal variability. To interpret these data, we developed an enhanced implementation of UVMultifit utilizing nested sampling for robust visibility fitting. This robust Bayesian framework allowed us to meticulously measure the brightness temperature profile, separating the subtle deviations driven by atmospheric dynamics from frequency-dependent opacity trends.
Our results map the evolution of the radio photosphere: low-frequency emission aligns with shock-heated convective layers, while higher frequencies reveal a significant surface expansion into a molecular pseudo-continuum. By characterizing this opacity shift, we provide critical empirical constraints on the radial structure and temperature profile for dynamic stellar models.
science theme: Advances in Models and Observations of Atmospheres in Cool Objects
schedule: Tue, 11:45 (contributed)
Rachael Roettenbacher
Univerisity of Michigan
Authors: Rachael Roettenbacher; MIRC-X/MYSTIC Collaboration; EXPRES Collaboration
Affiliations: Univerisity of Michigan
Cool, dark starspots are caused by the suppression of convection in the stellar photosphere due to strong magnetic fields. These features are readily observed, often as they rotate into and out of view. With indirect methods that require inverse techniques, information on the exact starspot location, number, and size can be lost due to degeneracies that cannot be broken. Long-baseline optical interferometry, however, provides the opportunity to unambiguously observe the stars and their spots as they appear on the sky. With the 330-m Center for High Angular Resolution Astronomy (CHARA) Array and the H-band MIRC-X and K-band MYSTIC beam combiners, we achieve sub-milliarcsecond resolution and directly image the spotted surfaces of active giant and main-sequence stars. We present interferometric images of stellar surfaces at multiple epochs showing short- and long-term evolution, which emphasizes the strength of this method for tracking changes that can go unobserved with other methods. We additionally highlight the utility of these images for modeling and isolating the radial velocity signature of the stellar surface in exoplanet searches, like the 100 Earths Survey using the EXPRES extreme precision radial velocity spectrograph.
science theme: Advances in Models and Observations of Atmospheres in Cool Objects
schedule: Tue, 12:00 (contributed)
Caroline Morley
University of Texas at Austin
Authors: Caroline Morley (1); Brittany Miles (2); Harshil Kothari (3); James Mang (1); Brianna Lacy (4); Andy Skemer (5)
Affiliations: (1) University of Texas at Austin; (2) University of Arizona; (3) University of Toledo; (4) NASA Ames Research Center; (4) University of California Santa Cruz
With a temperature less than 300 K, the brown dwarf WISE 0855–07 occupies a regime where atmospheric chemistry, clouds, and dynamics operate on truly planetary scales. This object serves as a tantalizing analogue for cold exoplanets, yet our understanding of its atmosphere has been shaped almost entirely by time-averaged spectra—static snapshots of what is, in reality, a dynamic world.
JWST has changed that picture. In this talk, I will present an 11-hour, time-resolved spectroscopic sequence of WISE 0855–07 obtained with JWST/NIRSpec, revealing a richly structured pattern of variability across the 3–5 µm window. The variability is not uniform: it is strongest within specific molecular bands, with carbon monoxide dominating the signal and additional contributions from carbon dioxide and phosphine. These modulations reach amplitudes of several percent, indicating that the atmosphere is neither chemically nor thermally homogeneous.
By applying atmospheric retrievals to individual rotational phases, we move beyond qualitative variability and instead track how the atmospheric state itself changes with time. We find that much of the observed spectral variability can be driven by modulations of the thermal structure, but that the abundances of species in chemical disequilibrium must also change. Complementary self-consistent modeling shows that these chemical modulations are naturally explained in the presence of patchy water clouds interacting with vertically mixed gas, linking horizontal weather patterns to vertical transport in the coldest brown dwarf known.
These results offer the first time-resolved view of weather on a free-floating atmosphere at cold temperatures. WISE 0855–07 emerges not as a static benchmark, but as an active climate system, where clouds, chemistry, and dynamics leave measurable fingerprints in JWST spectra. This work provides a critical bridge between brown dwarf variability studies and the emerging era of spectroscopy of cold exoplanets.
science theme: Advances in Models and Observations of Atmospheres in Cool Objects
schedule: Tue, 12:15 (contributed)
Tadafumi Matsuno
ZAH-ARI, Heidelberg Univ.
Authors: T. Matsuno (1); A. Kemp (2); A. Tanikawa (3); H.-N. Li (4); W. Aoki (5); C. E. Shariat (6); K. J. El-Badry (6); E. Dodd (7); A. Helmi (8); A. J. Koch-Hansen (1); N. Yamaguchi (6); H.-L. Yan (4)
Affiliations: (1) Heidelberg Univ.; (2) KU Leuven; (3) Fukui Prefectural Univ. ; (4) NAOC; (5) NAOJ; (6) Caltech; (7) Durham Univ.; (8) Univ. of Groningen
About one in a hundred stars shows Li-enhancement compared to other stars at similar metallicity and evolutionary status. Among them, those unevolved and before the first dredge-up are the most enigmatic ones since the internal Li production, which is the promising scenario for those evolved and on the red giant branch, is unlikely to provide an explanation for such stars. While about 2/3 of the known unevolved Li-rich stars show no sign of radial velocity variation, all three metal-poor stars that were found to have massive (>1.2 Msun) compact objects as companions were found to be Li-rich. We report on additional radial velocity observations for the first group of stars, confirming the lack of companions. Nonetheless, we discuss potential external polluters that could have enriched the surface of the currently observed stars with Li through a comparison of observed detail chemical abundances and nucleosynthesis models. Classical novae of a massive ONe white dwarf and intermediate AGB stars are found to be the most promising polluters from a chemical perspective. While the binary status poses a challenge to the AGB scenario, we show via a binary population synthesis model that capture of novae ejecta in triple-star systems can provide a viable explanation for the chemical abundance and the binary status of the unevolved Li-rich stars. We further show that there is a potential smoking gun that can confirm the nova scenario by future observations in the ultraviolet.
