# On the Origin of Sub-Subgiant Stars II: Binary Mass Transfer, Envelope Stripping, and Magnetic Activity

## TL;DR

This paper explores three potential mechanisms—binary mass transfer, envelope stripping, and magnetic activity—for the formation of sub-subgiant stars, using stellar evolution models and observational data from open clusters.

## Contribution

It develops evolutionary models for each hypothesis and compares their predictions with observed SSGs, highlighting the likely dominant role of magnetic activity in open clusters.

## Key findings

- Magnetic activity likely produces the most SSGs in open clusters.
- All three mechanisms can generate SSGs, but with different expected frequencies.
- Mass transfer and envelope stripping may be more relevant in globular clusters.

## Abstract

Sub-subgiant stars (SSGs) lie to the red of the main-sequence and fainter than the red giant branch in cluster color-magnitude diagrams (CMDs), a region not easily populated by standard stellar evolution pathways. While there has been speculation on what mechanisms may create these unusual stars, no well-developed theory exists to explain their origins. Here we discuss three hypotheses of SSG formation: (1) mass transfer in a binary system, (2) stripping of a subgiant's envelope, perhaps 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 six known SSGs in the two open clusters M67 and NGC 6791. All three of these mechanisms can create stars or binary systems in the SSG CMD domain. We also 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 envelope stripping have lower expected formation frequencies, but may nevertheless create occasional SSGs in open clusters. They may also be important mechanisms to create SSGs in higher mass globular clusters.