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1. A New Asteroseismic Kepler Benchmark Constrains the Onset of Weakened Magnetic Braking in Mature Sun-like Stars

   https://doi.org/10.3847/1538-4357/ad4eb1  

   Bhalotia, Vanshree; Huber, Daniel; van_Saders, Jennifer L; Metcalfe, Travis S; Stassun, Keivan G; White, Timothy R; Aguirre_Børsen-Koch, Víctor; Ball, Warrick H; Basu, Sarbani; Serenelli, Aldo M; et al (July 2024, The Astrophysical Journal)

   Abstract Stellar spin down is a critical yet poorly understood component of stellar evolution. In particular, results from the Kepler Mission imply that mature age, solar-type stars have inefficient magnetic braking, resulting in a stalled spin-down rate. However, a large number of precise asteroseismic ages are needed for mature (≥3 Gyr) stars in order to probe the regime where traditional and stalled spin-down models differ. In this paper, we present a new asteroseismic benchmark star for gyrochronology discovered using reprocessed Kepler short cadence data. KIC 11029516 (Papayu) is a bright (Kp= 9.6 mag) solar-type star with a well-measured rotation period (21.1 ± 0.8 days) from spot modulation using 4 yr of Kepler long-cadence data. We combine asteroseismology and spectroscopy to obtainT_eff= 5888 ± 100 K, [Fe/H] = 0.30 ± 0.06 dex,M= 1.24 ± 0.05M_⊙,R= 1.34 ± 0.02R_⊙, and age of 4.0 ± 0.4 Gyr, making Papayu one of the most similar stars to the Sun in terms of temperature and radius with an asteroseismic age and a rotation period measured from spot modulation. We find that Papayu sits at the transition of where traditional and weakened spin-down models diverge. A comparison with stars of similar zero-age main-sequence temperatures supports previous findings that weakened spin-down models are required to explain the ages and rotation periods of old solar-type stars. 

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2. J-PLUS: Beyond Spectroscopy. III. Stellar Parameters and Elemental-abundance Ratios for Five Million Stars from DR3

   https://doi.org/10.3847/1538-4357/ad6b94  

   Huang_黄, Yang 样; Beers, Timothy C; Xiao_肖, Kai 凯; Yuan_苑, Haibo 海波; Lee, Young Sun; Gu_顾, Hongrui 弘睿; Hong, Jihye; Liu_刘, Jifeng 继峰; Fan_范, Zhou 舟; Coelho, Paula; et al (October 2024, The Astrophysical Journal)

   Abstract We present a catalog of stellar parameters (effective temperatureT_eff, surface gravity $\log g$, age, and metallicity [Fe/H]) and elemental-abundance ratios ([C/Fe], [Mg/Fe], and [α/Fe]) for some five million stars (4.5 million dwarfs and 0.5 million giant stars) in the Milky Way, based on stellar colors from the Javalambre Photometric Local Universe Survey (J-PLUS) DR3 and Gaia EDR3. These estimates are obtained through the construction of a large spectroscopic training set with parameters and abundances adjusted to uniform scales, and trained with a kernel principal component analysis. Owing to the seven narrow/medium-band filters employed by J-PLUS, we obtain precisions in the abundance estimates that are as good as or better than those derived from medium-resolution spectroscopy for stars covering a wide range of the parameter space: 0.10–0.20 dex for [Fe/H] and [C/Fe], and 0.05 dex for [Mg/Fe] and [α/Fe]. Moreover, systematic errors due to the influence of molecular carbon bands on previous photometric-metallicity estimates (which only included two narrow/medium-band blue filters) have now been removed, resulting in photometric-metallicity estimates down to [Fe/H] ∼ −4.0, with typical uncertainties of 0.40 dex and 0.25 dex for dwarfs and giants, respectively. This large photometric sample should prove useful for the exploration of the assembly and chemical-evolution history of our Galaxy. 

