On the Origins of Sub-Subgiants: Mass Transfer, Dynamical Encounters, and Magnetic Fields

Sub-subgiant stars (SSGs) lie red of the main-sequence and fainter than the red giant branch in optical color-magnitude diagrams, a region not easily populated by standard stellar evolution pathways. Discoveries of dozens of SSGs in both open and globular clusters solidifies these stars as a unique class sharing several properties: X-ray emission, photometric variability, and, where binary status is known, orbital periods between 2-18 days. While there has been speculation on what mechanisms may create these unusual stars, no theory has yet been developed to explain their origins. Here we focus on three hypotheses of SSG formation: (1) mass transfer in a binary system, (2) tidally induced mass loss from a subgiant star during a dynamical encounter, and (3) reduced luminosity due to magnetic fields that lower convective efficiency and produce large star spots. Using the stellar evolution code MESA, we develop evolutionary tracks for each of these hypotheses, and compare the expected stellar and orbital properties of these models with the six known SSGs in the two open clusters M67 and NGC 6791. We calculate the frequency with which each of these mechanisms may create SSG systems, and find that the magnetic field hypothesis is expected to create SSGs with the highest frequency in open clusters. Mass transfer and dynamical encounters may also create SSGs, but have lower expected formation frequencies. They may be more important mechanisms in globular clusters with higher encounter rates and larger populations of subgiants.