Investigating the structure of the accretion disc in WZ Sge from multiwaveband time-resolved spectroscopic observations — I

Abstract

We present the first of two papers describing an in-depth study of multiwaveband phaseresolved spectroscopy of the unusual dwarf nova WZ Sge. In this paper we present an extensive set of Doppler maps of WZ Sge covering optical and infrared emission lines, and describe a new technique for studying the accretion discs of cataclysmic variables using ratioed Doppler maps. Applying the ratioed Doppler map technique to our WZ Sge data shows that the radial temperature profile of the disc is unlike that predicted for a steady state a disc. Time-averaged spectra of the accretion disc line flux (with the bright spot contribution removed) show evidence in the shapes of the line profiles for the presence of shear broadening in a quiescent non-turbulent accretion disc. From the positions of the bright spots in the Doppler maps of different lines, we conclude that the bright spot region is elongated along the ballistic stream, and that the density of the outer disc is low. The velocity of the outer edge of the accretion disc measured from the Ha line is found to be 723^23 km s 21 X Assuming that the accretion disc reaches to the 3:1 tidal resonance radius, we derive a value for the primary star mass of 0.82 M ( . We discuss the implications of our results on the present theories of WZ Sge type dwarf nova outbursts.

Discovery of a cataclysmic variable with a sub-stellar companion

“…Note the striking similarity in the structure of the ratio map of 1RXS J105010.3-140431 to that of WZ Sge shown in Fig. 6 of Skidmore et al (2000).…”

“…In quiescent dwarf novae the disk radial temperature profile is usually observed to depart from the r −3/4 law and to have a much flatter profile (Wood et al 1992;Skidmore et al 2000). The departure from T ∼ r …”

WZ Sge-type dwarf novae

“…Good examples are seen in WZ Sge and AL Com ), V455 And (Araujo-Betancor et al 2005aKato et al 2009a), V386 Ser (Mukadam et al 2010, not included in table 6 since no outburst has been recorded), EZ Lyn Zharikov et al 2013), BW Scl Kato et al 2013a). The classically accepted interpretation of this phenomenon is a result of a semi-transparent accretion disk (due to the low mass-transfer rate) which allows the light from the hot spot to escape in two directions (Skidmore et al 2000). Zharikov et al (2008) suggested an interpretation that the disk can reach the 2:1 resonance to produce these double-wave modulations, although Zharikov et al (2006) initially considered that in quiescence there is no chance that the disc extends as far as the 2:1 resonance.…”

Short Orbital Period Cataclysmic Variables

“…The main characteristics of the prototype of the subclass (WZ Sge) are summarized below: the short orbital period of 81.6min, close to the predicted period minimum; the spectra in quiescence show the strong double-peaked Balmer emission lines from the accretion disk surrounded by broad absorptions, formed by the primary white dwarf (see for an example [25]); infrequent ∼20-30 years and the large-amplitude (∼ 8 m ; 1913,1946,1978,2001) superoutbursts succeeded by echo outbursts, there are not normal outbursts; optical light curves during "s" -during super-outbursts; "q" -during quiescence; "-" absent of double-humps in LCs. Recent bounce-back candidates: OT J075418.7+381225, OT J230425.8+062546 [35], SSS J122221.7-311523 [36,37] a super-outburst show long-lasting super-humps [26]; there are double-humped in the light curves during super-outbursts and in quiescence [27,26]; there are evidence of forming of spiral arms in the disk during super-outbursts [28,29]; the accretion disc is asymmetric in quiescence, and the bright spot region is to be extended along the mass transfer stream [30,31]; the outer layers of the accretion disc is a low density and a low temperature of ∼3000K [32]; there is some evidence that a cavity was formed in the inner part of the disk during quiescence implying an annulus-shaped accretion disc [33]. The number of WZ Sge-type stars increases with decreasing orbital periods and they seem to be a continuation of the evolution of SU UMa-type stars when mass transfer rates begin to drop closely to the period minimum (Fig.…”