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3. 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; Triaud, A H; Sousa, S G; Olofsson, G; Beck, M; Billot, N; Hoyer, S; Gill, S; et al (January 2023, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT 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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4. APOKASC-3: The Third Joint Spectroscopic and Asteroseismic Catalog for Evolved Stars in the Kepler Fields

   https://doi.org/10.3847/1538-4365/ad9fef  

   Pinsonneault, Marc H; Zinn, Joel C; Tayar, Jamie; Serenelli, Aldo; García, Rafael A; Mathur, Savita; Vrard, Mathieu; Elsworth, Yvonne P; Mosser, Benoit; Stello, Dennis; et al (February 2025, The Astrophysical Journal Supplement Series)

   Abstract In the third APOKASC catalog, we present data for the complete sample of 15,808 evolved stars with APOGEE spectroscopic parameters and Kepler asteroseismology. We used 10 independent asteroseismic analysis techniques and anchor our system on fundamental radii derived from GaiaLand spectroscopicT_eff. We provide evolutionary state, asteroseismic surface gravity, mass, radius, age, and the data used to derive them for 12,418 stars. This includes 10,036 exceptionally precise measurements, with median fractional uncertainties in ${\nu }_{\max }$, Δν, mass, radius, and age of 0.6%, 0.6%, 3.8%, 1.8%, and 11.1%, respectively. We provide more limited data for 1624 additional stars that either have lower-quality data or are outside of our primary calibration domain. Using lower red giant branch (RGB) stars, we find a median age for the chemical thick disk of 9.14 ± 0.05(ran) ± 0.9(sys) Gyr with an age dispersion of 1.1 Gyr, consistent with our error model. We calibrate our red clump (RC) mass loss to derive an age consistent with the lower RGB and provide asymptotic GB and RGB ages for luminous stars. We also find a sharp upper-age boundary in the chemical thin disk. We find that scaling relations are precise and accurate on the lower RGB and RC, but they become more model dependent for more luminous giants and break down at the tip of the RGB. We recommend the use of multiple methods, calibration to a fundamental scale, and the use of stellar models to interpret frequency spacings. 

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5. The James Webb Space Telescope NIRSpec-PRISM Transmission Spectrum of the Super-puff, Kepler-51d

   https://doi.org/10.3847/1538-3881/ae33c0  

   Libby-Roberts, Jessica E; Bello-Arufe, Aaron; Berta-Thompson, Zachory K; Cañas, Caleb I; Chachan, Yayaati; Hu, Renyu; Kawashima, Yui; Murray, Catriona; Ohno, Kazumasa; Tokadjian, Armen; et al (March 2026, The Astronomical Journal)

   Abstract Kepler-51 is a 500 Myr G dwarf hosting three “super-puffs” and one low-mass nontransiting planet. Kepler-51d, the coolest (T_eq∼ 350 K) transiting planet in this system is also one of the lowest-density super-puffs known to date (ρ_p= 0.038 ± 0.009 g cm⁻³). With a planetary mass ofM_p= 5.6 ± 1.2M_⊕and a radius ofR_p= 9.32 ± 0.18R_⊕, the observed properties of this planet are not readily explained by most planet formation theories. Hypotheses explaining Kepler-51d’s low density range from a substantial H/He envelope comprising >30% its mass, a high-altitude haze layer, to a tilted ring system. To test these hypotheses, we present the NIRSpec-PRISM 0.6–5.3μm transmission spectrum of Kepler-51d observed by the James Webb Space Telescope. We find a spectrum best fit by a sloped line covering the entire wavelength range. Based on forward modeling and atmosphere retrievals, Kepler-51d likely possesses a low-metallicity atmosphere with high-altitude hazes of submicron particle sizes spanning pressures of 1–100μbar. However, the spectrum could also be explained by a tilted ring with an estimated lifetime on the order of ∼0.1 Myr. We also investigate the stellar activity of this young Sun-like star, extracting a spot temperature significantly hotter than sunspots and spot covering fractions on the order of 0.1%–10% depending on assumed spot parameters. 

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