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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. The maximum stellar surface density due to the failure of stellar feedback

   https://doi.org/10.1093/mnras/sty3386  

   Grudić, Michael Y; Hopkins, Philip F; Quataert, Eliot; Murray, Norman (December 2018, Monthly Notices of the Royal Astronomical Society)
3. Detection of a Spatially Extended Stellar Population in M33: A Shallow Stellar Halo?

   https://doi.org/10.3847/1538-4357/ad5445  

   Ogami, Itsuki; Komiyama, Yutaka; Chiba, Masashi; Tanaka, Mikito; Guhathakurta, Puragra; Kirby, Evan N; Wyse, Rosemary_F G; Filion, Carrie; Kirihara, Takanobu; Ishigaki, Miho N; et al (August 2024, The Astrophysical Journal)

   Abstract We analyze the outer regions of M33, beyond 15 kpc in projected distance from its center, using Subaru/Hyper Suprime-Cam multicolor imaging. We identify red giant branch (RGB) stars and red clump (RC) stars using the surface-gravity-sensitiveNB515filter for the RGB sample and a multicolor selection for both samples. We construct the radial surface density profiles of these RGB and RC stars and find that M33 has an extended stellar population with a shallow power-law index ofα> −3, depending on the intensity of the contamination. This result represents a flatter profile than the stellar halo that was detected by the previous study focusing on the central region, suggesting that M33 may have a double-structured halo component, i.e., inner/outer halos or a very extended disk. Also, the slope of this extended component is shallower than those typically found for halos in large galaxies, implying intermediate-mass galaxies may have different formation mechanisms (e.g., tidal interaction) from large spirals. We also analyze the radial color profiles of RC/RGB stars and detect a radial gradient, consistent with the presence of an old and/or metal-poor population in the outer region of M33, thereby supporting our proposal that the stellar halo extends beyond 15 kpc. Finally, we estimate that the surface brightness of this extended component isμ_V= 35.72 ± 0.08 mag arcsec⁻². If our detected component is the stellar halo, this estimated value is consistent with the detection limit of previous observations. 

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4. NGC 147 Corroborates the Break in the Stellar Mass–Stellar Metallicity Relation for Galaxies

   https://doi.org/10.3847/1538-4357/ac1340  

   Zhuang, Zhuyun; Kirby, Evan N.; Leethochawalit, Nicha; de los Reyes, Mithi A. (October 2021, The Astrophysical Journal)
5. ROBO-AO Kepler Asteroseismic Survey. II. Do Stellar Companions Inhibit Stellar Oscillations?

   https://doi.org/10.3847/1538-4357/ab50c3  

   Schonhut-Stasik, Jessica; Huber, Daniel; Baranec, Christoph; Lamman, Claire; Salama, Maïssa; Jensen-Clem, Rebecca; Duev, Dmitry A.; Riddle, Reed; Kulkarni, S. R.; Law, Nicholas M. (January 2020, The Astrophysical Journal)

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