The origin of single low-mass WDs: another problem that consequential angular momentum loss in CVs might solve

Abstract

Low-mass helium-core white dwarfs (WDs) with masses below 0.5 M_{⊙} are known to be formed in binary star systems but unexpectedly, a significant fraction of them seems to be single. On the other hand, in cataclysmic variables (CVs), a large number of low-mass WD primary stars is predicted but not observed. We recently showed that the latter problem can be solved if consequential angular momentum loss causes especially CVs with low-mass WDs to merge and form single stars. Here we simulate the population of single WDs resulting from single-star evolution and from binary star mergers taking into account these new merging CVs. We show that according to the revised model of CV evolution, merging CVs might be the dominant channel leading to the formation of low-mass single WDs and that the predicted relative numbers are consistent with observations. This can be interpreted as further evidence for the revised model of CV evolution we recently suggested. This model includes consequential angular momentum loss that increases with decreasing WD mass and might not only explain the absence of low-mass WD primaries in CVs but also the existence of single low-mass WDs.