What We Learn from Quantitative Ultraviolet Spectroscopy of Naked White D warfs in Cataclysmic Variables

Using the Hopkins Ultraviolet Telescope and Hubble Space Telescope, observers have now obtained UV spectra with sufficient signal to noise and resolution to allow quantitative spectroscopic analyses of the WDs in several DNe. In the ``cleanest'' DNe, such as U Gem, the observations are allowing the basic physical parameters of the WD -- temperature, radius, gravity, rotation rate, and surface abundances -- to be established. A second component also exists in these systems, which may either be the disk or may be related to the WD itself. Here I summarize the current state of the observations and our understanding of the data, highlighting some of the uncertainties in the analyses as well the prospects for fundamentally advancing our understanding of DNe and WDs with future observations.Comment: 6 pages, 8 figures, to be published in the proc. for Cataclysmic Variables: A 60th Birthday Symposium in Honour of Brian Warne

The Geometry and Ionization Structure of the Wind in the Eclipsing Nova-like Variables RW Tri and UX UMa

The UV spectra of nova-like variables are dominated by emission from the accretion disk, modified by scattering in a wind emanating from the disk. Here we model the spectra of RW Tri and UX UMa, the only two eclipsing nova-likes which have been observed with the Hubble Space Telescope in the far-ultraviolet, in an attempt to constrain the geometry and the ionization structure of their winds. Using our Monte Carlo radiative transfer code we computed spectra for simply-parameterized axisymmetric biconical outflow models and were able to find plausible models for both systems. These reproduce the primary UV resonance lines - N V, Si IV, and C IV - in the observed spectra in and out of eclipse. The distribution of these ions in the wind models is similar in both cases as is the extent of the primary scattering regions in which these lines are formed. The inferred mass loss rates are 6% to 8% of the mass accretion rates for the systems. We discuss the implication of our point models for our understanding of accretion disk winds in cataclysmic variables.Comment: 13 pages, 15 figures and 4 tables. Published in Ap

The Impact of Accretion Disk Winds on the Optical Spectra of Cataclysmic Variables

Many high-state non-magnetic cataclysmic variables (CVs) exhibit blue-shifted absorption or P-Cygni profiles associated with ultraviolet (UV) resonance lines. These features imply the existence of powerful accretion disk winds in CVs. Here, we use our Monte Carlo ionization and radiative transfer code to investigate whether disk wind models that produce realistic UV line profiles are also likely to generate observationally significant recombination line and continuum emission in the optical waveband. We also test whether outflows may be responsible for the single-peaked emission line profiles often seen in high-state CVs and for the weakness of the Balmer absorption edge (relative to simple models of optically thick accretion disks). We find that a standard disk wind model that is successful in reproducing the UV spectra of CVs also leaves a noticeable imprint on the optical spectrum, particularly for systems viewed at high inclination. The strongest optical wind-formed recombination lines are H$\alpha$ and He II $\lambda4686$. We demonstrate that a higher-density outflow model produces all the expected H and He lines and produces a recombination continuum that can fill in the Balmer jump at high inclinations. This model displays reasonable verisimilitude with the optical spectrum of RW Trianguli. No single-peaked emission is seen, although we observe a narrowing of the double-peaked emission lines from the base of the wind. Finally, we show that even denser models can produce a single-peaked H$\alpha$ line. On the basis of our results, we suggest that winds can modify, and perhaps even dominate, the line and continuum emission from CVs.Comment: 15 pages, 13 figures. Accepted to MNRA

The Compact UV Nucleus of M33

The most luminous X-ray source in the Local Group is associated with the nucleus of M33. This source, M33 X-8, appears modulated by ~20% over a ~106 day period, making it unlikely that the combined emission from unresolved sources could explain the otherwise persistent ~1e39 erg/s X-ray flux (Dubus et al. 1997, Hernquist et al. 1991). We present here high resolution UV imaging of the nucleus with the Planetary Camera of the HST undertaken in order to search for the counterpart to X-8. The nucleus is bluer and more compact than at longer wavelength images but it is still extended with half of its 3e38 erg/s UV luminosity coming from the inner 0.14". We cannot distinguish between a concentrated blue population and emission from a single object.Comment: 3 figures, accepted for publication in ApJ Letter