Hot subdwarfs formed from the merger of two He white dwarfs

TL;DR

This study models the evolution of double helium white dwarf mergers into hot subdwarfs, analyzing hydrogen retention, rotational dynamics, and how these relate to observed subdwarf properties.

Contribution

It provides the first detailed stellar evolution models of post-merger hot subdwarfs derived from hydrodynamic merger simulations, linking progenitor systems to observed characteristics.

Findings

Hydrogen survival in mergers is limited to less than 10^{-4} solar masses.

Mass loss reduces angular momentum, resulting in moderate surface rotation velocities.

Models do not fully explain all observed isolated hot subdwarfs, indicating other formation channels or model limitations.

Abstract

We perform stellar evolution calculations of the remnant of the merger of two He white dwarfs (WDs). Our initial conditions are taken from hydrodynamic simulations of double WD mergers and the viscous disc phase that follows. We evolve these objects from shortly after the merger into their core He-burning phase, when they appear as hot subdwarf stars. We use our models to quantify the amount of H that survives the merger, finding that it is difficult for $\gtrsim 10^{-4}\;{\rm M}_\odot$ of H to survive, with even less being concentrated in the surface layers of the object. We also study the rotational evolution of these merger remnants. We find that mass loss over the $\sim 10^4\;\rm yr$ following the merger can significantly reduce the angular momentum of these objects. As hot subdwarfs, our models have moderate surface rotation velocities of $30-100\; {\rm km\,s^{-1}}$. The properties of our models are not representative of many apparently-isolated hot subdwarfs, suggesting that those objects may form via other channels or that our modelling is incomplete. However, a sub-population of hot subdwarfs are moderate-to-rapid rotators and/or have He-rich atmospheres. Our models help to connect the observed properties of these objects to their progenitor systems.