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1. The EBLM project – IX. Five fully convective M-dwarfs, precisely measured with CHEOPS and TESS light curves

   https://doi.org/10.1093/mnras/stac2565  

   Sebastian, D. ; Swayne, M. I. ; Maxted, P. F. L. ; Triaud, A. H. M. J. ; Sousa, S. G. ; Olofsson, G. ; Beck, M. ; Billot, N. ; Hoyer, S. ; Gill, S. ; et al ( September 2022 , Monthly Notices of the Royal Astronomical Society)

   Eclipsing binaries are important benchmark objects to test and calibrate stellar structure and evolution models. This is especially true for binaries with a fully convective M-dwarf component for which direct measurements of these stars’ masses and radii are difficult using other techniques. Within the potential of M-dwarfs to be exoplanet host stars, the accuracy of theoretical predictions of their radius and effective temperature as a function of their mass is an active topic of discussion. Not only the parameters of transiting exoplanets but also the success of future atmospheric characterization relies on accurate theoretical predictions. We present the analysis of five eclipsing binaries with low-mass stellar companions out of a subsample of 23, for which we obtained ultra-high-precision light curves using the CHEOPS satellite. The observation of their primary and secondary eclipses are combined with spectroscopic measurements to precisely model the primary parameters and derive the M-dwarfs mass, radius, surface gravity, and effective temperature estimates using the PYCHEOPS data analysis software. Combining these results to the same set of parameters derived from TESS light curves, we find very good agreement (better than 1 per cent for radius and better than 0.2 per cent for surface gravity). We also analyse the importance of precise orbits from radial velocity measurements and find them to be crucial to derive M-dwarf radii in a regime below 5 per cent accuracy. These results add five valuable data points to the mass–radius diagram of fully convective M-dwarfs.

    

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2. On the Possibility of an Upper Limit on Magnetically Induced Radius Inflation in Low-mass Stars

   https://doi.org/10.3847/1538-4357/ad234e  

   MacDonald, J. ; Mullan, D. J. ( February 2024 , The Astrophysical Journal)

   The radii of low-mass stars are observed to beinflatedabove standard model predictions, especially in magnetically active stars. Typically, the empirical relative radius inflations ΔR/Rare ≤10% but in (rare) cases may be ≥20%. Our magneto-convective stellar models have already replicated many empirical ΔR/Rvalues. Here, we ask: is there any theoreticalupper limiton the amount of such inflation? We use our magneto-convective model to compute ΔR/Rusing empirically plausible values of thesurfacefield strength parameterδ. Inside each model, the maximum internal field is set to a particular value:B_ceil= 10, or 100 kG, or 1 MG. WhenB_ceil= 10 kG, peak inflation with ΔR/R≈ 90% occurs in stars with masses of 0.7M_⊙. WithB_ceil= 100 kG, peak inflation with ΔR/R≈ 140% occurs in stars withM≈ 0.5M_⊙. But withB_ceil= 1 MG, we findno peakin ΔR/Ras a function ofδ; instead, the largerδis, the larger ΔR/Rbecomes, reaching 300%–350% in the case of the largestδconsidered. Thus, magneto-convective modeling can accommodate ΔR/Rvalues which are considerably larger than any reported empirical inflations. We find that a maximum occurs in ΔR/Ras a function ofδonly in model stars where the field reaches its maximum strengthB_ceilinside the convective envelope. Moreover, our models of completely convective stars undergosmalleramounts of relative radius inflation than models with radiative cores, a result consistent with some previous reports.

    

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3. The LHS 1678 System: Two Earth-sized Transiting Planets and an Astrometric Companion Orbiting an M Dwarf Near the Convective Boundary at 20 pc

   https://doi.org/10.3847/1538-3881/ac32e3  

   Silverstein, Michele L. ; Schlieder, Joshua E. ; Barclay, Thomas ; Hord, Benjamin J. ; Jao, Wei-Chun ; Vrijmoet, Eliot Halley ; Henry, Todd J. ; Cloutier, Ryan ; Kostov, Veselin B. ; Kruse, Ethan ; et al ( March 2022 , The Astronomical Journal)