science theme: Magnetic Activity and Large-scale Events
schedule: Wed, 09:00 (invited)
Kosuke Namekata
Kyoto University / Catholic University of America
The growing number of exoplanet discoveries has emphasized the importance of stellar magnetic activity in shaping exoplanetary atmospheres. Kepler and TESS have revealed that some exoplanet-hosting stars produce frequent flares, highlighting flare-related XUV radiation and particles as potential key drivers of atmospheric chemistry and escape. Not only M dwarfs but also solar-type stars are capable of producing superflares, giving an indication of extreme solar activity in the past. However, much less is known about whether coronal mass ejections (CMEs)—a major factor of star–planet interaction—accompany these events, especially for solar-type stars. Motivated by this gap, over the last six years we have pursued spectroscopic searches for CMEs, primarily using time-resolved H$\alpha$ spectroscopy with the 3.8-m Seimei telescope. From extensive monitoring, we discovered the first evidence of gigantic filament eruptions, interpreted as the initial phase of CMEs, associated with superflares on young solar-type stars, supported by comparisons with Sun-as-a-star observations and solar models. These observations demonstrate that superflare-related eruptions can be far more massive and energetic than the largest solar events. By accumulating multiple events, we have constrained the characteristic frequency, mass, and velocity scales of CMEs, which are critical for evaluating their impact on young exoplanetary systems. Recent studies on M dwarfs and close binaries have also reported CME-related signatures using different wavelength diagnostics, and together these efforts are opening a broader stellar-mass perspective. We have recently extended these to multi-wavelength observations and identified simultaneous signatures in H$\alpha$, X-ray, and far-UV, marking a transition from detection toward physical characterization of stellar eruptions. I will also discuss future prospects for advancing stellar CME studies through coordinated o
science theme: Magnetic Activity and Large-scale Events
schedule: Wed, 09:30 (contributed)
Yuto Kajikiya
Science Tokyo
Authors: Yuto Kajikiya (1); Kosuke Namekata (2); Yuta Notsu (3); Kai Ikuta (4); Hiroyuki Maehara (5); Bunei Sato (1); Daisaku Nogami (2)
Affiliations: (1) Science Tokyo; (2) Kyoto Univ.; (3) Univ. of Colorado Boulder; (4) Hitotsubashi Univ.; (5) National Astronomical Observatory of Japan
M-dwarfs produce frequent flares, and their associated mass ejections are expected to significantly affect the habitability of close-in exoplanets. Recent spectroscopic observations have revealed several prominence eruptions---indicative of stellar mass ejections---on M-dwarfs through Doppler shifts of the H$\alpha$ line. However, systematic and statistical studies, particularly regarding their association with white-light flares, have been limited due to the lack of intensive and continuous simultaneous photometric and spectroscopic monitoring of the same target star. We conducted one month of continuous spectroscopic observations of the active M-dwarf YZ CMi using the 3.8-m Seimei telescope with an unprecedentedly high time cadence of $\sim$1~min, simultaneously with TESS. We detected four prominence eruptions, among which two events showed rapid, short-duration eruptions with velocities of $\sim$300--500~km~s$^{-1}$ and durations of approximately 5~min (Kajikiya et al. 2025a, ApJ). Such short-duration events may have been missed in previous observations due to insufficient time cadence. We further performed a systematic analysis using a total of 35 H$\alpha$ flares on YZ CMi observed simultaneously with TESS. Notably, most prominence eruptions (6 out of 7) were not associated with detectable white-light flares (Kajikiya et al. 2025b, ApJ). This result suggests that most observed prominence eruptions on M-dwarfs may have occurred near the stellar limb, where white-light flares are difficult to detect, which may imply a potential observational bias in their detectability due to low contrast with the background emission. These first statistical constraints, together with the discovery of rapid, short-duration prominence eruptions, indicate that previous observations may have underestimated both the frequency and velocities of mass ejections on M-dwarfs due to observational biases and highlight the necessity of reassessing their impact on close-in exoplanets.
science theme: Magnetic Activity and Large-scale Events
schedule: Wed, 09:45 (contributed)
Joseph Callingham
ASTRON / University of Amsterdam
Authors: J. R. Callingham (1,2); C. Tasse (3,4,5); R. Keers (6,7); R. D. Kavanagh (1,2); H. K. Vedantham (1,8); P. Zarka (5,9); S. Bellotti (10,11); P. I. Cristofari (12); S. Bloot (1,8); D. C. Konijn (1,8); M. J. Hardcastle (13); L. Lamy (5,9,14); E. K. Pass (12,15); B. J. S. Pope (16); H. Reid (17); H. J. A. Röttgering (10); T. W. Shimwell (1,10); P. Zucca (1)
Affiliations: (1) ASTRON, Netherlands Institute for Radio Astronomy; (2) Anton Pannekoek Institute for Astronomy, University of Amsterdam; (3) LUX, Observatoire de Paris, Université PSL, CNRS; (4) Centre for Radio Astronomy Techniques and Technologies (RATT), Department of Physics and Electronics, Rhodes University; (5) Observatoire Radioastronomique de Nançay (ORN), Observatoire de Paris, CNRS, PSL, Université d'Orléans, OSUC; (6) Max Planck Institute for Solar System Research; (7) Faculty of Electrical Engineering, Information Technology, and Physics, Technische Universität Braunschweig; (8) ASTRON, Netherlands Institute for Radio Astronomy; (9) Anton Pannekoek Institute for Astronomy, University of Amsterdam; (10) Kapteyn Astronomical Institute, University of Groningen; (11) Observatoire Radioastronomique de Nançay (ORN), Observatoire de Paris, CNRS, PSL, Université d'Orléans, OSUC; (12) LIRA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris; (13) Leiden Observatory, Leiden University; (14) Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse, CNRS; (15) Center for Astrophysics, Harvard & Smithsonian; (16) Centre for Astrophysics Research, University of Hertfordshire; (17) LAM, Aix-Marseille Université, CNRS, CNES; (18) Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology; (19) Department of Physics and Astronomy, Macquarie University; (20) Mullard Space Science Laboratory, University College London.
Coronal Mass Ejections are predicted to play an important role in planetary atmospheric erosion, especially for planets that are close to their host star. However, this conclusion remains controversial because, despite decades of searches, there has not been an unambiguous detection of a CME from a star beyond our Sun. A clear signature of a CME is a type II radio burst, which is emitted from the shock wave produced as the CME travels through the stellar corona into interplanetary space. In this talk, I will present the first detection of a stellar Type II radio burst, originating from the early M dwarf StKM 1-1262 (Callingham et al., Nature, 2025). I will show how the burst constrains the CME shock properties and the coronal environment of a low-mass star, and discuss the implications for space weather and atmospheric loss for a hypothetical planet in the star’s habitable zone. I will conclude with new radio-burst discoveries from our ongoing wide-field survey of the northern sky, opening a path toward population-level constraints on stellar CMEs.
