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1. Seven classes of rotational variables from a study of 50 000 spotted stars with ASAS-SN, Gaia , and APOGEE

   https://doi.org/10.1093/mnras/stad3564  

   Phillips, Anya; Kochanek, C S; Jayasinghe, Tharindu; Cao, Lyra; Christy, Collin T; Rowan, D M; Pinsonneault, Marc (November 2023, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We examine the properties of ∼50 000 rotational variables from the ASAS-SN survey using distances, stellar properties, and probes of binarity from Gaia DR3 and the SDSS APOGEE survey. They have higher amplitudes and span a broader period range than previously studied Kepler rotators. We find they divide into three groups of main sequence stars (MS1, MS2s, MS2b) and four of giants (G1/3, G2, G4s, and G4b). MS1 stars are slowly rotating (10–30 d), likely single stars with a limited range of temperatures. MS2s stars are more rapidly rotating (days) single stars spanning the lower main sequence up to the Kraft break. There is a clear period gap (or minimum) between MS1 and MS2s, similar to that seen for lower temperatures in the Kepler samples. MS2b stars are tidally locked binaries with periods of days. G1/3 stars are heavily spotted, tidally locked RS CVn stars with periods of 10s of days. G2 stars are less luminous, heavily spotted, tidally locked sub-subgiants with periods of ∼10 d. G4s stars have intermediate luminosities to G1/3 and G2, slow rotation periods (approaching 100 d), and are almost certainly all merger remnants. G4b stars have similar rotation periods and luminosities to G4s, but consist of sub-synchronously rotating binaries. We see no difference in indicators for the presence of very wide binary companions between any of these groups and control samples of photometric twin stars built for each group. 

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2. Close substellar-mass companions in stellar wide binaries: Discovery and characterization with APOGEE and Gaia DR2

   https://doi.org/10.1093/mnras/stab2349  

   Lewis, Hannah M; Anguiano, Borja; Majewski, Steven R; Nidever, David L; Badenes, Carles; De Lee, Nathan; Hasselquist, Sten; Daher, Christine Mazzola; Stassun, Keivan G; Bizyaev, Dmitry; et al (September 2021, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   Abstract We present a search for close, unresolved companions in a subset of spatially resolved Gaia wide binaries containing main-sequence stars within 200 pc of the Sun, utilizing the APOGEE–Gaia Wide Binary Catalog. A catalog of 37 wide binaries was created by selecting pairs of stars with nearly identical Gaia positions, parallaxes, and proper motions, and then confirming candidates to be gravitationally-bound pairs using APOGEE radial velocities. We identify close, unresolved stellar and substellar candidate companions in these multiple systems using (1) the Gaia binary main-sequence and (2) observed periodic radial velocity variations in APOGEE measurements due to the influence of a close substellar-mass companion. The studied wide binary pairs reveal a total of four stellar-mass close companions in four different wide binaries, and four substellar-mass close companion candidates in two wide binaries. The latter are therefore quadruple systems, with one substellar mass companion orbiting each wide binary component in an S-type orbit. Taken at face value, these candidate systems represent an enhancement of an order of magnitude over the expected occurrence rate of ∼2 per cent of stars having substellar companions >2 MJup within ∼100 day orbits; we discuss implications and possible explanations for this result. Finally, we compare chemical differences between the components of the wide binaries and the components of the candidate higher-order systems and find that any chemical influence or correlation due to the presence of close companions to wide binary stars is not discernible. 

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3. Population Demographics of White Dwarf Binaries with Intermediate Separations: Gaia Constraints on post-AGB Mass Transfer

   https://doi.org/10.1088/1538-3873/ae0d30  

   Yamaguchi, Natsuko; El-Badry, Kareem; Shahaf, Sahar (October 2025, Publications of the Astronomical Society of the Pacific)

