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1. Stellar multiplicity and stellar rotation: insights from APOGEE

   https://doi.org/10.1093/mnras/stac590  

   Daher, Christine Mazzola; Badenes, Carles; Tayar, Jamie; Pinsonneault, Marc; Koposov, Sergey E; Kratter, Kaitlin; Moe, Maxwell; Anguiano, Borja; Godoy-Rivera, Diego; Majewski, Steven; et al (March 2022, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We measure rotational broadening in spectra taken by the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey to characterize the relationship between stellar multiplicity and rotation. We create a sample of 2786 giants and 24 496 dwarfs with stellar parameters and multiple radial velocities from the APOGEE pipeline, projected rotation speeds vsin i determined from our own pipeline, and distances, masses, and ages measured by Sanders & Das. We use the statistical distribution of the maximum shift in the radial velocities, ΔRVmax, as a proxy for the close binary fraction to explore the interplay between stellar evolution, rotation, and multiplicity. Assuming that the minimum orbital period allowed is the critical period for Roche Lobe overflow and rotational synchronization, we calculate theoretical upper limits on expected vsin i and ΔRVmax values. These expectations agree with the positive correlation between the maximum ΔRVmax and vsin i values observed in our sample as a function of log(g). We find that the fast rotators in our sample have a high occurrence of short-period [log(P/d) ≲ 4] companions. We also find that old, rapidly rotating main-sequence stars have larger completeness-corrected close binary fractions than their younger peers. Furthermore, rapidly rotating stars with large ΔRVmax consistently show differences of 1–10 Gyr between the predicted gyrochronological and measured isochronal ages. These results point towards a link between rapid rotation and close binarity through tidal interactions. We conclude that stellar rotation is strongly correlated with stellar multiplicity in the field, and caution should be taken in the application of gyrochronology relations to cool stars. 

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2. Magnetic Fields in M-dwarf Members of the Pleiades Open Cluster Using APOGEE Spectra

   https://doi.org/10.3847/1538-4357/ad571f  

   Wanderley, Fábio; Cunha, Katia; Kochukhov, Oleg; Smith, Verne V; Souto, Diogo; Cao, Lyra; Covey, Kevin; Majewski, Steven R; Martinez, Cintia; Muirhead, Philip S; et al (August 2024, The Astrophysical Journal)

   Abstract Average magnetic field measurements are presented for 62 M-dwarf members of the Pleiades open cluster, derived from Zeeman-enhanced Feilines in theHband. A Markov Chain Monte Carlo methodology was employed to model magnetic filling factors using Sloan Digital Sky Survey (SDSS) IV APOGEE high-resolution spectra, along with the radiative transfer code Synmast, MARCS stellar atmosphere models, and the APOGEE Data Release 17 spectral line list. There is a positive correlation between mean magnetic fields and stellar rotation, with slow-rotator stars (Rossby number, Ro > 0.13) exhibiting a steeper slope than rapid rotators (Ro < 0.13). However, the latter sample still shows a positive trend between Ro and magnetic fields, which is given by 〈B〉 = 1604 × Ro^−0.20. The derived stellar radii when compared with physical isochrones show that, on average, our sample shows radius inflation, with median enhanced radii ranging from +3.0% to +7.0%, depending on the model. There is a positive correlation between magnetic field strength and radius inflation, as well as with stellar spot coverage, correlations which together indicate that stellar spot-filling factors generated by strong magnetic fields might be the mechanism that drives radius inflation in these stars. We also compare our derived magnetic fields with chromospheric emission lines (Hα, Hβ, and CaiiK), as well as with X-ray and Hαto bolometric luminosity ratios, and find that stars with higher chromospheric and coronal activity tend to be more magnetic. 

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3. Spectroscopic Follow-up of Young High- α Dwarf Star Candidates: Still Likely Genuinely Young

   https://doi.org/10.3847/1538-3881/ae6590  

   Lu, Yuxi Lucy; Manea, Catherine; McKenzie, Madeleine; Sayeed, Maryum; Douglas, Stephanie T; Rowan, Dominick; Ilyin, Ilya; Wheeler, Adam; Buder, Sven; Amard, Louis; et al (July 2026, The Astronomical Journal)