science theme: Magnetic Activity and Large-scale Events
schedule: Wed, 10:00 (contributed)
Meredith MacGregor
Johns Hopkins University
Authors: Meredith A. MacGregor(1); Kiana Burton (2); Rachel A. Osten (3,1); Guadalupe Tovar Mendoza (1); Kylee Carden (1); Ward S. Howard (2); J. Sebastian Pineda(2); Evgenya Shkolnik (4); Laura D. Vega (5,6); David J. Wilner (7); Jan Forbrich (7,8); Elisa Quintana (5); Thomas Barclay (5;9); Alycia J. Weinberger (10)
Affiliations: (1) Johns Hopkins University; (2) University of Colorado Boulder; (3) Space Telescope Science Institute; (4) Arizona State University; (5) NASA Goddard Space Flight Center; (6) University of Maryland; (7) Center for Astrophysics | Harvard & Smithsonian; (8)University of Hertfordshire; (9) University of Maryland Baltimore County; (10) Earth & Planets Laboratory, Carnegie Institution for Science
M dwarf stars are the most abundant stars in the galaxy and have a high frequency of Earth-sized planets, making them favored targets of upcoming missions to detect and characterize exoplanets. However, these stars are known to exhibit high levels of activity and flaring, which can deplete a planet's atmosphere of ozone over time, raising questions about the habitability of planets around these stars. We detected the first millimeter flaring event from Proxima Cen using the Atacama Large Millimeter/submillimeter Array (ALMA) during which the star brightened by a factor of 1000x over 1 minute (MacGregor et al. 2018). Since then, numerous millimeter flares have been discovered with both ALMA and other facilities suggesting that these events are actually common and allowing for construction of the first flare frequency distribution (FFD) at millimeter wavelengths (Burton et al. 2025). This emphasizes the need for observing campaigns across the entire electromagnetic spectrum targeted at catching and characterizing stellar flares. I will present recent results from ALMA, JWST, TESS, HST, Swift, Chandra, and other ground-based facilities showing the multi-wavelength behavior of stellar flares. This work is especially time critical to inform the interpretation of JWST observations of exoplanet atmospheric spectra and ultimately target selection for HWO.
science theme: Magnetic Activity and Large-scale Events
schedule: Wed, 10:15 (contributed)
Lauren Doyle
University of Warwick
Authors: L. Doyle (1); G. W. King (2); G. Ramsay (3); L. R. Corrales (2); S. Bagnulo (3); J. G. Doyle (3); P. Hakala (4)
Affiliations: (1)University of Warwick; (2) University of Michigan; (3) Armagh Observatory and Planetarium; (4) Finnish Centre for Astronomy with ESO
Ultra-fast rotating (UFR) M dwarfs with rotation periods below one day occupy an extreme and poorly understood regime of stellar magnetism. Despite expectations from the activity–rotation relation, several UFRs exhibit surprisingly weak optical flaring, raising fundamental questions about how magnetic energy is stored and released at the fastest rotation rates. We present a multi-wavelength study of ten UFR M dwarfs (M2–M6) combining TESS photometry, optical spectropolarimetry, and X-ray observations to directly test leading explanations for this behaviour. Using TESS light curves spanning up to seven years, we identify more than 950 flares with energies between ~10$^{31}$ and 10$^{35}$ erg and quantify changes in flare rates and light-curve morphology on multi-year timescales. While some targets show evolving spot-modulated variability indicative of changes in large-scale magnetic structure, we find no consistent evidence for long-term flare suppression or coherent activity cycles. Kilogauss-strength magnetic fields are detected in half of the sample, preferentially in the more flare-active stars, demonstrating that strong fields alone are insufficient to guarantee efficient flare production. X-ray luminosities from Swift and XMM-Newton place these stars at or above the saturated activity level, ruling out supersaturation as the primary mechanism limiting flaring. If neither rotation rate, magnetic field strength, nor supersaturation controls flare efficiency in ultra-fast rotators, is it the magnetic field topology, or even the underlying dynamo behaviour, which ultimately governs magnetic energy release in these stars?
science theme: Magnetic Activity and Large-scale Events
schedule: Wed, 11:00 (invited)
Aline Vidotto
Leiden University
TBA
science theme: Magnetic Activity and Large-scale Events
schedule: Wed, 11:30 (contributed)
Kevin France
University of Colorado
Authors: Kevin France and the MUSCLES Collaboration
Affiliations: (1) University of Colorado; (2) MUSCLES
The extreme-ultraviolet (EUV; 100 -- 911 \AA) spectra of F, G, K, and M stars provide diagnostics of the stellar chromosphere through the corona, with line and continuum formation temperatures spanning roughly 10$^{4}$ - 10$^{7}$ K. The EUV stellar spectrum in turn drives atmospheric photochemistry and numerous escape processes on orbiting planets. We present a new study of the EUV history of solar-type stars, using new and archival {\it Hubble Space Telescope} observations of solar analogs (T$_{\odot}$ $\pm$ 150 K for stars older than 100 Myr) and ``Young Suns" (age < 100 Myr) that will evolve into main sequence early G-type stars to predict the 90 -- 360 \AA\ EUV flux from a sample of 23 stars. We find that the EUV activity evolution for solar-type stars follows a two-component behavior: a saturated L(EUV)/L$_{bol}$ plateau (at a level of about 10$^{-4}$) followed by a power law decay ($\alpha$ $\approx$ $-$1.1) after ages of $\approx$ 50 -- 100 Myr. Consequently, the EUV flux incident at 1 AU around solar analogs varies over the lifetime of the Sun, ranging from 100 $\times$ the present day UV irradiance at 10 Myr to 0.3 $\times$ the present-day level at 10 Gyr. We find that the EUV luminosity is approximately the same as the soft X-ray luminosity up to approximately 1 Gyr, after which the EUV luminosity of the stars dominate. In comparison to Sun-like stars, the EUV saturation level of early/mid M dwarfs is several times higher and lasts $\sim$10 -- 20 times longer.
science theme: Magnetic Activity and Large-scale Events
schedule: Wed, 11:45 (contributed)
Katja Poppenhaeger
Leibniz Institute for Astrophysics AIP
The rotational spin-down of cool main-sequence stars governs their long-term magnetic evolution. In recent years, substructure in the age-rotation relationship of cool stars has been identified, showing deviations from the historical Skumanich law. A weakened magnetic braking regime and potentially a break-down of the magnetic dynamo have been proposed as reasons for this deviation. We have investigated a sample of mature main-sequence stars with solar-like masses which display faster rotation than expected. We collected deep observations of those stars in soft X-rays, yielding information on their coronal magnetic activity, and in particular on their X-ray surface fluxes and average coronal temperatures. We detected the majority of the sample in X-rays, at flux levels that are at the low end of the empirical rotation-activity relationship, but not indicative of a full break-down of the magnetic dynamo itself. However, we do find a trend that the more over-rotating a star is for its age, the more under-active it is for its rotation period. We also find that the strongest outliers from the empirical rotation-activity relationship are characterized by very low X-ray fluxes through the stellar surface and very low coronal temperatures. Therefore, angular momentum loss through the stellar wind may be impeded for such stars, while overall magnetic activity can still be observed. An open question is why there are no rotational outliers detected so far in middle-aged open clusters such as M67, challenging an interpretation that relies purely on a reduction of the angular momentum loss rate for middle-aged suns.
science theme: Magnetic Activity and Large-scale Events
schedule: Wed, 12:00 (contributed)
Shin Toriumi
Japan Aerospace Exploration Agency
Authors: S. Toriumi (1); K. Namekata (2,3); Y. Notsu (4,5); M. Yamashita (6); V. S. Airapetian (3,7)
Affiliations: (1) JAXA; (2) Kyoto Univ.; (3) NASA GSFC; (4) CU Boulder; (5) NSO; (6) Shimane Univ.; (7) American Univ.
