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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.

 

Abstract We use photometry and proper motions from Gaia DR2 to determine the blue straggler star (BSS) populations of 16 old (1–10 Gyr), nearby ( d < 3500 pc) open clusters. We find that the fractional number of BSS compared to red giant branch stars increases with age, starting near zero at 1 Gyr and flattening to ∼0.35 by 4 Gyr. Fitting stellar evolutionary tracks to these BSSs, we find that their mass distribution peaks at a few tenths of a solar mass above the main-sequence turnoff. BSSs more than 0.5 M ⊙ above the turnoff make up only ∼25% of the sample, and BSSs more than 1.0 M ⊙ above the turnoff are rare. We compare this to Compact Object Synthesis and Monte Carlo Investigation Code population synthesis models of BSSs formed via mass transfer. We find that standard population synthesis assumptions dramatically under-produce the number of BSS in old open clusters. We also find that these models overproduce high-mass BSSs relative to lower-mass BSSs. The expected number of BSSs formed through dynamics do not fully account for this discrepancy. We conclude that in order to explain the observed BSS populations from Roche lobe overflow, mass transfer from giant donors must be more stable than assumed in canonical mass-transfer prescriptions, and including nonconservative mass transfer is important in producing realistic BSS masses. Even with these modifications, it is difficult to achieve the large number of BSSs observed in the oldest open clusters. We discuss some additional physics that may explain the large number of observed blue stragglers among old stellar populations.