Christian Knigge

A Self-occulting Accretion Disk in the SW Sextantis Star DW Ursae Majoris*

We present the ultraviolet spectrum of the SW Sextantis star and nova-like variable DW Ursae Majoris in an optical low state, as observed with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope (HST). The data are well described by a synthetic white dwarf (WD) spectrum with Teff = 46,000 ± 1000 K, log g = 7.60 ± 0.15, v sin i = 370 ± 100 km s-1, and Z/Z☉ = 0.47 ± 0.15. For this combination of Teff and log g, WD models predict MWD = 0.48 ± 0.06 M☉ and RWD = (1.27 ± 0.18) × 109 cm. Combining the radius estimate with the normalization of the spectral fit, we obtain a distance estimate of d = 830 ± 150 pc. During our observations, DW UMa was approximately 3 mag fainter in V than in the high state. A comparison of our low-state HST spectrum with a high-state spectrum obtained with the International Ultraviolet Explorer shows that the former is much bluer and has a higher continuum level shortward of 1450 A. Since DW UMa is an eclipsing system, this suggests that an optically thick accretion disk rim blocks our view of the WD primary in the high state. If self-occulting accretion disks are common among the SW Sex stars, we can account for (1) the preference for high-inclination systems within the class and (2) their V-shaped continuum eclipses. Moreover, even though the emission lines produced by a self-obscured disk are generally still double-peaked, they are weaker and narrower than those produced by an unobscured disk. This may allow a secondary line emission mechanism to dominate and produce the single-peaked, optical lines that are a distinguishing characteristic of the SW Sex stars.

Unveiling the Core of the Globular Cluster M15 in the Ultraviolet

We have obtained deep FUV and NUV images of the inner region of the dense globular cluster M15 with the HST ACS. The FUV - NUV color-magnitude diagram shows a well-defined track of horizontal branch stars, as well as a trail of blue stragglers and white dwarfs. The main-sequence turnoff is clearly visible at FUV 23.5 mag and FUV - NUV 3 mag, and the main-sequence stars form a prominent track that extends at least 2 mag below the main-sequence turnoff. As such, this is the deepest FUV - NUV color-magnitude diagram of a globular cluster presented so far. Cataclysmic variable and blue straggler candidates are the most centrally concentrated stellar populations, which might either be an effect of mass segregation or reflect the preferred birthplace in the dense cluster core of such dynamically formed objects. We find 41 FUV sources that exhibit significant variability. We classify the variables based on an analysis of their UV colors and variability properties. We find four previously known RR Lyrae and 13 further RR Lyrae candidates, one known Cepheid and six further candidates, six cataclysmic variable candidates, one known and one probable SX Phoenicis star, and the well-known low-mass X-ray binary AC 211. Our analysis represents the first detection of SX Phoenicis pulsations in the FUV. We find that Cepheids, RR Lyrae stars, and SX Phoenicis exhibit massive variability amplitudes in this wave band (several magnitudes).

The effective temperature distribution of steady‐state, mass‐losing accretion discs

Mass loss appears to be a common phenomenon among disk-accreting astrophysical systems. An outflow emanating from an accretion disk can act as a sink for mass, angular momentum and energy and can therefore alter the dissipation rates and effective temperatures across the disk. Here, the radial distributions of dissipation rate and effective temperature across a Keplerian, steady-state, mass-losing accretion disk are derived, using a simple, parametric approach that is sufficiently general to be applicable to many types of dynamical disk wind models. Effective temperature distributions for mass-losing accretion disks in cataclysmic variables are shown explicitly, with parameters chosen to describe both radiation-driven and centrifugally-driven outflows. For realistic wind mass-loss rates of a few percent, only centrifugally-driven outflows -- particularly those in which mass loss is concentrated in the inner disk -- are likely to alter the disks effective temperature distribution significantly. Accretion disks that drive such outflows could produce spectra and eclipse light curves that are noticeably different from those produced by standard, conservative disks.

A far-ultraviolet variability survey of the globular cluster M 80: A far-ultraviolet variability survey of M 80

We have searched for variable sources in the core region of M80, using far ultra-violet data taken with the Advanced Camera for Surveys on board the Hubble Space Telescope. We found three sources that exhibit strong signs of variability in our data. Among these is source TDK1, which we believe to be an RR Lyrae star that reached maximum brightness during our observations. The light curve shows a >3 mag FUV brightening over the course of ~5 hours, with an estimated peak brightness of ~16.7 mag, followed by a decrease to ~20 mag. Archival optical data obtained with WFPC2 confirm that TDK1 is variable in all wavebands. TDK1s SED is reasonably fit by a star with temperature T(eff)=6700K and radius R=4.2R(sun), consistent with the suggestion that it is an RR Lyrae. Based on the photometric and variability characteristics of the other two variables, we suggest that TDK2 is likely to be an SX Phoenicis star with ~55 minutes period, and TDK3 is likely another RR Lyrae. Finally, we briefly discuss the FUV counterparts to two previously known variables in M80, the classical nova T Sco and the dwarf nova DN1.

Eclipse Mapping of the Accretion Disk Wind in the Cataclysmic Variable UX UMa

We present the results of an effort to model recent HST eclipse observations of the wind-formed C, ɪv 1550A resonance line in the high-inclination nova-like variable UX UMa (Mason et al. 1995, Baptista et al. 1995). Within the framework of a simple kinematic model, in which the outflow is described as a rotating, biconical accretion disk wind (Knigge et al. 1995), we are able to reproduce not only the shapes and strengths of the observed line profiles both away from and during eclipse, but also most of the detailed behaviour of different parts of the line (blue wing, line centre and red wing) as a function of orbital phase (Figure 1). The most important result of our modeling is that it strongly suggests the presence of a vertically extended ( H ~ few × 10 R WD ), relatively dense ( n e ~ 10 13 cm −3 ) and only slowly-outflowing ( υ poloidal υ escape ) transition region between the disk photosphere and the fast-moving wind. Thus the best current data already demand that the simplified picture of accretion disks and winds as quasi-independent physical phenomena should be replaced by a unified theory of accretion disk atmospheres. It may be important in this context that, barring strong contamination of the observed C ɪv wind line by an underlying disk-formed component, the evidence for rotation in the outflow is unambiguous: the observed line flux light curves clearly exhibit a rotational disturbance that is well matched by our simple model of a rotating disk wind.