Examinando por Autor "Stassun, Keivan G."
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Ítem Stellar Characterization and Radius Inflation of Hyades M-dwarf Stars from the APOGEE Survey(Institute of Physics, 2023-07-01) Wanderley, Fábio; Cunha, Katia; Souto, Diogo; Smith, Verne V.; Cao, Lyra; Pinsonneault, Marc; Allende Prieto, C.; Covey, Kevin; Masseron, Thomas; Pascucci, Ilaria; Stassun, Keivan G.; Terrien, Ryan; Bergsten, Galen J.; Bizyaev, Dmitry; Fernández-Trincado, José G.; Jönsson, Henrik; Hasselquist, Sten; Holtzman, Jon A.; Lane, Richard R.; Mahadevan, Suvrath; Majewski, Steven R.; Minniti, Dante; Pan, Kaike; Serna, Javier; Sobeck, Jennifer; Stringfellow, Guy S.We present a spectroscopic analysis of a sample of 48 M-dwarf stars (0.2 M ⊙ < M < 0.6 M ⊙) from the Hyades open cluster using high-resolution H-band spectra from the Sloan Digital Sky Survey/Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey. Our methodology adopts spectrum synthesis with LTE MARCS model atmospheres, along with the APOGEE Data Release 17 line list, to determine effective temperatures, surface gravities, metallicities, and projected rotational velocities. The median metallicity obtained for the Hyades M dwarfs is [M/H] = 0.09 ± 0.03 dex, indicating a small internal uncertainty and good agreement with optical results for Hyades red giants. Overall, the median radii are larger than predicted by stellar models by 1.6% ± 2.3% and 2.4% ± 2.3%, relative to a MIST and DARTMOUTH isochrone, respectively. We emphasize, however, that these isochrones are different, and the fractional radius inflation for the fully and partially convective regimes have distinct behaviors depending on the isochrone. Using a MIST isochrone there is no evidence of radius inflation for the fully convective stars, while for the partially convective M dwarfs the radii are inflated by 2.7% ± 2.1%, which is in agreement with predictions from models that include magnetic fields. For the partially convective stars, rapid rotators present on average higher inflation levels than slow rotators. The comparison with SPOTS isochrone models indicates that the derived M-dwarf radii can be explained by accounting for stellar spots in the photosphere of the stars, with 76% of the studied M dwarfs having up to 20% spot coverage, and the most inflated stars with ∼20%-40% spot coverage.Ítem Stellar Properties for a Comprehensive Collection of Star-forming Regions in the SDSS APOGEE-2 Survey* Based on SDSS Data Releases 16 and 17.(American Astronomical Society, 2023-02) Román-Zúñiga, Carlos G.; Kounkel, Marina; Hernández, Jesús; Peña Ramírez, Karla; López-Valdivia, Ricardo; Covey, Kevin R.; Stutz, Amelia M.; Roman-Lopes, Alexandre; Campbell, Hunter; Khilfeh, Elliott; Tapia, Mauricio; Stringfellow, Guy S.; Downes, Juan José; Stassun, Keivan G.; Minniti, Dante; Bayo, Amelia; Kim, Jinyoung Serena; Suárez, Genaro; Ybarra, Jason E.; Fernández-Trincado, José G.; Longa-Peña, Penélope; Ramírez-Preciado, Valeria; Serna, Javier; Lane, Richard R.; García-Hernández D.A.; Beaton, Rachael L.; Bizyaev, Dmitry; Pan, KaikeThe Sloan Digital Sky Survey IV APOGEE-2 primary science goal was to observe red giant stars throughout the Galaxy to study its dynamics, morphology, and chemical evolution. The APOGEE instrument, a high-resolution 300-fiber H-band (1.55-1.71 μm) spectrograph, is also ideal to study other stellar populations in the Galaxy, among which are a number of star-forming regions and young open clusters. We present the results of the determination of six stellar properties (T eff, log g , [Fe/H], L/L ⊙, M/M ⊙, and age) for a sample that is composed of 3360 young stars, of subsolar to supersolar types, in 16 Galactic star formation and young open cluster regions. Those sources were selected by using a clustering method that removes most of the field contamination. Samples were also refined by removing targets affected by various systematic effects of the parameter determination. The final samples are presented in a comprehensive catalog that includes all six estimated parameters. This overview study also includes parameter spatial distribution maps for all regions and Hertzsprung-Russell ( log L / L ⊙ vs. T eff) diagrams. This study serves as a guide for detailed studies on individual regions and paves the way for the future studies on the global properties of stars in the pre-main-sequence phase of stellar evolution using more robust samples. © 2023. The Author(s). Published by the American Astronomical Society.Ítem The APO-K2 Catalog. I. ∼7500 Red Giants with Fundamental Stellar Parameters from APOGEE DR17 Spectroscopy and K2-GAP Asteroseismology(American Astronomical Society, 2024-02-01) Schonhut-Stasik, Jessica; Zinn, Joel C.; Stassun, Keivan G.; Pinsonneault, Marc; Johnson, Jennifer A.; Warfield, Jack T.; Stello, Dennis; Elsworth, Yvonne; García, Rafael A.; Mathur, Savita; Mosser, Benoit; Hon, Marc; Tayar, Jamie; Stringfellow, Guy S.; Beaton, Rachael L.; Jönsson, Henrik; Minniti, DanteWe present a catalog of fundamental stellar properties for ∼7500 evolved stars, including stellar radii and masses, determined from the combination of spectroscopic observations from the Apache Point Observatory Galactic Evolution Experiment, part of the Sloan Digital Sky Survey IV, and asteroseismology from K2. The resulting APO-K2 catalog provides spectroscopically derived temperatures and metallicities, asteroseismic global parameters, evolutionary states, and asteroseismically derived masses and radii. Additionally, we include kinematic information from Gaia. We investigate the multidimensional space of abundance, stellar mass, and velocity with an eye toward applications in Galactic archaeology. The APO-K2 sample has a large population of low-metallicity stars (∼288 with [M/H] ≤ −1), and their asteroseismic masses are larger than astrophysical estimates. We argue that this may reflect offsets in the adopted fundamental temperature scale for metal-poor stars rather than metallicity-dependent issues with interpreting asteroseismic data. We characterize the kinematic properties of the population as a function of α enhancement and position in the disk and identify those stars in the sample that are candidate components of the Gaia-Enceladus merger. Importantly, we characterize the selection function for the APO-K2 sample as a function of metallicity, radius, mass, ν max , color, and magnitude referencing Galactic simulations and target selection criteria to enable robust statistical inferences with the catalog.