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Register a new userDwarf Nova Oscillations and Quasi-Periodic Oscillations in Cataclysmic Variables - VIII. VW Hyi in outburst observed with the Southern African Large Telescope
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We analyse four light curves obtained at high time resolution (~ 0.1 s) with the 11-m Southern African Large Telescope, at the ends of two normal outbursts and one superoutburst of the dwarf nova VW Hyi. All of these contain at least some Dwarf Nova Oscillations (DNOs), which, when at their highest amplitudes, are seen in unprecedented detail. In addition to the expected DNOs with periods > 20 s we find a previously unknown modulation at 13.39 s, but none at shorter periods. The various DNOs and their interaction with the longer period Quasi-periodic Oscillations are interpreted in terms of the model of magnetically controlled flow from an accretion disc proposed earlier in this series of papers. Our observations include rare DNOs very late in outburst; we find that the fundamental period does not increase beyond ~ 90 s, which is the same value that the independent ``longer period DNOs converge on.
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We have observed dwarf nova oscillations (DNOs) in OY Car during outburst, down through decline and beyond; its behaviour is similar to what we have previously seen in VW Hyi, making it only the second dwarf nova to have DNOs late in outburst that continue well into quiescence. There are also occasional examples of DNOs in deep quiescence, well away from outburst - they have properties similar to those during outburst, indicating similar physical causes and structures. We discuss the occurrence of DNOs in other dwarf novae and conclude that DNOs during quiescence are more common than often supposed and exhibit properties similar to those seen in outburst.
We examine published observations of dwarf nova oscillations (DNOs) on the rise and decline of outbursts and show that their rates of change are in reasonable agreement with those predicted from the magnetic accretion model. We find evidence for propellering in the late stages of outburst of several dwarf novae, as shown by reductions in EUVE fluxes and from rapid increases of the DNO periods. Reanalysis of DNOs observed in TY PsA, which had particularly large amplitudes, shows that the apparent loss of coherence during late decline is better described as a regular switching between two nearby periods. It is partly this and the rapid deceleration in some systems that make the DNOs harder to detect. We suggest that the 28.95 s periodicity in WZ Sge, which has long been a puzzle, is caused by heated regions in the disc, just beyond the corotation radius, which are a consequence of magnetic coupling between the primary and gas in the accretion disc. This leads to a possible new interpretation of the `longer period DNOs (lpDNOs) commonly observed in dwarf novae and nova-like variables.
We present the first release of a large-scale study of relatively bright (V < 13.5) metal-poor stars observed with the Southern African Large Telescope (SALT), based on high-resolution spectra of 50 stars with a resolving power of R ~ 40,000 and S/N ~ 20 per pixel at 4300 Angstrom. The elemental abundances of C, Sr, Ba, and Eu are reported, as well as several alpha-elements (Mg, Ca, Sc, Ti, V) and iron-peak elements (Mn, Co, Ni, Zn). We find a diverse array of abundance patterns, including several consistent with the signatures of carbon-enhanced metal-poor CEMP-i and CEMP-r stars. We find that 15 of 50 (30%) are carbon enhanced (with [C/Fe] >+ 0.70), and that a large fraction (26 of 50, 52%) are enhanced in r-process elements, among the r-process-enhanced stars, five are strongly enhanced r-II ([Eu/Fe] >+ 1.0) stars (two of which are newly discovered) and 21 are newly discovered moderately enhanced r-I (+0.3 <= [Eu/Fe] <=+ 1..0) stars. There are eight stars in our sample that, on the basis of their abundances and kinematics, are possible members of the metal-weak thick-disk population. We also compare our measured abundances to progenitor-enrichment models, and find that the abundance patterns for the majority of our stars can be attributed to a single (rather than multiple) enrichment event.
We present the results of our spectroscopic study of the dwarf nova SS Cygni, using Roche tomography to map the stellar surface and derive the system parameters. Given that this technique takes into account the inhomogeneous brightness distribution on the surface of the secondary star, our derived parameters are (in principle) the most robust yet found for this system. Furthermore, our surface maps reveal that the secondary star is highly spotted, with strongly asymmetric irradiation on the inner hemisphere. Moreover, by constructing Doppler tomograms of several Balmer emission lines, we find strong asymmetric emission from the irradiated secondary star, and an asymmetric accretion disc that exhibits spiral structures.
We present an analysis of X-ray and ultra-violet data of the dwarf nova VW Hyi that were obtained with XMM-Newton during the quiescent state. The X-ray spectrum indicates the presence of an optically thin plasma in the boundary layer that cools as it settles onto the white dwarf. The plasma has a continuous temperature distribution that is well described by a power-law or a cooling flow model with a maximum temperature of 6-8 keV. We estimate from the X-ray spectrum a boundary layer luminosity of 8*10^30 erg/s, which is only 20 per cent of the disk luminosity. The rate of accretion onto the white dwarf is 5*10^-12 solar masses per year, about half of the rate in the disk. From the high-resolution X-ray spectra, we estimate that the X-ray emitting part of the boundary layer is rotating with a velocity of 540 km/s, which is close the rotation velocity of the white dwarf but significantly smaller than the Keplerian velocity. We detect a 60-s quasi-periodic oscillation of the X-ray flux that is likely due to the rotation of the boundary layer. The X-ray and the ultra-violet flux show strong variability on a time scale of ~1500 s. We find that the variability in the two bands is correlated and that the X-ray fluctuations are delayed by ~100 s. The correlation indicates that the variable ultra-violet flux is emitted near the transition region between the disk and the boundary layer and that accretion rate fluctuations in this region are propagated to the X-ray emitting part of the boundary layer within ~100 s. An orbital modulation of the X-ray flux suggests that the inner accretion disk is tilted with respect to the orbital plane. The elemental abundances in the boundary layer are close to their solar values.
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