On the origins of sub-subgiants: Mass transfer, dynamical encounters, and magnetic fields

In an optical color-magnitude diagram sub-subgiant stars (SSGs) lie redward of the mainsequenc and fainter than the red giant branch in a region not easily populated by standard stella evolution pathways. The first SSGs were discovered in the open cluster M67 [1; 2]. Subsequen discoveries of dozens of SSGs in both open and globular clusters solidifies these stars as thei own unique class sharing several properties: X-ray emission, photometric variability, and, wher binary status is known, orbital periods between 2-18 days. While there has been speculation o what mechanisms may create these unusual stars, no well-developed theory has yet been propose to explain their origins. Here we focus on three possible theories of SSG formation: (1 mass transfer in a binary system, (2) mass loss in a dynamical encounter, (3) a reduced luminosit due to magnetic activity that lowers convective efficiency. We compare the expected stellar an orbital properties of these models with the six known SSGs in the two open clusters M67 an NGC 6791.