  We have obtained observations of the ultraviolet spectrum of AM CVn, an
ultra-short-period helium cataclysmic variable, using the Space Telescope
Imaging Spectrograph (STIS) aboard the Hubble Space Telescope (HST). We
obtained data in time-tag mode during two consecutive orbits of HST, covering
1600-3150 and 1140-1710 Angstrom, respectively. The mean spectrum is
approximately flat in f-nu. The absorption profiles of the strong lines of N V,
Si IV, C IV, He II, and N IV are blue-shifted and in some cases asymmetric,
evidencing a wind that is partly occulted by the accretion disk. There is weak
red-shifted emission from N V and He II. The profiles of these lines vary
mildly with time. The light curve shows a decline of ~20% over the span of the
observations. There is also flickering and a 27 s (or 54 s) "dwarf nova
oscillation", revealed in a power-spectrum analysis. The amplitude of this
oscillation is larger at shorter wavelengths. We assemble and illustrate the
spectral energy distribution (s.e.d.) of AM CVn from the ultraviolet to the
near-infrared. Modeling the accretion phenomenon in this binary system can in
principle lead to a robust estimate of the mass accretion rate on to the
central white dwarf, which is of great interest in characterizing the
evolutionary history of the binary system. Inferences about the mass accretion
rate depend strongly on the local radiative properties of the disk, as we
illustrate. Uncertainty in the distance of AM CVn and other parameters of the
binary system presently limit the ability to confidently infer the mass
accretion rate.
