White dwarf masses in cataclysmic variables

Abstract

Context. The white dwarf (WD) mass distribution of cataclysmic variables (CVs) has recently 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 cannot be attributed to selection effects. Two possibilities have been put forward to solve this issue: either the WD grows in mass during CV evolution, or in a significant fraction of cases, CV formation is preceded by a (short) phase of thermal time-scale mass transfer (TTMT) in which the WD gains a sufficient amount of mass. Aims. Here we investigate if and under which conditions a phase of TTMT before CV formation or mass growth in CVs can bring theoretical predictions and observations into agreement. Methods. We employed binary population synthesis models using the binary_c/nucsyn code to simulate the present intrinsic CV population. To that end we incorporated aspects specific to CV evolution such as an appropriate mass-radius relation of the donor star and a more detailed prescription for the critical mass ratio for dynamically unstable mass transfer. We have also implemented a previously suggested wind from the surface of the WD during TTMT and tested the idea of WD mass growth during the CV phase by arbitrarily changing the accretion efficiency. We compare the model predictions of the TTMT and the mass growth model with the characteristics of CVs derived from observed samples. Results. We find that mass growth of the WDs in CVs fails to reproduce the observed WD mass distribution. In the case of TTMT, we are able to produce a large number of massive WDs if we assume significant mass loss from the surface of the WD during the TTMT phase. However, the model still produces too many CVs with helium WDs. Moreover, the donor stars are evolved in many of these post-TTMT CVs, which contradicts the observations. Conclusions. We conclude that in our current framework of CV evolution neither TTMT nor WD mass growth can fully explain either the observed WD mass or the period distribution in CVs.

Three in one go: consequential angular momentum loss can solve major problems of CV evolution

“…This is because low-mass WDs are found in large numbers in samples of detached post common envelope binaries (Rebassa-Mansergas et al 2011) and the observed WD mass distribution of these objects is in good agreement with the model predictions (Toonen & Nelemans 2013;Zorotovic et al 2014;Camacho et al 2014). Finally, in Wijnen, Zorotovic & Schreiber (2015) we have shown recently that the two scenarios suggested by ZSG11, i.e. mass growth in CVs or a preceding phase of thermal time scale mass transfer for many CVs cannot solve the WD mass problem in CVs either.…”

“…The sample of generated MS binary stars is then evolved until the end of the common envelope (CE) phase using the binary-star evolution (BSE) code from Hurley, Tout & Pols (2002) assuming a CE efficiency of α = 0.25 (Zorotovic et al 2010). The generated population of zero age post-CE binaries is subsequently evolved to obtain the current population of CVs following the approach used in Wijnen, Zorotovic & Schreiber (2015). We simply assume the Mass-Radius relation for CV secondary stars and systemic angular momentum loss prescriptions as derived from observational constraints by Knigge, Baraffe & Patterson (2011).…”

NuSTAR HARD X-RAY SURVEY OF THE GALACTIC CENTER REGION. II. X-RAY POINT SOURCES

“…Hailey et al (2016) argue that the excess B-star population needed to explain high WD masses is within the large uncertainty of the expected population in the GC region. On the other hand, the average mass of the WD in the nonmagnetic CVs or isolated magnetic WDs are about 0.8M e (Ferrario et al 2015;Wijnen et al 2015). In addition, there is no clear evidence for high mass progenitors for the WDs in CVs (Zorotovic et al 2011).…”

Short Orbital Period Cataclysmic Variables

“…evolution [8]. WDs in the short-period CVs are also hotter (10000-20000 K, [9]) than expected for their age.…”