Cool dwarf stars, including the Sun, commonly host outer atmospheres composed of $10^4$-Kelvin chromospheres, transition regions, and million-Kelvin coronae emitting X-ray and extreme ultraviolet (XUV) radiation that affects planetary atmospheres and habitability. Understanding the heating mechanisms operating in these outer stellar atmospheres is a major challenge in solar and stellar physics. For the Sun, it is thought that magnetic fields transport energy upward; however, whether this process is universal among stars has long been uncertain. To address this, we analyzed decade-long multi-wavelength solar observations and compared them with stellar data. Our findings reveal that power-law scaling relations between magnetic flux and emission-line luminosities across temperature regimes from $10^4$ to $10^7 K$ demonstrate that solar scaling laws extend to Sun-like stars aged 50 Myr to 4.5 Gyr. This suggests that magnetically driven heating operates universally, independently of age or activity. We then derived empirical laws that link magnetic fluxes to XUV spectra, enabling reconstruction of stellar XUV outputs from observed magnetic fluxes. Although the XUV radiation is crucial for planetary atmospheric escape, EUV is currently unobservable due to strong interstellar absorption and thus reconstruction is necessary. Our modeled spectra serve as critical inputs for photochemical modeling of planetary atmospheres and habitability assessments, including conditions on early Earth and rocky exoplanets around young Sun-like stars. Furthermore, we find that the mean stellar magnetic field strength and its variation follow the same scaling law for pre-main-sequence stars, zero-age main-sequence stars, and main-sequence stars. This work establishes a universal framework for stellar atmospheric heating and XUV prediction, essential for exoplanet habitability studies and future missions.
science theme: Magnetic Activity and Large-scale Events
schedule: Wed, 12:15 (contributed)
Masahiro Tsujimoto
JAXA ISAS
Authors: M. Tsujimoto (1,2); M. Kurihara (1,2)
Affiliations: (1) JAXA ISAS; (2) Univ. of Tokyo
High-resolution X-ray spectroscopy is the most direct probe of the hottest plasma
produced by stellar flares. For the Sun, pioneering results were obtained with
Hinotori/SOX, SMM/BCS, and Yohkoh/BCS in 1980-2000, but no comparable mission has been
realized since then. For distant stars, diverse results were obtained with
Chandra/LETGm, HETG and XMM-Newton/RGS since 1999. The launch of XRISM in 2023
introduced Resolve, the first high-resolution X-ray microcalorimeter spectrometer for
distant stars. Its resolving power in the 2-12 keV band enables, for the first time,
direct measurements of fine-structure and satellite lines in giant stellar flares.
We present XRISM observations of two RS CVn-type binaries, GT Mus and HR 1099 (Kurihara
et al. 2025, 2026). In GT Mus, we resolved the Fe XXIV-XXV K-shell main and satellite
lines for the first time in a stellar source, enabling quantitative tests of collisional
ionization equilibrium and of the Maxwellian electron energy distribution. In HR 1099,
XRISM captured the first stellar flare ever observed with a microcalorimeter, lasting
approximately 100 ks. The detection of inner-shell Fe XIX-XXIV and outer-shell Fe
XXV-XXVI lines allowed precise reconstruction of the differential emission measure and
elemental abundances. We find a significant enhancement of low-FIP elements,
particularly Ca and Fe, during the flare, providing the first direct evidence of
FIP-related fractionation in stellar flares and demonstrating that XRISM opens a new
regime of quantitative stellar flare plasma physics.
science theme: Cool Stars with Protoplanetary Disks and Exoplanetary Systems
schedule: Thu, 09:00 (invited)
TBA
TBA
TBA
science theme: Cool Stars with Protoplanetary Disks and Exoplanetary Systems
schedule: Thu, 09:30 (contributed)
Songhu Wang
Indiana University
Authors: Songhu Wang (1); Xian-Yu Wang (1); Joel Ong (2)
Affiliations: (1) Indiana University; (2) University of Sydney
Hot Jupiters are sometimes found on strongly spin–orbit misaligned orbits around hot stars, but tend to be aligned around cool stars. This ``stellar obliquity transition" is often assumed to coincide with the Kraft break in stellar rotation. Yet the oft-quoted obliquity transition in the literature (6100K or 6250K) lies several hundred kelvin cooler than the well-determined rotation break near 6500 K, creating an apparent inconsistency. We show that this mismatch arises largely from different treatment of stellar multiplicity: rotation studies typically exclude binaries, whereas obliquity samples historically could not do so uniformly — because there was no systematic, homogeneous way to identify stellar companions of the sample prior to Gaia. When we restrict the obliquity sample to single-star systems, the inferred obliquity transition shifts upward to ~6500K, matching the single-star rotation break. This revision has two immediate consequences. First, there are very few Rossiter–McLaughlin measurements for planets beyond hot Jupiters around truly hot stars (Teff>6500K), limiting current tests of whether large misalignments are specific to hot-Jupiter-like planets (e.g., high-e migration) or instead reflect a broader property of hot-star hosts across architectures. Second, the 6500K coincidence favors convective-envelope tidal dissipation that weakens in tandem with magnetic braking. It disfavors resonance locking as the dominant explanation, which instead predicts a cooler transition near 6100K, at the onset of a convective core.
science theme: Cool Stars with Protoplanetary Disks and Exoplanetary Systems
schedule: Thu, 09:45 (contributed)
Claudia Aguilera-Gómez
Pontificia Universidad Católica de Chile
Authors: C. Aguilera-Gómez(1); L.K. Rogers(2); A.Bonsor(3); P. Jofré(4); S. Blouin;O. Shorttle; A. Buchan; Y. Li; S. Xu
Affiliations: (1)Pontificia Universidad Católica de Chile;(2)NSF NOIRLab;(3)University of Cambridge;(4) Universidad Diego Portales
"Planets form from the same molecular gas and dust as their host stars. However, the extent to which rocky planetary bodies preserve the elemental composition of their natal material remains uncertain, and key questions persist: Do planets retain refractory elements? Are volatile and moderately refractory elements depleted during formation? Polluted white dwarfs, i.e., those with atmospheres containing metals from recently accreted rocky material, provide a unique window into the bulk composition of exoplanetesimals. I will discuss how polluted white dwarfs in wide binary systems can be used to compare the composition of planetary material with that of the host star. In these systems, the main-sequence companion, often an M dwarf, serves as a proxy for the composition of the natal molecular cloud while the polluted white dwarf traces the composition of the disrupted planetary body. Through detailed spectroscopic abundance analyses of both components, we can directly test whether planets preserve stellar abundance patterns. Using this framework, we find that planetary material can show differences relative to the host star, with lower abundances of elements such as Mg, Si, and Fe compared to more refractory species like Ca and Ti. Considering elements as a function of their condensation temperature helps link stellar and planetary compositions, showing that processes during planet formation and early evolution can modify element ratios. The observed behavior is similar to the difference between the bulk Earth and the Sun, indicating a common chemical processing mechanism in rocky planets. I will highlight the importance of wide binaries containing polluted white dwarfs as laboratories to link stellar abundances and the interior composition of rocky exoplanets."