   Abstract The question of whether genuinely young high-αstars exist has been discussed for more than a decade since their discovery from asteroseismology of giant stars, as it is challenging to break the degeneracy between the binary interaction and the genuinely young scenarios. Young high-αstars are hard to explain with a traditional chemical evolution model, as the high-αdisk is typically associated with the early epoch of star formation in the Milky Way. Combined with recent advances of gyrochronology, and that⁷Li can serve as an unambiguous indicator for identifying merger products in dwarfs (thanks to its low burning temperature), we identified young high-αdwarf candidate stars through their fast rotation in a previous study. For this paper, we performed high-resolution spectroscopic follow-up on these candidates using the Potsdam Echelle Polarimetric and Spectroscopic Instrument, and confirm three additional stars that are most likely genuinely young. Together with the star from the earlier paper, we find three out of four of them center around [Fe/H] = −0.5 dex, are ∼5 Gyr old, and have a similar amount of elevated Li (∼0.5 dex) and Al (∼0.1 dex) compared to stars with matching $\log g$ ,T_eff, Mg, and Fe within observational uncertainties, hinting at their common formation pathway. 

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4. The TIME Table: rotation and ages of cool exoplanet host stars

   https://doi.org/10.1093/mnras/stad343  

   Gaidos, Eric; Claytor, Zachary; Dungee, Ryan; Ali, Aleezah; Feiden, Gregory A (February 2023, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Age is a stellar parameter that is both fundamental and difficult to determine. Among middle-aged M dwarfs, the most prolific hosts of close-in and detectable exoplanets, gyrochronology is the most promising method to assign ages, but requires calibration by rotation-temperature sequences (gyrochrones) in clusters of known ages. We curated a catalogue of 249 late K- and M-type (Teff = 3200–4200 K) exoplanet host stars with established rotation periods, and applied empirical, temperature-dependent rotation–age relations based on relevant published gyrochrones, including one derived from observations of the 4-Gyr-old open cluster M67. We estimated ages for 227 of these stars, and upper limits for eight others, excluding 14 which are too rapidly rotating or are otherwise outside the valid parameter range of our gyrochronology. We estimated uncertainties based on observed scatter in rotation periods in young clusters, error in the gyrochrones, and uncertainties in temperature and non-solar metallicity. For those stars with measured metallicities, we provide but do not incorporate a correction for the effects of deviation from solar-metallicity. The age distribution of our sample declines to near zero at 10 Gyr, the age of the Galactic disc, with the handful of outliers explainable by large uncertainties. Continued addition or extension of cluster rotation sequences to more thoroughly calibrate the gyrochronology in time and temperature space, more precise and robust measurement of rotation periods, and more accurate stellar parameter measurements will enable continued improvements in the age estimates of these important exoplanet host stars. 

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5. Deciphering the Milky Way’s star formation at cosmic noon with high proper-motion stars: A precursor to the merger-driven starburst

   https://doi.org/10.1051/0004-6361/202555510  

   An, Deokkeun; Lee, Young Sun; Hirai, Yutaka; Beers, Timothy C (August 2025, Astronomy & Astrophysics)

   Context. Evidence suggests that the Milky Way (MW) underwent a major collision with the Gaia–Sausage/Enceladus (GSE) dwarf galaxy around cosmic noon. While GSE has since been fully disrupted, it brought in ex situ stars and dynamically heated in situ stars into the halo. In addition, the gas-rich merger may have triggered a burst of in situ star formation, potentially giving rise to a chemically distinct stellar component. Aims. We investigated the region of phase space where stars formed during the GSE merger likely reside, and retain distinct chemical and dynamical signatures. Methods. Building on our previous investigation of metallicity ([Fe/H]) and vertical angular momentum (L_Z) distributions, we analysed spectroscopic samples from GALAH, APOGEE, SDSS, and LAMOST, combined withGaiakinematics. We focused on high proper-motion stars as effective tracers of the phase-space volume likely influenced by the GSE merger. To correct for selection effects, we incorporated metallicity estimates derived from SDSS and SMSS photometry. Results. Our analysis reveals that low-αstars with GSE-like kinematics exhibit bimodality in [Na/Fe] and [Al/Fe] at −1.0 ≲[Fe/H] ≲ −0.4. One group follows the low light-element abundances of GSE stars, while another exhibits enhanced values. These low-α, high-Na stars have eccentric orbits but are more confined to the inner MW. Eos overlaps with a high-eccentricity subset of these stars, implying that it constitutes a smaller structure nested within the broader population. After correcting for sampling biases, we estimated a population ratio of approximately 1:10 between the low-α, high-Na stars and the GSE debris. Conclusions. These results suggest that the low-α, high-Na stars formed in a compact region, likely fuelled by gas from the GSE progenitor, analogous to clumpy star-forming clouds seen in high-redshift galaxies. Such stars may trace the first sparks of more extensive merger-driven starburst activity. 

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