# Evolution models of helium white dwarf--main sequence star merger remnants

## TL;DR

This paper models the evolution of helium white dwarf and main-sequence star mergers, revealing pathways to hot subdwarfs and explaining observed intermediate helium-rich hot subdwarfs through detailed stellar evolution simulations.

## Contribution

It provides the first detailed post-merger evolutionary models of helium white dwarf-main sequence star mergers, identifying conditions leading to hot subdwarfs.

## Key findings

- Some mergers produce stars with intermediate helium-rich surfaces.
- Merger remnants can develop helium-poor surfaces due to diffusion.
- Model results align with observed intermediate helium-rich hot subdwarfs.

## Abstract

It is predicted that orbital decay by gravitational-wave radiation and tidal interaction will cause some close-binary stars to merge within a Hubble time. The merger of a helium-core white dwarf with a main-sequence star can produce a red giant branch star that has a low-mass hydrogen envelope when helium is ignited and thus become a hot subdwarf. Because detailed calculations have not been made, we compute post-merger models with a stellar evolution code. We find the evolutionary paths available to merger remnants and find the pre-merger conditions that lead to the formation of hot subdwarfs. We find that some such mergers result in the formation of stars with intermediate helium-rich surfaces. These stars later develop helium-poor surfaces owing to diffusion. Combining our results with a model population and comparing to observed stars, we find that some observed intermediate helium-rich hot subdwarfs can be explained as the remnants of the mergers of helium-core white dwarfs with low-mass main-sequence stars.