science theme: Cool Stars with Protoplanetary Disks and Exoplanetary Systems
schedule: Thu, 10:00 (contributed)
Takato Tokuno
Univ. of Osaka
A recent asteroseismic analysis suggests that Kepler-56 ─ a planet-hosting red giant ─ exhibits a unique spin structure: (1) the spin axes of the core and envelope are misaligned; and (2) the envelope rotates approximately an order of magnitude faster than typical red giants. In this paper, we investigate a feasible scenario to reproduce this spin structure by estimating the amount of the angular momentum (AM) supply from the planets through the simplified calculation of the time evolution of AM. As a result, unless the tidal efficiency is extremely high, we show that the tidal interactions between the known close-in planets (Kepler-56 b and c) are insufficient to supply the AM required to accelerate Kepler-56 from the spin rate observed in typical red giants. We also show that the engulfment of a hot Jupiter can be expected to provide sufficient AM supply for the acceleration and that the mass and orbit of the engulfed hot Jupiter are constrained by a mass of 0.5─2 Jupiter masses and an orbital period of 1─6 days. On the other hand, if Kepler 56 was already rapidly spinning before entering the RG stage and requires no acceleration, the obliquity damping by the known close-in planets can reproduce the spin structure of Kepler-56. Even in such cases, planetary engulfment during the MS stage might be involved in achieving rapid spin before the tidal alignment. These discussions demonstrate the importance of Kepler-56 as a candidate for planetary engulfment that may leave traces of its spin structure.
science theme: Cool Stars with Protoplanetary Disks and Exoplanetary Systems
schedule: Thu, 10:15 (contributed)
Ekaterina Ilin
Netherlands Institute for Radio Astronomy (ASTRON)
Authors: E. Ilin (1); H. K. Vedantham (1); K. Poppenhaeger (2,3); S. Bloot (1,4); J.R. Callingham (1,5); A. Brandeker (6); H. Chakraborty (7)
Affiliations: (1) Netherlands Institute for Radio Astronomy (ASTRON); (2) Leibniz Institute for Astrophysics Potsdam (AIP); (3) Institute for Physics and Astronomy, University of Potsdam; (4) Kapteyn Astronomical Institute, Univeristy of Groningen; (5) Anton Pannekoek Institute (API), University of Amsterdam; (6) Department of Astronomy, Stockholm University; (7) Geneva Observatory, University of Geneva
In the past decade, hundreds of exoplanets have been discovered in extremely short orbits below 10 days. Unlike in the Solar System, planets in these systems orbit their host stars close enough to disturb the stellar magnetic field lines. The interaction can enhance the star's magnetic activity, such as its chromospheric and radio emission, or flaring. So far, the search for magnetic star-planet interactions has remained inconclusive. In this talk, I present the first detection of planet-induced flares on HIP 67522, a 17 million-year-old G dwarf star with two known close-in planets. Combining space-borne photometry from TESS and dedicated CHEOPS observations over a span of 5 years, we find that the 15 flares in HIP 67522 cluster near the innermost planet's transit phase, indicating persistent magnetic star-planet interaction in the system. The stability of interaction implies that the innermost planet is continuously self-inflicting a six times higher flare rate than it would experience without interaction. The subsequent flux of energetic radiation and particles bombarding HIP 67522 b may explain the planet's remarkably extended atmosphere, recently detected with the James Webb Space Telescope, potentially doubling its mass loss rate. Our results establish HIP 67522 as the archetype system for flaring star-planet interaction, and urge further characterization of the dynamic magnetism of this and similar star-planet systems.
science theme: Cool Stars with Protoplanetary Disks and Exoplanetary Systems
schedule: Thu, 11:00 (invited)
Adina Feinstein
Michigan State University
Young planetary systems offer a unique advantage in that their atmospheric compositions are more representative of primordial formation conditions. This is particularly advantageous when compared to more evolved planets which have experienced significant stellar irradiation processing and pollution. The 23 Myr M star AU Mic and its transiting exoplanets are an ideal laboratory for understanding the impact of stellar activity on young planetary atmospheres. In this talk, I will present a comprehensive analysis of four transit observations of the young Neptune-sized planet AU Mic b as observed simultaneously with JWST/NIRCam (2-5 micron) and Hubble/COS (1060 - 1360 Angstrom). I will present a characterization of AU Mic the star at these wavelengths, specifically highlighting the measured flare temperatures and unique features observed. Despite the challenge of disentangling a constant background of flares and starspot crossing events, I will present the detected suite of CNOS-bearing molecules in the atmosphere of AU Mic b. These results represent the first atmospheric detection of a young planet orbiting an M star. I will contextualize our measured abundances against more evolved exoplanets, which suggest that young planetary atmospheres are chemically distinct from their mature counterparts, likely reflecting their evolution processes.
science theme: Cool Stars with Protoplanetary Disks and Exoplanetary Systems
schedule: Thu, 11:30 (contributed)
Patrick McCreery
Johns Hopkins University
Authors: Patrick McCreery (1); Kevin Schlaufman (1); Henrique Reggiani (2); Zafar Rustamkulov (3)
Affiliations: (1) Johns Hopkins University; (2) NOIRLab, Gemini South; (3) IPAC at Caltech
Determining the age of an isolated field star is one of the most challenging inferences in astronomy, especially for hot Jupiter systems that may have experienced angular momentum exchange that violates the assumptions of most age–stellar rotation calibrations (i.e., gyrochronology). One promising alternative approach to the age inference problem is to use 1% precision host star mean density measurements based on James Webb Space Telescope (JWST) mid infrared transit light curves of hot Jupiters that are substantially unaffected by stellar limb darkening. Using these density measurements as constraints on state-of-the-art stellar age inferences based on all available Gaia astrometry, multiwavelength photometry from the ultraviolet to the mid infrared, and high-resolution spectroscopy results in age precisions exceeding similar approaches based on short-cadence Kepler asteroseismology of its LEGACY sample. Supporting its accuracy, our approach produces 1% precision stellar masses and radii consistent with star–planet SB2 solutions and interferometric measurements for nearby stars, along with precise photospheric elemental abundances. Applying our approach to the sample of 12 hot Jupiter hosting stars observed in the JWST Grand Tour program, we obtain unparalleled age precisions. As an example, for the JWST Cycle 1 target HD 189733, we find an age consistent with previous inferences but nearly an order of magnitude more precise. HD 189733 is rotating more quickly than open cluster K dwarfs of similar masses and ages, and we interpret its excess angular momentum as evidence of approximately 0.04 AU of inward tidal migration by HD 189733 b. Over its expected 10-year lifetime, we expect JWST light curves for hundreds of transiting giant planets to yield similarly precise age inferences and enable precise studies of the time evolution of giant planet systems.
