Program

14:00 - 14:20
Determining the chemical composition of cool dwarf stars
Ulrike Heiter (Invited) - Uppsala University, Sweden

Recent developments for determining the chemical composition of cool dwarf stars are reviewed, focusing on the determination of abundances of individual elements for M dwarfs from high-resolution near-infrared spectra. Abundances based on different spectral regions (Y/J- vs H-band) are found to agree within uncertainties. Binaries consisting of an M dwarf and an F/G/K dwarf are being used to verify the abundance determinations. Several caveats are pointed out, which need to be taken into account or which need further development.

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14:20 - 14:30
Stars and their Planets: How the analysis of host-star can influence our
knowledge of rocky planets’ interior structures?
Henrique Reggiani - Gemini South/NOIRLAB, Chile

Abstract: Planets and host-stars are connected. Both form from the same molecular cloud, and stellar properties and evolution affect planetary formation and evolution. Planetary mass and radius inferences, in particular, are a strong function of stellar mass and radius. However, due to a limited sample of rocky exoplanets with homogeneously analyzed host stars, the fitted relationships suffer from unaccounted-for uncertainties due to sampling and selection biases. To overcome this, we perform a homogeneous analysis of 25 systems. We analyze the host stars to obtain their photospheric (Teff, logg, and [Fe/H]) and fundamental (mass, radius) stellar parameters, as well as their chemistry. We use this homogeneous dataset to re-estimate the rocky planets’ masses and radii. We compare our results to previous analyzes to show how different datasets and analyzes techniques influence the inference of the planetary parameters, and the empirical relations between stars and planets. With the stellar composition we construct priors to constrain the planetary interior structures. We calculate their core-mass fractions (CMF) within three different paradigms: based on the inferred planetary radii and masses alone, and including priors based on the stellar Fe/Mg and Fe/Si ratios. Compared to previous analyzes, at up to 14% of our sample the planetary masses and radii changes have influenced the CMFs to the point at which the planetary classification (earth-like, super earth, etc) changed. We find no strong correlation among all methodologies and show how small deviations on stellar abundances can change the inferred stellar-planetary correlations. We also show there is a weak correlation among the super-Earths.

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14:30 - 14:40
Advancing chemical analysis of cool dwarfs with CARMENES
David Montes - Complutense University of Madrid, Spain

Abstract: The chemical connection between stars and exoplanets is key to modern planetary science. To make progress in measuring the chemical composition of cool dwarfs requires good high-resolution spectroscopic data. The CARMENES spectrograph has produced high-quality, R~90,000 spectra in the optical and NIR for hundreds of nearby -late K and M dwarfs. The wide wavelength coverage (5600 — 17100 A) of the CARMENES channels offers the possibility of characterising many chemical species. These spectra have been combined into a single high-quality and telluric-free spectrum per target, making them suitable for detailed abundance analysis. Chemical elements that have already been investigated in detail using this data include Mg, Si, Rb, and V for many M dwarfs in our sample, which are relevant for understanding planetary structure and Galactic chemical evolution. We preview ongoing work using CARMENES spectra to study a number of other elements and benchmark them using wide binaries. We demonstrate that the ability to work with a large number of atomic features and stars enables better calibration of abundance measurements and characterisation of their precision for cool stars.

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14:40 - 14:50
Stellar Parameter Estimation for Over Half a Million M-Type Stars from LAMOST
DR10 with Transfer Learning
‍Shuo Zhang - Tsinghua University, China

Abstract: The accurate determination of stellar parameters in M-type stars is of significant importance for various astronomical studies. In our study, we employed a transfer learning method to estimate fundamental atmospheric parameters (Teff, log g, [M/H], [ /M]) and radial velocities (RV) for an extensive dataset encompassing 518,653 M dwarfs obtained from the Low-Resolution Spectroscopic Survey (LRS) within LAMOST DR10. By utilizing the domain-adaptation approach Cycle-StarNet, we successfully bridged the gap between observed and synthetic spectra. PHOENIX BT-Settl synthetic spectra and information from PARSEC isochrones were combined and used to construct the training sample in the synthetic domain, and the L-BFGS algorithm was subsequently employed to identify the best-fit transferred synthetic spectra. For spectra with signal-to-noise ratios (S/Ns) exceeding 5, the accuracy derived from a comparison to high-resolution results for Teff, log g, [M/H], [ /M], and RV is 11 71 K, 0.03 0.13 dex, 0.05 0.2 dex, 0.08 0.07 dex, and 3 10 km/s, respectively. The typical precisions are 61 K, 0.07 dex, 0.15 dex, 0.06 dex, and 9 km/s for Teff, log g, [M/H], [ /M], and RV, respectively, estimated from the duplicate observations of the same stars.