   Abstract Astrometry from the Gaia mission has revealed a large population of white dwarf (WD) + main sequence (MS) binaries with periods of 100–1000 days. These systems have separations intermediate to predictions from standard binary evolution scenarios, challenging models of binary interaction and mass transfer. Because the selection function of Gaia astrometric catalogs is complex, the underlying population demographics of WD+MS binaries remain imperfectly understood. We present a forward-model of the AU-scale WD+MS binary population probed by Gaia that begins with a realistic binary population and incorporates a full model of Gaia mock observations and astrometric model fitting, as well as cuts employed in producing the Gaia astrometric catalog and selecting WD+MS binary candidates. We model the formation of AU-scale WD+MS binaries as the result of interaction when the WD progenitor is an AGB star. We test several models for the binaries’ formation, including stable mass transfer with theoretically predicted stability criteria and two different formalisms for common envelope evolution. None of these models succeed in reproducing the observed component mass distributions or the absolute number of WD+MS binaries. The data are best reproduced by a model in which post-AGB binaries remain wide only if the accretor-to-donor mass ratio exceeds ∼0.4. Our model allows us to constrain the intrinsic population demographics of intermediate-separation WD+MS binaries. The inferred period distribution is close to flat, with $dN/d{P}_{\mathrm{orb}}\propto P_{\mathrm{orb}}^{0.12}$, while the WD mass distribution is sharply peaked at 0.6M_⊙. The model implies that ∼0.4% of solar-type stars have WD companions with periods of 100–1000 days. 

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4. Revealing the Field Sub-subgiant Population Using a Catalog of Active Giant Stars and Gaia EDR3

   https://doi.org/10.3847/1538-4357/ac53b1  

   Leiner, Emily M.; Geller, Aaron M.; Gully-Santiago, Michael A.; Gosnell, Natalie M.; Tofflemire, Benjamin M. (March 2022, The Astrophysical Journal)

   Abstract Sub-subgiant stars (SSGs) fall below the subgiant branch and/or red of the giant branch in open and globular clusters, an area of the color–magnitude diagram (CMD) not populated by standard stellar evolution tracks. One hypothesis is that SSGs result from rapid rotation in subgiants or giants due to tidal synchronization in a close binary. The strong magnetic fields generated inhibit convection, which in turn produces large starspots, radius inflation, and lower-than-expected average surface temperatures and luminosities. Here we cross-reference a catalog of active giant binaries (RS CVns) in the field with Gaia EDR3. Using the Gaia photometry and parallaxes, we precisely position the RS CVns in a CMD. We identify stars that fall below a 14 Gyr, metal-rich isochrone as candidate field SSGs. Out of a sample of 1723 RS CVn, we find 448 SSG candidates, a dramatic expansion from the 65 SSGs previously known. Most SSGs have rotation periods of 2–20 days, with the highest SSG fraction found among RS CVn with the shortest periods. The ubiquity of SSGs among this population indicates that SSGs are a normal phase in evolution for RS CVn-type systems, not rare by-products of dynamical encounters found only in dense star clusters as some have suggested. We present our catalog of 1723 active giants, including Gaia photometry and astrometry, and rotation periods from the Transiting Exoplanet Survey Satellite and International Variable Star Index (VSX). This catalog can serve as an important sample to study the impacts of magnetic fields in evolved stars. 

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5. Testing Models for Fully and Partially Stripped Low-mass Stars with Gaia: Implications for Hot Subdwarfs, Binary RR Lyrae, and Black Hole Impostors

   https://doi.org/10.1088/1538-3873/ae5f48  

   Nagarajan, Pranav; El-Badry, Kareem; Bobrick, Alexey; Iorio, Giuliano; Molina, Francisco; Vos, Joris; Vučković, Maja (April 2026, Publications of the Astronomical Society of the Pacific)

   Abstract When low-mass (≲2M_⊙) red giants lose their envelopes to a companion just before the helium flash, the resulting mass transfer can produce binaries hosting hot subdwarfs, horizontal branch stars, and undermassive red clump stars. Recent work predicts a continuum of such products, from fully stripped hot subdwarfs to partially stripped horizontal branch and red clump stars, and suggests that young, metal-rich RR Lyrae can form when partial stripping leaves a helium-burning star in the instability strip. To enable direct comparison with observations, we model these binaries in a simulated Milky Way–like galaxy with a realistic metallicity-dependent star formation history and 3D dust map, generate epoch astrometry using Gaia’s scanning law, and fit it with the cascade of astrometric models applied in Gaia DR3. We compare the simulated population to DR3 observations of hot subdwarfs, RR Lyrae, and red giants with high astrometric mass functions. The model significantly overpredicts the number of hot subdwarfs with astrometric binary solutions, partly because the predicted flux ratios are more unequal than observed. It also predicts ≳100 RR Lyrae with DR3 astrometric orbital solutions, while none are observed. We conclude that RR Lyrae in au-scale binaries may be substantially rarer than predicted. In contrast, the model plausibly explains the population of red clump stars with high astrometric mass functions, which we interpret as potential black hole impostors. We predict that ∼10 ×  more stripped-star binaries will be detectable in DR4, whose sensitivity to longer periods will more strongly test wide-orbit systems. 

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