science theme: Cool Stars with Protoplanetary Disks and Exoplanetary Systems
schedule: Thu, 11:45 (contributed)
Aida Behmard
Flatiron Institute
Authors: Aida Behmard (1); Casey Brinkman (2); Soichiro Hattori (3); Ryan Rubenzahl (1); Megan Bedell (1)
Affiliations: (1) Flatiron Institute; (2) McGill University; (3) Columbia University
Planets and their host stars form from the same cloud of gas and dust, so we assume that their chemical compositions are linked. However, a clear correlation between rocky planet interior properties and host star chemistry remains elusive for planets around FGK dwarfs, and non-existent for planets around M dwarfs because cool stars frequently lack detailed chemical information. Here, we investigate the relationship between small ($R_{P}$ $\leq$ 1.8 $R_{\oplus}$) planet densities and host star chemical abundances for both FGK and M dwarfs. We use datasets from the Sloan Digital Sky Survey-V and an accompanying data-driven framework to obtain abundances for FGK and M dwarf hosts of 22 rocky planets. We find that planet densities exhibit a strong, inverse relationship to [Mg/Fe] abundances of FGK hosts (p = 0.001). This correlation becomes more significant with the addition of M dwarf hosts (p = 0.0005). If we assume that rocky planets have terrestrial-like compositions, this suggests that low [Mg/Fe] environments form planets with larger Fe-rich cores and thus higher densities around both solar-like and cool stars. Rocky planets located in the Milky Way's thick disk help anchor this trend, illustrating the importance of sampling exoplanet properties across a range of host star populations. This finding highlights the connection between Galactic chemical evolution and rocky planet formation, and indicates that Earth-like planet compositions may vary significantly across different regions of our Galaxy.
science theme: Cool Stars with Protoplanetary Disks and Exoplanetary Systems
schedule: Thu, 12:00 (contributed)
Manoj Puravankara
Tata Institute of Fundamental Research, Mumbai, India
Authors: M. Puravankara (1); S. T. Megeath (2); Himanshu Tyagi (1); Sam Federman (3); Mayank Narang (4); Dan Watson (5); Alessio Caratti o Garatti (3); David Neufeld (6); Ewine van Dishoeck (7); IPA & HEFE Teams
Affiliations: (1)TIFR, Mumbai; (2)Univ. of Toledo; (3) INAF-Osservatorio Astronomico di Capodimonte, Italy; (4) JPL/CalTech; (5) Univ. of Rochester; (6) Johns Hokins Univ.; (7) Leiden Observatory
Jets and winds are fundamental to star formation, enabling accretion through angular-momentum removal while simultaneously providing powerful feedback that shapes disks, envelopes, and the surrounding molecular cloud. They regulate how stars assemble their mass and set the initial conditions for disk evolution and planet formation. Despite their central role, the physical origin, launching mechanisms, and evolution of jets and winds remain poorly constrained, particularly during the deeply embedded protostellar phase when accretion and mass loss are most intense. The protostellar stage represents a critical yet observationally challenging phase of early stellar evolution, during which disks are assembled, disk–outflow coupling is established, and much of the final stellar mass is accumulated. Infrared diagnostics are essential for probing this phase, as the key tracers of shocks, molecular winds, and accretion-powered activity are heavily obscured at optical wavelengths. In this talk, we will present results from two of the largest JWST General Observer programs targeting the youngest protostars: Investigating Protostellar Accretion (IPA), a 67-hour Cycle 1 program probing accretion and outflows across a broad mass range (0.1–10 M⊙), and High Angular Resolution Observations of Stellar Emergence in Filamentary Environments (HEFE), a 180-hour Cycle 3 program providing an extensive spectral-imaging survey of protostars emerging from dense filaments. Spatially resolved JWST imaging and spectroscopy reveal wide-angle molecular winds traced by warm and hot H₂, highly collimated jets, and stratified velocity and excitation structures that directly link accretion, disk winds, and feedback. These observations place strong new constraints on disk-launched wind models and offer a coherent picture of how accretion and ejection operate at the earliest stages of stellar evolution.
science theme: Cool Stars with Protoplanetary Disks and Exoplanetary Systems
schedule: Thu, 12:15 (contributed)
Nick Tusay
University of Washington
Authors: N. Tusay (1); M. Hon (2); S. Rappaport (3); A. Shporer (3); A. Vanderburg (4); J. Wright (5)
Affiliations: (1) University of Washington; (2) National University of Singapore; (3) Massachusetts Institute of Technology; (4) Harvard University; (5) Pennsylvania State University
Disintegrating planets, a unique subset of ultrashort-period planets, produce tails of gas and dust extending far out into space that can be studied with transmission spectroscopy.BD+05 4868 Ab is the latest of these objects to be discovered, and the only one definitively identified with TESS, so far. It orbits a K5V star with an apparent magnitude of V~10,making it the brightest target for the known objects, every 1.27 days. Using the MIRI low-resolution spectrograph in a Cycle 4 JWST Program in October 2025, we observed a fullphase curve of BD+05 4868 Ab, including 2 transits of the central core where the planet is believed to be embedded. By comparing the resulting spectra of the transiting effluents inthe tails to dust extinction models for various likely minerals we seek to determine information on the outflowing material. The high signal-to-noise of these observations offers thebest opportunity to precisely infer the composition of a rocky planet around a main sequence star, providing insights into the formation of terrestrial planets and their potentialhabitability. Here we present the preliminary results of our analysis thus far.
