Spiral Structure in WZ Sagittae around the 2001 Outburst Maximum

Abstract

Intermediate resolution phase-resolved spectra of WZ Sge were obtained on five consecutive nights (July 23 -27) covering the initial stage of the 2001 superoutburst. Double-peaked emission lines of HeII at 4686 A, which were absent on July 23, emerged on July 24 together with emission lines of CIII / NIII Bowen blend. Analyses of the HeII emission lines using the Doppler tomography revealed an asymmetric spiral structure on the accretion disk. This finding demonstrates that spiral shocks with a very short orbital period can arise during the initial stage of an outburst and may be present in all SU UMa stars.

First detection of the growing humps at the rapidly rising stage of dwarf novae AL Com and WZ Sge

“…This is clearly L44 R. Ishioka et al: First detection of the growing humps contrary to our observation. This is also supported by Doppler tomography of the earliest stage of the superoutburst (Baba et al 2001b;Kuulkers et al 2002). The strongest emission on velocity maps reside on the opposite side of the expected hot spot, excluding an increased mass-transfer as the origin of the strong emissions.…”

“…On July 23, just after the outburst onset, He ii λ4686 shows no emission componemt, but on July 24, the line was observed as a strong emission line. The Doppler maps of He ii λ4686 constructed using time-resolved spectra on July 24 and July 28 show that the accretion-disk emission is dominated by two spiral arms (Ishioka et al 2001;Steeghs et al 2001;Baba et al 2001a;Baba et al 2001c;Kuulkers et al 2002). This result indicates that some relation exists between the growth of He ii emission line and the spiral structure, and the observed growth of early superhumps.…”

Is evidence for enhanced mass transfer during dwarf-nova outbursts well substantiated?

“…During the early stage of the 2001 outburst of WZ Sge, a twoarmed arch-like pattern was observed in the Doppler maps of emission lines of He II 4686 Å and C III/N III on the second day of the outburst, July 24, (Baba et al 2002) and on July 28 (Steeghs et al 2001a). Kato (2002) has proposed that this and the early hump phenomenon can be understood in terms of irradiation of the disk by the central radiation source.…”

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.…”