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1. Cool stars in the Galactic center as seen by APOGEE: M giants, AGB stars, and supergiant stars and candidates

   https://doi.org/10.1051/0004-6361/202038327  

   Schultheis, M. ; Rojas-Arriagada, A. ; Cunha, K. ; Zoccali, M. ; Chiappini, C. ; Zasowski, G. ; Queiroz, A. B. ; Minniti, D. ; Fritz, T. ; García-Hernández, D. A. ; et al ( October 2020 , Astronomy & Astrophysics)

   The Galactic center region, including the nuclear disk, has until recently been largely avoided in chemical census studies because of extreme extinction and stellar crowding. Large, near-IR spectroscopic surveys, such as the Apache Point Observatory Galactic Evolution Experiment (APOGEE), allow the measurement of metallicities in the inner region of our Galaxy. Making use of the latest APOGEE data release (DR16), we are able for the first time to study cool Asymptotic Giant branch (AGB) stars and supergiants in this region. The stellar parameters of five known AGB stars and one supergiant star (VR 5-7) show that their location is well above the tip of the red giant branch. We studied metallicities of 157 M giants situated within 150 pc of the Galactic center from observations obtained by the APOGEE survey with reliable stellar parameters from the APOGEE pipeline making use of the cool star grid down to 3200 K. Distances, interstellar extinction values, and radial velocities were checked to confirm that these stars are indeed situated in the Galactic center region. We detect a clear bimodal structure in the metallicity distribution function, with a dominant metal-rich peak of [Fe/H] ∼ +0.3 dex and a metal-poor peak around {Fe/H] = −0.5more »dex, which is 0.2 dex poorer than Baade’s Window. The α -elements Mg, Si, Ca, and O show a similar trend to the Galactic bulge. The metal-poor component is enhanced in the α -elements, suggesting that this population could be associated with the classical bulge and a fast formation scenario. We find a clear signature of a rotating nuclear stellar disk and a significant fraction of high-velocity stars with v gal  >  300 km s −1 ; the metal-rich stars show a much higher rotation velocity (∼200 km s −1 ) with respect to the metal-poor stars (∼140 km s −1 ). The chemical abundances as well as the metallicity distribution function suggest that the nuclear stellar disk and the nuclear star cluster show distinct chemical signatures and might be formed differently.« less
2. Gravitational interactions of stars with supermassive black hole binaries – II. Hypervelocity stars

   https://doi.org/10.1093/mnras/sty2786  

   Darbha, Siva ; Coughlin, Eric R ; Kasen, Daniel ; Quataert, Eliot ( October 2018 , Monthly Notices of the Royal Astronomical Society)
3. The Stars of the HETDEX Survey. I. Radial Velocities and Metal-poor Stars from Low-resolution Stellar Spectra

   https://doi.org/10.3847/1538-4357/abe9bd  

   Hawkins, Keith ; Zeimann, Greg ; Sneden, Chris ; Cooper, Erin Mentuch ; Gebhardt, Karl ; Bond, Howard E. ; Carrillo, Andreia ; Casey, Caitlin M. ; Castanheira, Barbara G. ; Ciardullo, Robin ; et al ( April 2021 , The Astrophysical Journal)
4. The ASAS-SN catalogue of variable stars VI: an all-sky sample of δ Scuti stars

   https://doi.org/10.1093/mnras/staa499  

   Jayasinghe, T ; Stanek, K Z ; Kochanek, C S ; Vallely, P J ; Shappee, B J ; Holoien, T W-S ; Thompson, Todd A ; Prieto, J L ; Pejcha, O ; Fausnaugh, M ; et al ( April 2020 , Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We characterize an all-sky catalogue of ∼8400 δ Scuti variables in ASAS-SN, which includes ∼3300 new discoveries. Using distances from Gaia DR2, we derive period–luminosity relationships for both the fundamental mode and overtone pulsators in the WJK, V, Gaia DR2 G, J, H, Ks, and W1 bands. We find that the overtone pulsators have a dominant overtone mode, with many sources pulsating in the second overtone or higher order modes. The fundamental mode pulsators have metallicity-dependent periods, with log10(P) ∼ −1.1 for $\rm [Fe/H]\lt -0.3$ and log10(P) ∼ −0.9 for $\rm [Fe/H]\gt 0$, which leads to a period-dependent scale height. Stars with $P\gt 0.100\, \rm d$ are predominantly located close to the Galactic disc ($\rm |\mathit{ Z}|\lt 0.5\, kpc$). The median period at a scale height of $Z\sim 0\, \rm kpc$ also increases with the Galactocentric radius R, from log10(P) ∼ −0.94 for sources with $R\gt 9\, \rm kpc$ to log10(P) ∼ −0.85 for sources with $R\lt 7\, \rm kpc$, which is indicative of a radial metallicity gradient. To illustrate potential applications of this all-sky catalogue, we obtained 30 min cadence, image subtraction TESS light curves for a sample of 10 fundamental mode and 10 overtone δ Scuti stars discoveredmore »by ASAS-SN. From this sample, we identified two new δ Scuti eclipsing binaries, ASASSN-V J071855.62−434247.3 and ASASSN-V J170344.20−615941.2 with short orbital periods of Porb = 2.6096 and 2.5347 d, respectively.« less
5. Observations of Binary Stars with the Differential Speckle Survey Instrument. IX. Observations of Known and Suspected Binaries, and a Partial Survey of Be Stars

   https://doi.org/10.3847/1538-3881/ab87a6  

   Horch, Elliott P. ; van Belle, Gerard T. ; Davidson, James W. ; Willmarth, Daryl ; Fekel, Francis C. ; Muterspaugh, Matthew ; Casetti-Dinescu, Dana I. ; Hahne, Frederick W. ; Granucci, Nicole M. ; Clark, Catherine ; et al ( May 2020 , The Astronomical Journal)