He-Accreting WDs: accretion regimes and final outcomes

TL;DR

This study explores how carbon-oxygen white dwarfs respond thermally to helium accretion across various regimes, identifying conditions leading to different burning behaviors and their implications for binary systems and He-novae.

Contribution

It provides a detailed analysis of accretion regimes, transition conditions, and mass retention efficiencies for WDs accreting helium, with polynomial fits for population synthesis.

Findings

Identified regimes: red-giant-like, steady burning, flashes.

Derived mass retention efficiencies as functions of mass and accretion rate.

Provided polynomial fits for use in binary evolution models.

Abstract

The behaviour of carbon-oxygen white dwarfs (WDs) subject to direct helium accretion is extensively studied. We aim to analyze the thermal response of the accreting WD to mass deposition at different time scales. The analysis has been performed for initial WDs masses and accretion rates in the range (0.60 - 1.02) Msun and 1.e-9 - 1.e-5 Msun/yr, respectively. Thermal regimes in the parameters space M_{WD} - dot{M}_{He}, leading to formation of red-giant-like structure, steady burning of He, mild, strong and dynamical flashes have been identified and the transition between those regimes has been studied in detail. In particular, the physical properties of WDs experiencing the He-flash accretion regime have been investigated in order to determine the mass retention efficiency as a function of the accretor total mass and accretion rate. We also discuss to what extent the building-up of a He-rich layer via H-burning could be described according to the behaviour of models accreting He-rich matter directly. Polynomial fits to the obtained results are provided for use in binary population synthesis computations. Several applications for close binary systems with He-rich donors and CO WD accretors are considered and the relevance of the results for the interpretation of He-novae is discussed.