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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. Wide Post-common Envelope Binaries from Gaia: Orbit Validation and Formation Models

   https://doi.org/10.1088/1538-3873/ad6809  

   Yamaguchi, Natsuko; El-Badry, Kareem; Rees, Natalie_R; Shahaf, Sahar; Mazeh, Tsevi; Andrae, Renŕ (August 2024, Publications of the Astronomical Society of the Pacific)

   Abstract Astrometry from Gaia DR3 has enabled the discovery of a sample of 3000+ binaries containing white dwarfs (WD) and main-sequence (MS) stars in relatively wide orbits, with orbital periodsP_orb= (100–1000) days. This population was not predicted by binary population synthesis models before Gaia and—if the Gaia orbits are robust—likely requires very efficient envelope ejection during common envelope evolution (CEE). To assess the reliability of the Gaia solutions, we measured multi-epoch radial velocities (RVs) of 31 WD+MS binary candidates withP_orb= (40–300) days andAstroSpectroSB1orbital solutions. We jointly fit the RVs and astrometry, allowing us to validate the Gaia solutions and tighten constraints on component masses. We find a high success rate for the Gaia solutions, with only 2 out of the 31 systems showing significant discrepancies between their Gaia orbital solutions and our RVs. Joint fitting of RVs and astrometry allows us to directly constrain the secondary-to-primary flux ratio $\mathcal{S}$, and we find $\mathcal{S}\lesssim 0.02$for most objects, confirming the companions are indeed WDs. We tighten constraints on the binaries’ eccentricities, finding a mediane≈ 0.1. These eccentricities are much lower than those of normal MS+MS binaries at similar periods, but much higher than predicted for binaries formed via stable mass transfer. We present MESA single and binary evolution models to explore how the binaries may have formed. The orbits of most binaries in the sample can be produced through CEE that begins when the WD progenitor is an AGB star, corresponding to initial separations of 2–5 au. Roughly 50% of all post-common envelope binaries are predicted to have first interacted on the AGB, ending up in wide orbits like these systems. 

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3. 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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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. The Companion Mass Distribution of Post Common Envelope Hot Subdwarf Binaries: Evidence for Boosted and Disrupted Magnetic Braking?

   https://doi.org/10.1088/1538-3873/ad94a2  

   Blomberg, Lisa; El-Badry, Kareem; Breivik, Katelyn; Caiazzo, Ilaria; Nagarajan, Pranav; Rodriguez, Antonio; van_Roestel, Jan; Vanderbosch, Zachary_P; Yamaguchi, Natsuko (December 2024, Publications of the Astronomical Society of the Pacific)

   Abstract We measure the mass distribution of main-sequence (MS) companions to hot subdwarf B stars (sdBs) in post-common envelope binaries (PCEBs). We carried out a spectroscopic survey of 14 eclipsing systems (“HW Vir binaries”) with orbital periods of 3.8 < P_orb < 12 hr, resulting in a well-understood selection function and a near-complete sample of HW Vir binaries withG < 16. We constrain companion masses from the radial velocity curves of the sdB stars. The companion mass distribution peaks atM_MS ≈ 0.15M_⊙and drops off atM_MS > 0.2M_⊙, with only two systems hosting companions above the fully convective limit. There is no correlation betweenP_orbandM_MSwithin the sample. A similar drop-off in the companion mass distribution of white dwarf (WD) + MS PCEBs has been attributed to disrupted magnetic braking (MB) below the fully convective limit. We compare the sdB companion mass distribution to predictions of binary evolution simulations with a range of MB laws. Because sdBs have short lifetimes compared to WDs, explaining the lack of higher-mass MS companions to sdBs with disrupted MB requires MB to be boosted by a factor of 20–100 relative to MB laws inferred from the rotation evolution of single stars. We speculate that such boosting may be a result of irradiation-driven enhancement of the MS stars’ winds. An alternative possibility is that common envelope evolution favors low-mass companions in short-period orbits, but the existence of massive WD companions to sdBs with similar periods disfavors this scenario. 

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