science theme: Cool Stars in the Context of Cosmic Evolution
schedule: Fri, 09:00 (invited)
Keith Hawkins
The University of Texas at Austin
TBA
science theme: Cool Stars in the Context of Cosmic Evolution
schedule: Fri, 09:30 (contributed)
Dionysios Gakis
The University of Texas at Austin
Authors: D. Gakis (1); K. Hawkins (1); Z. Maas (2); C. Sneden (1)
Affiliations: (1) The University of Texas at Austin; (2) Indiana University
Stars serve as long-lived fossil records of chemical enrichment across cosmic time, encoding the formation and evolutionary histories of their host galaxies through their atmospheric abundances. Among the elements traced in stellar atmospheres, phosphorus (P) remains one of the least constrained despite its importance for both galactic chemical evolution and astrobiology. This is primarily due to the intrinsic weakness of P I lines, for which the only viable stellar diagnostics are found in the UV or near-infrared spectra of cool stars, requiring high-resolution and high–signal-to-noise observations. In this work, we present the first measurements of P abundances in FGK stars belonging to the accreted Gaia–Enceladus–Sausage (GES) merger remnant, together with the largest homogeneous sample of P abundances in in-situ Milky Way stars to date, using high-resolution near-infrared spectra obtained with the HPF instrument on the Hobby–Eberly Telescope. By applying detailed spectral synthesis techniques, we directly compare [P/Fe] trends in in-situ Milky Way stars and ex-situ stars originating from a dwarf-galaxy environment. We find that GES stars exhibit systematically lower [P/Fe] than in-situ Milky Way stars at fixed [Fe/H], with an offset of order ≈0.3 dex. This behavior points to a distinct phosphorus enrichment pathway in accreted systems, consistent with slower chemical evolution and reduced star formation efficiency. Current nucleosynthetic models struggle to reproduce observed phosphorus abundances, and our results provide new empirical constraints on phosphorus production in different galactic environments. Given phosphorus’s role as a bio-essential element, these results also inform the chemical conditions under which planetary systems form in dwarf-galaxy environments, emphasizing the unique role of cool stars’ abundances in linking galactic assembly history to the broader question of habitability across cosmic environments.
science theme: Cool Stars in the Context of Cosmic Evolution
schedule: Fri, 09:45 (contributed)
Dario Gonzalez Picos
Leiden University
Authors: Dario Gonzalez Picos (1); Ignas Snellen (1); Sam de Regt (1)
Affiliations: (1) Leiden University
Elements heavier than hydrogen and helium, or metals, are synthesised in stars and dispersed into the interstellar medium during the final stages of stellar evolution. Minor isotopes are also produced in specific nucleosynthetic processes, providing key insights into the chemical evolution of our Galaxy, yet robust measurements in cool stars remain scarce. We present the first systematic study of rare carbon and oxygen isotopes in 32 nearby M dwarfs spanning the metallicity range of the solar neighbourhood. Using high-resolution K-band spectroscopy, we employ a novel approach that simultaneously fits atmospheric parameters and isotope ratios. Our retrievals extend methods developed for young planets and brown dwarfs to M dwarfs (3000–4000 K), using state-of-the-art linelists and on-the-fly radiative transfer (petitRADTRANS; Mollière et al. 2019). We obtain precise abundances for CO, H₂O, OH, CN, HF, Fe, Ti, Ca, and Sc, and isotope ratios ¹²C/¹³C and ¹⁶O/¹⁸O from CO isotopologues. We find a clear decline in ¹²C/¹³C with metallicity, consistent with progressive ¹³C enrichment, and values below solar point to additional sources, such as novae. Metal-rich M dwarfs show ¹⁶O/¹⁸O ratios well below solar (~200), with tentative evidence for a turnover at subsolar metallicity, matching galactic chemical evolution predictions of enhanced primary ¹⁸O production in massive, low-metallicity stars (e.g. Romano 2022). These results establish M dwarfs as powerful tracers of isotopic enrichment, opening a new observational window onto Galactic chemical evolution. I will present our state-of-the-art modelling, ongoing efforts to expand isotopic surveys across metallicities, and how GCE models link isotope ratios to the enrichment history of the solar neighbourhood and beyond.
science theme: Cool Stars in the Context of Cosmic Evolution
schedule: Fri, 10:00 (contributed)
Ankita Mondal
Swinburne University of Technology
Authors: Ankita Mondal (1); Michael T. Murphy (1); Chris Flynn (1,2)
Affiliations: (1) Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia; (2) Centre of Excellence for Gravitational Wave Discovery (OzGrav), Australia
K-dwarf stars offer a unique window into the formation and chemical evolution of the Milky Way. Their slow evolution and long lifetimes allow them to retain the chemical signatures of their birth material, making them useful tracers of the Galaxy’s enrichment history. In this work, we present a study using ∼25,000 K-dwarfs from the GALAH DR4 survey to achieve two key goals: (1) to improve the precision of fundamental stellar parameters (Teff, log g, [Fe/H]) using a differential approach, and (2) to constrain the helium-to-metal enrichment ratio (ΔY/ΔZ), which has implications for stellar evolution, galactic chemical evolution, and big bang nucleosynthesis. We apply the EPIC code – previously used tested for G-dwarfs – to our K-dwarf sample, employing a differential equivalent width analysis that leverages the existing GALAH stellar parameters. This differential approach with EPIC significantly improves the internal precision of stellar parameter estimates while minimizing systematic errors. With precise metallicities and bolometric luminosities derived from Gaia DR3 parallaxes, we then fit GARSTEC stellar evolution models across a grid of helium and metal abundances to infer individual helium mass fractions (Y) for K-dwarfs. This allows us to estimate the ΔY/ΔZ ratio and extrapolate to Z = 0 to determine the primordial helium abundance (Yₚ). Our results will provide new observational constraints on both stellar evolution theory and cosmological predictions of Big Bang Nucleosynthesis.
science theme: Cool Stars in the Context of Cosmic Evolution
schedule: Fri, 10:15 (contributed)
Marc H. Pinsonneault
Ohio State University
Authors: Marc H. Pinsonneault(1); Tim Bedding(2); Noah Downing (3); Scott Gaudi (1); Marc Hon (4); Dan Huber (5); Matt Penny (6); Sanjib Sharma (7); Dennis Stello (8); Trevor Weiss (9); Joel C. Zinn (9)
Affiliations: (1) Ohio State University; (2) Sydney University; (3) Yale University; (4) University of SIngapore; (5) University of Hawaii; (6) Louisiana State University; (7) Johns Hopkins University; (8) University of New South Wales; (9) California State University at Long Beach
The Roman mission, schedule for launch in September 2026, is ideally suited for cool star science. Here we focus on the Galactic Bulge Time Domain Survey, a core community survey. We discuss its properties, and demonstrate that it is capable of discovering large numbers of asteroseismic signals from cool evolved giants. More broadly, we argue that this survey with a total dwell time in excess of one year, sampled at a 12 minute cadence, will enable a broad range of cool star discoveries, including at the Galactic center.
