FUSE and IUE Spectroscopy of the Prototype Dwarf Nova ER Ursa Majoris During Quiescence

Abstract

ER Ursae Majoris is the prototype for a subset of SU UMa-type dwarf novae characterized by short cycle times between outburst, high outburst frequency, and "negative" superhumps. It suffers superoutbursts every 43 days, lasting 20 days, normal outbursts every 4 days and has an outburst amplitude of 3 magnitudes. We have carried out a far ultraviolet (FUV) spectral analysis of ER UMa in quiescence, by fitting Far Ultraviolet Spectroscopic Explorer (FUSE) and International Ultraviolet Explorer (IUE) spectra with model accretion disks and high gravity photosphere models. Using the Gaia parallax distance and an orbital inclination of 50 • , we find that during the brief quiescence of only four days, the accretion rate is 7.3 × 10 −11 M ⊙ /yr, with the ER UMa white dwarf contributing 55% of the FUV flux and the accretion disk contributing the remaining 45% of the flux. The white dwarf in ER UMa is markedly hotter (32,000 K) than the other white dwarfs in dwarf novae below the CV period gap which have typical temperatures ∼15,000 K. For a higher inclinations of 60 to 75 degrees, the accretion rates that we derive are roughly an order of magnitude higher 1 − 3 × 10 −10 M ⊙ /yr.