Archival [ITAL]HST[/ITAL] and [ITAL]IUE[/ITAL] Study of the Dwarf Novae AH Herculis and CM Delphini: Exposed White Dwarfs?

Abstract

We have analyzed Hubble Space Telescope (HST) archival and IUE NEWSIPS archival SWP spectra of the dwarf novae CM Del and AH Her obtained when AAVSO light-curve data indicated the occurrence of dwarf nova quiescence. We have computed synthetic, high-gravity spectra in LTE with solar composition using TLUSTY195 and SYNSPEC42 and have carried out fits of these models to the far-UV continuum and narrow absorption line spectra. For CM Del, we find that the far-UV spectrum is dominated by a hot (Teff = 22,000 K and log g = 8) white dwarf, but reliable information on the metal abundances is unavailable because of the quality of the data. For AH Her, we find evidence for a hot white dwarf with Teff = 27,000 K and log g = 8, with subsolar abundance for Si. An HST spectrum (GHRS G160M) of AH Her reveals blue-asymmetric Si IV profiles. If this is due to wind outflow, we derive a single-scattering, upper limit, mass-loss rate of 10-9 M⊙ yr-1. Synthetic profile fits to 12 co-added HST medium-resolution G160M spectra of the Si IV (1393, 1402 A) resonance doublet of CM Del yield a white dwarf rotational velocity estimate of v sin i = 400 km s-1 and Si/H = 1 times solar. The value of v sin i for CM Del is discussed in the context of angular momentum transfer and rotation rates for other cataclysmic variable, primary degenerate stars.