science theme: Cool Stars in the Context of Cosmic Evolution
schedule: Fri, 11:00 (invited)
Wako Aoki
National Astronomical Observatory of Japan
TBA
science theme: Cool Stars in the Context of Cosmic Evolution
schedule: Fri, 11:30 (contributed)
Jiadong Li
MPIA Heidelberg
Authors: Jiadong Li (1); Hans-Walter Rix (1); Yuan-Sen Ting (1,2,3); Yu-Ting Wang (1,4,5); Szabolcs Mészáros (6,7,8); Ilija Medan (9); Chao Liu (4,5); Zhiqiang Yan (10,11); Peter J. Smith (1,12); Dan Qiu (4); Alexandre Roman-Lopes (13); Gregory M. Green (1); Danny Horta (14); Zachary Way (15); Tadafumi Matsuno (16); Stefano O. Souza (1); José G. Fernández-Trincado (17)
Affiliations: (1) Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany; (2) Department of Astronomy, The Ohio State University, Columbus, USA; (3) Center for Cosmology and AstroParticle Physics (CCAPP), The Ohio State University, Columbus, OH 43210, USA; (4) Key Laboratory of Space Astronomy and Technology, National Astronomical Observatories, CAS, Beijing 100101, China; (5) University of Chinese Academy of Sciences, Beijing 100049, China; (6) ELTE Eötvös Loránd University, Gothard Astrophysical Observatory, 9700 Szombathely, Hungary; (7) MTA-ELTE Lendület “Momentum” Milky Way Research Group, 9700 Szombathely, Hungary; (8) HUN-REN CSFK, Konkoly Observatory, Konkoly Thege Miklós út 15–17, 1121 Budapest, Hungary; (9) Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, USA; (10) School of Astronomy and Space Science, Nanjing University, Nanjing 210000, China; (11) Key Laboratory of Modern Astronomy and Astrophysics, Nanjing University, Ministry of Education, Nanjing 210093, China; (12) Fakultät für Physik und Astronomie, Universität Heidelberg, Im Neuenheimer Feld 226, 69120 Heidelberg, Germany; (13) Department of Astronomy, Universidad de La Serena, Av. Raul Bitran #1302, La Serena, Chile; (14) Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK; (15) Department of Physics and Astronomy, Georgia State University, 25 Park Place, Atlanta, GA 30303, USA; (16) Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstraße 12–14, 69120 Heidelberg, Germany; (17) Universidad Católica del Norte, Núcleo UCN en Arqueología Galáctica – Instituto de Astronomía, Av. Angamos 0610, Antofagasta, Chile.
We present the first determination of the Galactic stellar mass function (MF) for low-mass, cool stars (0.2–0.5 M$_\odot$) at metallicities [Fe/H] ≲ −1. A sample of ∼53,000 stars was selected as metal-poor based on both halo-like orbits and spectroscopic [Fe/H] inferred from Gaia DR3 BP/RP (XP) spectra. Reliable metallicity estimates for low-mass stars were enabled by calibrating Gaia XP spectra with stellar parameters from SDSS-V. For −1.5 < [Fe/H] < −1, we find that the MF below 0.5 M$_\odot$ exhibits a bottom-heavy power-law slope of α ≈ −1.6. At even lower metallicities, the MF becomes markedly bottom-light, with a near-flat slope of α ≈ 0, implying a strong deficit of low-mass stars. This metallicity-dependent variation is insensitive to the adopted stellar evolution model, indicating that the Galactic low-mass MF is not universal in the metal-poor regime. Further calibration of XP metallicities for M < 0.5 M$_\odot$ and [Fe/H] < −1.5 will be essential to verify these tentative low-metallicity trends.
science theme: Cool Stars in the Context of Cosmic Evolution
schedule: Fri, 11:45 (contributed)
Jonathan Gagné
Université de Montréal
Authors: J. Gagné; L. Moranta; A. Ruiz Diaz; L.-P. Coulombe
Affiliations: Université de Montréal
Since Gaia, the census of nearby stellar associations has expanded rapidly, enabling precise age-dating of substellar objects, stars and their exoplanets. However, this has also made it much harder to keep the census up to date. Recent discoveries also include spatial extensions of known associations, such as coronae around several nearby open clusters that display complex shapes, making it more challenging to identify their members, especially when they lack parallaxes or radial velocities. To address these challenges, my team built the Montreal Open Clusters and Associations (MOCA) database, which collects members of 10,000 young associations and their properties. I will present our most recent results enabled by MOCAdb, including a new version of BANYAN $\Sigma$ that captures complex association shapes, a population of 50 newly identified isolated planetary-mass candidates, 50 exoplanet hosts with newly calibrated ages, along with 200 new substellar objects with membership-based age calibration. In parallel, we are developing Bayesian age-inference tools to place all nearby associations within 500 pc of the Sun on a uniform age scale by combining empirical isochrones with lithium, activity and rotation measurements from the literature. Our inclusion of many recent surveys such as eROSITA and large-scale high-resolution spectroscopic programs also enables homogeneous studies of trends in stellar properties with age and chemistry.
science theme: Cool Stars in the Context of Cosmic Evolution
schedule: Fri, 12:00 (contributed)
Roman Gerasimov
University of Notre Dame
Globular clusters (GCs) are among the very few objects in the Galaxy for which precise relative ages can be measured, making them powerful probes of the Milky Way’s formation and evolution. Recent analyses combining GC ages, kinematics, and chemical abundances have mapped out the Galaxy’s merger tree and provided a timeline for the emergence of its present-day structure. Since the announcement of the first discovery of brown dwarf members in a GC at Cool Stars 22, new JWST observations of these unique objects have delivered independent constraints on GC ages and chemistries, and offered new insights into the poorly understood processes the drive the internal chemical evolution of GCs. In this talk, I will review the current state of this rapidly developing field and highlight new approaches to long-standing problems enabled by observations of cool stars and brown dwarfs in GCs.
science theme: Cool Stars in the Context of Cosmic Evolution
schedule: Fri, 12:15 (contributed)
Emma Softich
Astronomy & Astrophysics Department at University of California San Diego
Authors: Emma Softich(1);Emma Softich(1); Adam J. Burgasser(1); Anna Lueber(2); Julia Haynes(3)
Affiliations: (1) Astronomy & Astrophysics Department at University of California San Diego; (2)University of Bern, Center for Space and Habitability, Gesellschaftsstrasse 6, CH-3012, Bern, Switzerland; (3)Halıcıoğlu Data Science Institute University of California San Diego
Brown dwarf spectra are challenging to model as their typically low temperatures result in strong molecular features, and complicated gas and condensate chemistry. Atmosphere model grids have steadily increased in complexity to reproduce high-fidelity observations, resulting in more computationally expensive approaches in traditional spectral model fits. Machine learning models provide an alternative and efficient approach to spectral analysis, but may also produce worse fits or larger parameter uncertainties. We report our analysis of comparing Random Forest Retrieval approaches to direct spectral comparison through Markov Chain Monte Carlo techniques on a JWST spectral sample of nine T dwarf companions to FGK stars with known compositions and ages. We evaluate the quality of fits, precisions of inferred atmosphere parameters, agreement with the known properties of the primaries, and overall fit efficiency. We discuss potential improvements to both approaches that may facilitate more efficient model fits to large spectral samples of brown dwarfs being uncovered with JWST, Euclid, and SPHEREx.