Binary population models of CVs: post thermal timescale mass transfer CVs

Abstract

Context: Recently the white dwarf (WD) mass distribution of cataclysmic variables (CVs) has been found to dramatically disagree with the predictions of the standard CV formation model. The high mean WD mass among CVs is not imprinted in the currently observed sample of CV progenitors and can therefore not be attributed to selection effects. The standard CV formation model might thus miss an important ingredient that can explain the discrepancy between the WD masses among CVs and their progenitors. Two explanations have been put forward: either the WD grows in mass during CV evolution or the CV formation is preceded by a (short) phase of thermal timescale mass transfer (TTMT) in which the WD gains a sufficient amount of mass from its companion.

Aims: Here we investigate if and under which conditions a phase of TTMT prior to the CV formation, which has been considered a rare channel in previous works, can become a typical channel of CV formation and if the problem with the high WD masses can be solved in this way.

Methods: We perform binary population synthesis models using the Binary\_C code to simulate the present intrinsic CV population. We use different models to investigate how several key aspects of CV evolution can influence the effect of a TTMT phase on the WD mass distribution. We carry out a statistical analysis on the characteristics of each model and compare these with the characteristics of a sample of observed CVs.

Results: We are able to produce a large number of massive WDs if we assume significant mass loss due to wind from the surface of the WD. The models that include this wind predict that two-thirds of the intrinsic CV population had a phase of TTMT and produce a mean WD mass that agrees with the observed value. The most convincing agreement between observations and model predictions is reached if, in addition to the TTMT wind, mass loss during nova cycles is taken into account.

Conclusions: The high WD masses among CVs can be explained by a preliminary phase of TTMT if such a TTMT wind exists. An accurate prescription for the adiabatic mass-radius exponent and corresponding critical mass ratio is of crucial importance for the formation of WDs with a mass of $\sim 0.8\,\Msun$. Our models predict that the majority of massive WDs among CVs have experienced TTMT.