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

TL;DR

This study identifies a large population of field sub-subgiant stars (SSGs) among active giant binaries using Gaia data, revealing SSGs are common in such systems and expanding the known sample significantly.

Contribution

It introduces a catalog of 1723 active giant binaries with Gaia data and identifies 448 new field SSG candidates, demonstrating SSGs are a typical phase in RS CVn evolution.

Findings

448 SSG candidates identified among active giants

Most SSGs have rotation periods of 2-20 days

SSGs are common in RS CVn systems, not rare anomalies

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 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.