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1. 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 discovered 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. 

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2. 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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3. A unicorn in monoceros: the 3 M⊙ dark companion to the bright, nearby red giant V723 Mon is a non-interacting, mass-gap black hole candidate

   https://doi.org/10.1093/mnras/stab907  

   Jayasinghe, T; Stanek, K Z; Thompson, Todd A; Kochanek, C S; Rowan, D M; Vallely, P J; Strassmeier, K G; Weber, M; Hinkle, J T; Hambsch, F-J; et al (May 2021, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT We report the discovery of the closest known black hole candidate as a binary companion to V723 Mon. V723 Mon is a nearby ($$d\sim 460\, \rm pc$$), bright (V ≃ 8.3 mag), evolved (Teff, giant ≃ 4440 K, and Lgiant ≃ 173 L⊙) red giant in a high mass function, f(M) = 1.72 ± 0.01 M⊙, nearly circular binary (P = 59.9 d, e ≃ 0). V723 Mon is a known variable star, previously classified as an eclipsing binary, but its All-Sky Automated Survey, Kilodegree Extremely Little Telescope, and Transiting Exoplanet Survey Satellite light curves are those of a nearly edge-on ellipsoidal variable. Detailed models of the light curves constrained by the period, radial velocities, and stellar temperature give an inclination of $$87.0^{\circ ^{+1.7^\circ }}_{-1.4^\circ }$$, a mass ratio of q ≃ 0.33 ± 0.02, a companion mass of Mcomp = 3.04 ± 0.06 M⊙, a stellar radius of Rgiant = 24.9 ± 0.7 R⊙, and a giant mass of Mgiant = 1.00 ± 0.07 M⊙. We identify a likely non-stellar, diffuse veiling component with contributions in the B and V band of $${\sim }63{{\ \rm per\ cent}}$$ and $${\sim }24{{\ \rm per\ cent}}$$, respectively. The SED and the absence of continuum eclipses imply that the companion mass must be dominated by a compact object. We do observe eclipses of the Balmer lines when the dark companion passes behind the giant, but their velocity spreads are low compared to observed accretion discs. The X-ray luminosity of the system is $$L_{\rm X}\simeq 7.6\times 10^{29}~\rm ergs~s^{-1}$$, corresponding to L/Ledd ∼ 10−9. The simplest explanation for the massive companion is a single compact object, most likely a black hole in the ‘mass gap’. 

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4. Progress on the calibration of surface brightness–color relations for early- and late-type stars

   https://doi.org/10.1051/0004-6361/202140763  

   Salsi, A.; Nardetto, N.; Mourard, D.; Graczyk, D.; Taormina, M.; Creevey, O.; Hocdé, V.; Morand, F.; Perraut, K.; Pietrzynski, G.; et al (August 2021, Astronomy & Astrophysics)

   null (Ed.)

   Context. Surface brightness-color relations (SBCRs) are widely used for estimating angular diameters and deriving stellar properties. They are critical to derive extragalactic distances of early-type and late-type eclipsing binaries or, potentially, for extracting planetary parameters of late-type stars hosting planets. Various SBCRs have been implemented so far, but strong discrepancies in terms of precision and accuracy still exist in the literature. Aims. We aim to develop a precise SBCR for early-type B and A stars using selection criteria, based on stellar characteristics, and combined with homogeneous interferometric angular diameter measurements. We also improve SBCRs for late-type stars, in particular in the Gaia photometric band. Methods. We observed 18 early-type stars with the VEGA interferometric instrument, installed on the CHARA array. We then applied additional criteria on the photometric measurements, together with stellar characteristics diagnostics in order to build the SBCRs. Results. We calibrated a SBCR for subgiant and dwarf early-type stars. The RMS of the relation is σ F V 0  = 0.0051 mag, leading to an average precision of 2.3% on the estimation of angular diameters, with 3.1% for V − K < −0.2 mag and 1.8% for V − K > −0.2 mag. We found that the conversion between Johnson- K and 2MASS- K s photometries is a key issue for early-type stars. Following this result, we have revisited our previous SBCRs for late-type stars by calibrating them with either converted Johnson- K or 2MASS- K s photometries. We also improve the calibration of these SBCRs based on the Gaia photometry. The expected precision on the angular diameter using our SBCRs for late-type stars ranges from 1.0 to 2.7%. Conclusions. By reaching a precision of 2.3% on the estimation of angular diameters for early-type stars, significant progress has been made to determine extragalactic distances, such as M31 and M33 galaxies, using early-type eclipsing binaries. 

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5. Discovery of a highly eccentric, chromospherically active binary: ASASSN-V J192114.84+624950.8

   https://doi.org/10.1093/mnras/stac1343  

   Way, Zachary S.; Jayasinghe, T.; Kochanek, C. S.; Stanek, K. Z.; Vallely, Patrick; Thompson, Todd A.; Holoien, Thomas W-S; Shappee, Benjamin J. (May 2022, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT As part of an All-Sky Automated Survey for SuperNovae (ASAS-SN) search for sources with large flux decrements, we discovered a transient where the quiescent, stellar source ASASSN-V J192114.84+624950.8 rapidly decreased in flux by $$\sim 55{{\ \rm per\ cent}}$$ (∼0.9 mag) in the g band. The Transiting Exoplanet Survey Satellite light curve revealed that the source is a highly eccentric, eclipsing binary. Fits to the light curve using phoebe find the binary orbit to have e = 0.79, Porb = 18.462 d, and i = 88.6°, and the ratios of the stellar radii and temperatures to be R2/R1 = 0.71 and Te,2/Te,1 = 0.82. Both stars are chromospherically active, allowing us to determine their rotational periods of P1 = 1.52 d and P2 = 1.79 d, respectively. A Large Binocular Telescope/Multi-Object Double Spectrograph spectrum shows that the primary is a late-G- or early-K-type dwarf. Fits to the spectral energy distribution show that the luminosities and temperatures of the two stars are L1 = 0.48 L⊙, $$T_1= 5050\, \mathrm{K}$$, L2 = 0.12 L⊙, and $$T_{2} = 4190\, \mathrm{K}$$. We conclude that ASASSN-V J192114.84+624950.8 consists of two chromospherically active, rotational variable stars in a highly elliptical eclipsing orbit. 

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