August 11-15, 2014


CV WD masses and temperatures: a new synergy between HST/COS and VLT/X-Shooter

Anna Francesca Pala (University of Warwick)

Boris Gaensicke (University of Warwick)

The mass retention efficiency is a key question in both the theoretical and observational study of accreting white dwarfs (WDs) in interacting binaries, with important implications for their potential as progenitors for type Ia supernovae. Canonical wisdom is that classical nova eruptions erode the WD mass (e.g. Prialnik & Kovetz, 1995), and consequently, cataclysmic variables (CVs) have been excluded from the SNIa progenitor discussion. However, Zorotovic et al. (2011) showed that the average mass of WD in CVs is substantially higher (~0.83Msun) than that of single WDs (~0.64Msun), in stark contrast to expectations based on current classical nova models. This finding is based on a sample of ~30 CV WDs with accurate mass measurements, most of them in eclipsing systems.

Given the fundamental importance of the mass evolution of accreting WDs, we have begun a systematic study of 40 CVs using 122 orbits of HST ultraviolet spectroscopy and several nights of VLT/X-Shooter observations. Here, we present the first results from this project. We used a grid of synthetic spectra, calculated using the TLUSTY/SYNSPEC model atmosphere code, to determine the WD effective temperature (Teff) and surface gravity (log(g)). The HST spectra alone cannot resolve the degeneracy between Teff and log(g), and an additional dynamical constraint is needed. We use the phase-resolved X-Shooter observations to (i) measure the mass ratio of the CVs from the reflex motion of both the WD and the donor star (ii) to establish the spectral energy distribution (SED), which constrains both Teff and log(g). Combining the X-Shooter and HST data, we can measure the WD masses to a few percent, and will be able to answer the question whether accreting CV WDs grow in mass.

Mode of presentation: poster