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1. The fastest stars in the Galaxy

   https://doi.org/10.21105/astro.2306.03914  

   Jahaveri, Kareem; Shen, Ken J.; Chandra, Vedant; Bauer, Evan B.; Fuller, Jim; Strader, Jay; Chomiuk, Laura; Naidu, Rohan P.; Caiazzo, Ilaria; Rodriguez, Antonio C.; et al (July 2023, The Open Journal of Astrophysics)
2. 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] = −0.5 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. 

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3. The ASAS-SN catalogue of variable stars IX: The spectroscopic properties of Galactic variable stars

   https://doi.org/10.1093/mnras/stab114  

   Jayasinghe, T; Kochanek, C S; Stanek, K Z; Shappee, B J; Holoien, T W-S; Thompson, Todd A; Prieto, J L; Dong, Subo; Pawlak, M; Pejcha, O; et al (March 2021, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT The All-Sky Automated Survey for Supernovae provides long baseline (∼4 yr) V-band light curves for sources brighter than V≲ 17 mag across the whole sky. We produced V-band light curves for a total of ∼61.5 million sources and systematically searched these sources for variability. We identified ∼426 000 variables, including ∼219 000 new discoveries. Most ($${\sim }74{ per\ cent}$$) of our discoveries are in the Southern hemisphere. Here, we use spectroscopic information from LAMOST, GALAH, RAVE, and APOGEE to study the physical and chemical properties of these variables. We find that metal-poor eclipsing binaries have orbital periods that are shorter than metal-rich systems at fixed temperature. We identified rotational variables on the main-sequence, red giant branch, and the red clump. A substantial fraction ($${\gtrsim }80{ per\ cent}$$) of the rotating giants have large $$v$$rot or large near-ultraviolet excesses also indicative of fast rotation. The rotational variables have unusual abundances suggestive of analysis problems. Semiregular variables tend to be lower metallicity ($$\rm [Fe/H]{\sim }-0.5$$) than most giant stars. We find that the APOGEE DR16 temperatures of oxygen-rich semiregular variables are strongly correlated with the WRP − WJK colour index for $$\rm T_{eff}\lesssim 3800$$ K. Using abundance measurements from APOGEE DR16, we find evidence for Mg and N enrichment in the semiregular variables. We find that the Aluminum abundances of the semiregular variables are strongly correlated with the pulsation period, where the variables with $$\rm P\gtrsim 60$$ d are significantly depleted in Al. 

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4. Fate of twin stars on the unstable branch: Implications for the formation of twin stars

   https://doi.org/10.1103/PhysRevD.105.043014  

   Espino, Pedro L.; Paschalidis, Vasileios (February 2022, Physical Review D)
5. The ASAS-SN catalogue of variable stars – VIII. ‘Dipper’ stars in the Lupus star-forming region

   https://doi.org/10.1093/mnras/staa1588  

   Bredall, J W; Shappee, B J; Gaidos, E; Jayasinghe, T; Vallely, P; Stanek, K Z; Kochanek, C S; Gagné, J; Hart, K; Holoien, T W-S; et al (August 2020, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Some young stellar objects such as T Tauri-like ‘dipper’ stars vary due to transient partial occultation by circumstellar dust, and observations of this phenomenon inform us of conditions in the planet-forming zones close to these stars. Although many dipper stars have been identified with space missions such as Kepler/K2, ground-based telescopes offer longer term and multiwavelength perspectives. We identified 11 dipper stars in the Lupus star-forming region in data from the All-Sky Automated Survey for SuperNovae (ASAS-SN), and further characterized these using observations by the Las Cumbres Global Observatory Telescope (LCOGT) and the Transiting Exoplanet Survey Satellite (TESS), as well as archival data from other missions. Dipper stars were identified from a catalogue of nearby young stars and selected based on the statistical significance, asymmetry, and quasi-periodicity or aperiodicity of variability in their ASAS-SN light curves. All 11 stars lie above or redwards of the zero-age main sequence and have infrared (IR) excesses indicating the presence of full circumstellar discs. We obtain reddening–extinction relations for the variability of seven stars using our combined ASAS-SN-TESS and LCOGT photometry. In all cases, the slopes are below the ISM value, suggesting larger grains, and we find a tentative relation between the slope (grain size) and the $$K_\text{s}-[22 \, \mu \text{m}]$$ IR colour regarded as a proxy for disc evolutionary state. 

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