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14:50 - 15:00
Linking the primordial composition of stars to the present-day composition of
their rocky exoplanets
‍Vardan Adibekyan - Institute of Astrophysics and Space Sciences, Portugal

Abstract: Stellar atmospheres offer the only observational window into the remnants of planet-forming disk composition, given that disks, with lifetimes typically lasting only a few million years, have long dissipated around most discovered planet-hosting stars. When modeling the interiors of rocky planets an one-to-one relation between the composition of the planet and the host stars is typically assumed. However, Adibekyan et al (2021) showed that while there is a relation between the composition of rocky planets and their host stars, the relation is not one-to-one. The study by Adibekyan et al. (2021) has two potential limitations: i) The interiors of the planets were modeled assuming they are made only of core and mantle, i.e., no volatiles, and ii) the present-day stellar abundances were used as a proxy for the primordial protoplanetary disk composition. Some astrophysical processes, such as atomic diffusion, can influence the stellar composition as the star evolves. Thus, the present-day stellar abundances can be measurably different from their primordial composition. Over the last two years, we worked on the EXO-Terra project, which aimed at a comprehensive analysis of the star-planet compositional link by overcoming the two aforementioned limitations. Additionally, we increased the sample by about 50%. I propose an oral contribution to present the final results of the EXO-Terra project.

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15:00 - 15:10
Korg, a new code for spectral synthesis, and model atmosphere interpolation as a
precision bottleneck for M dwarf spectra
‍Adam Wheeler - Ohio State University, USA

Abstract: I will describe Korg, a package for spectral fitting and synthesis of cool star spectra. Recent upgrades to the chemical equilibrium solver include polyatomic and charged molecules, extending Korg's regime of applicability to M stars. Built-in functions to fit observed spectra via synthesis or equivalent widths make it easy to take advantage of Korg's automatic differentiation. In addition to demonstrating that Korg is a fast a accurate option for modeling cool star spectra, I will discuss the topic of model atmosphere interpolation. For stars hotter than Teff 4000 K, many approaches have been demonstrated to work well. But for cooler stars atmosphere interpolation is substantially complicated by the presence of molecules. I will demonstrate that naive methods can easily introduce catastrophic levels of error to synthesized spectra and discuss ad-hoc model atmosphere calculations and more sophisticated interpolation methods as possible solutions.

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15:10 - 15:20
SPHINX+ Flat transmission spectra for small planets with JWST---but first,
do we understand the host star well enough?
Aishwarya Iyer - NASA Goddard Space Flight Center, USA

Abstract: M-dwarfs are a dominant stellar class representing a significant population of our Galaxy. Spanning a wide range of masses to the hydrogen burning limit, they endure varying physical processes in their atmospheres. Characterizing dust/cloud condensation, convection, and activity-induced photospheric effects such as spots pose tremendous challenges. Notably, Iyer & Line 2020, showed that with JWST spectrophotometric precisions; despite star-planet joint retrievals and corrections for the Transit Light Source Effect, transmission spectra of small planets around spotty-M-dwarfs will still yield biased inferences on the planetary atmosphere—if we do not understand the disk-integrated spectrum of the host correctly. We address this with efforts to improve M-dwarf atmosphere models. We present SPHINX+, with a gray cloud model mimicking suppression of flux at shorter wavelengths and reddening at longer wavelengths. We improve the convection model with a reduced mixing length value expanding the region of the convective envelope relative to the sun. We perform validation tests for SPHINX+ by acquiring atmospheric properties for 32 mid-to-late-type M-dwarfs with low-resolution data taken from SpeX Prism Library and with SpeX IRTF spectra of 39 mid-to-late-type M-dwarfs that are companions to FGKM primaries observed by Mann et al. 2014. Our model-derived values have comparable precision as the empirical constraints of 0.078 dex in metallicities and 13% precision in M-dwarf C/O ratios.

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15:20 - 15:30
An Exploration of Optical Cool Dwarf Chemistry with Both Data-Driven and
Physical Models
Adam Rains - Uppsala University, Sweden

Abstract: Detailed chemical studies of F/G/K - or Solar-type stars have long been routine in stellar astrophysics, enabling studies in both Galactic chemodynamics, and exoplanet demographics. However, similar understanding of the chemistry of M and late-K dwarfs - the most common stars in the Galaxy—has been greatly hampered both observationally and theoretically by the complex molecular chemistry of their atmospheres. Here I present an extension to our recently published implementation of the data-driven Cannon model for cool dwarf Teff, log g, [Fe/H], and [Ti/Fe] as trained on low-to-medium resolution optical spectra (4000-7000 Å). With an updated training sample double the size of our published sample, we are able to provide more reliable parameters over a broader range of stellar Teff, [Fe/H], and [Ti/Fe]. Alongside this, we also investigated the sensitivity of optical wavelengths to various atomic and molecular species using both data-driven and theoretical means via a custom grid of MARCS synthetic spectra, and make recommendations for where MARCS struggles to reproduce cool dwarf fluxes. Under leave-one-out cross-validation, our published Cannon model is capable of recovering Teff, log g, [Fe/H], and [Ti/Fe] with precisions of 1.4%, 0.04 dex, 0.10 dex, and 0.06 dex respectively, with the recovery of [Ti/Fe] pointing to the as-yet mostly untapped potential of exploiting the abundant - but complex - chemical information within optical spectra of cool stars.

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