The accreting white dwarfs in BW Scl, BC UMa and SW UMa

TL;DR

This study uses Hubble Space Telescope data to analyze the far-ultraviolet spectra of three short-period dwarf novae, revealing white dwarf temperatures, elemental abundances, and variability characteristics.

Contribution

First detailed UV spectral analysis of BW Scl, BC UMa, and SW UMa, determining white dwarf temperatures, abundances, and variability, with implications for accretion processes.

Findings

White dwarf temperatures are around 14,000-15,200K.

Sub-solar abundances of C, O, Si; supra-solar Al.

SW UMa shows significant UV variability.

Abstract

We have observed the short-period dwarf novae BW Scl, BC UMa and SW UMa using the Hubble Space Telescope/Space Telescope Imaging Spectrograph. In all three systems, the white dwarf is the dominant source of far-ultraviolet flux, even though in BC UMa and SW UMa an additional continuum component contributes ~10% and ~20% of the 1400A flux, respectively. Fitting the data with detailed white dwarf model spectra, we determine the effective temperatures to be 14800+-900K (BW Scl), 15200+-1000K (BC UMa), and 13900+-900K (SW UMa). The additional continuum component in BC UMa and SW UMa is equally well described by either a blackbody or a power law, which could be associated with emission from the hot spot or from an optically thin accretion disk (or an optically thin layer on top of a colder optically thick disk), respectively. Modelling the narrow metal lines detected in the STIS spectra results in sub-solar abundances of carbon, oxygen and silicon for all three systems, and also suggests substantial supra-solar abundances of aluminium. The narrow absorption line profiles imply low white dwarf rotation rates, v sin(i)<=300km/s for the three white dwarfs. SW UMa is the only system that shows significant short-term variability in the far-ultraviolet range, which is primarily associated with the observed emission lines.