   Abstract We present the Transiting Exoplanet Survey Satellite (TESS) discovery of the LHS 1678 (TOI-696) exoplanet system, comprised of two approximately Earth-sized transiting planets and a likely astrometric brown dwarf orbiting a bright ( V J = 12.5, K s = 8.3) M2 dwarf at 19.9 pc. The two TESS-detected planets are of radius 0.70 ± 0.04 R ⊕ and 0.98 ± 0.06 R ⊕ in 0.86 day and 3.69 day orbits, respectively. Both planets are validated and characterized via ground-based follow-up observations. High Accuracy Radial Velocity Planet Searcher RV monitoring yields 97.7 percentile mass upper limits of 0.35 M ⊕ and 1.4 M ⊕ for planets b and c, respectively. The astrometric companion detected by the Cerro Tololo Inter-American Observatory/Small and Moderate Aperture Telescope System 0.9 m has an orbital period on the order of decades and is undetected by other means. Additional ground-based observations constrain the companion to being a high-mass brown dwarf or smaller. Each planet is of unique interest; the inner planet has an ultra-short period, and the outer planet is in the Venus zone. Both are promising targets for atmospheric characterization with the James Webb Space Telescope and mass measurements via extreme-precision radial velocity. A third planet candidate of radius 0.9 ± 0.1 R ⊕ in a 4.97 day orbit is also identified in multicycle TESS data for validation in future work. The host star is associated with an observed gap in the lower main sequence of the Hertzsprung–Russell diagram. This gap is tied to the transition from partially to fully convective interiors in M dwarfs, and the effect of the associated stellar astrophysics on exoplanet evolution is currently unknown. The culmination of these system properties makes LHS 1678 a unique, compelling playground for comparative exoplanet science and understanding the formation and evolution of small, short-period exoplanets orbiting low-mass stars. 

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4. The SPHINX M-dwarf Spectral Grid. I. Benchmarking New Model Atmospheres to Derive Fundamental M-dwarf Properties

   https://doi.org/10.3847/1538-4357/acabc2  

   Iyer, Aishwarya R. ; Line, Michael R. ; Muirhead, Philip S. ; Fortney, Jonathan J. ; Gharib-Nezhad, Ehsan ( February 2023 , The Astrophysical Journal)

   Abstract About 70%–80% of stars in our solar and Galactic neighborhood are M dwarfs. They span a range of low masses and temperatures relative to solar-type stars, facilitating molecule formation throughout their atmospheres. Standard stellar atmosphere models primarily designed for FGK stars face challenges when characterizing broadband molecular features in spectra of cool stars. Here, we introduce SPHINX —a new 1D self-consistent radiative–convective thermochemical equilibrium chemistry model grid of atmospheres and spectra for M dwarfs in low resolution ( R ∼ 250). We incorporate the latest precomputed absorption cross sections with pressure broadening for key molecules dominant in late-K, early/main-sequence-M stars. We then validate our grid models by determining fundamental properties ( T eff , log g , [M/H], radius, and C/O) for 10 benchmark M+G binary stars with known host metallicities and 10 M dwarfs with interferometrically measured angular diameters. Incorporating the Gaussian process inference tool Starfish , we account for correlated and systematic noise in low-resolution (spectral stitching of SpeX, SNIFS, and STIS) observations and derive robust estimates of fundamental M-dwarf atmospheric parameters. Additionally, we assess the influence of photospheric heterogeneity on inferred [M/H] and find that it could explain some deviations from observations. We also probe whether the adopted convective mixing length parameter influences inferred radii, effective temperature, and [M/H] and again find that may explain discrepancies between interferometric observations and model-derived parameters for cooler M dwarfs. Mainly, we show the unique strength in leveraging broadband molecular absorption features occurring in low-resolution M dwarf spectra and demonstrate the ability to improve constraints on fundamental properties of exoplanet hosts and brown-dwarf companions. 

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5. Characterization of Low-mass Companions to Kepler Objects of Interest Observed with APOGEE-N

   https://doi.org/10.3847/1538-4365/acbcbe  

   Cañas, Caleb I. ; Bender, Chad F. ; Mahadevan, Suvrath ; Bizyaev, Dmitry ; De Lee, Nathan ; Fleming, Scott W. ; Hearty, Fred ; Majewski, Steven R. ; Nitschelm, Christian ; Schneider, Donald P. ; et al ( April 2023 , The Astrophysical Journal Supplement Series)

   Abstract We report the characterization of 28 low-mass (0.02 M ⊙ ≤ M 2 ≤ 0.25 M ⊙ ) companions to Kepler objects of interest (KOIs), eight of which were previously designated confirmed planets. These objects were detected as transiting companions to Sunlike stars (G and F dwarfs) by the Kepler mission and are confirmed as single-lined spectroscopic binaries in the current work using the northern multiplexed Apache Point Observatory Galactic Evolution Experiment near-infrared spectrograph (APOGEE-N) as part of the third and fourth Sloan Digital Sky Surveys. We have observed hundreds of KOIs using APOGEE-N and collected a total of 43,175 spectra with a median of 19 visits and a median baseline of ∼1.9 yr per target. We jointly model the Kepler photometry and APOGEE-N radial velocities to derive fundamental parameters for this subset of 28 transiting companions. The radii for most of these low-mass companions are overinflated (by ∼10%) when compared to theoretical models. Tidally locked M dwarfs on short-period orbits show the largest amount of inflation, but inflation is also evident for companions that are well separated from the host star. We demonstrate that APOGEE-N data provide reliable radial velocities when compared to precise high-resolution spectrographs that enable detailed characterization of individual systems and the inference of orbital elements for faint ( H > 12) KOIs. The data from the entire APOGEE-KOI program are public and present an opportunity to characterize an extensive subset of the binary population observed by Kepler. 

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