Revealing the Field Sub-subgiant Population Using a Catalog of Active
  Giant Stars and Gaia EDR3
  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 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 TESS and VSX. This catalog can serve as an important sample to
study the impacts of magnetic fields in evolved stars.
