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Post Info TOPIC: WZ Sge-type dwarf nova


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Posts: 131433
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WZ Sge-Type Dwarf Novae
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Title: WZ Sge-Type Dwarf Novae
Author: Taichi Kato (Kyoto U.)

We have summarised the current understanding and recently obtained findings about WZ Sge-type dwarf novae. We also reviewed the historical development of the understanding of these objects, provided the modern criteria, and reviewed the past research in relation to superhumps, early superhumps and the outburst mechanism. We regard that the presence of early superhumps (reflecting the 2:1 resonance) and long or multiple rebrightenings are the best distinguishing properties of WZ Sge-type dwarf novae. We provided the updated list of nearly 100 WZ Sge-type dwarf novae mainly based on the data obtained by the VSNET Collaboration up to Kato et al. (2015, arXiv/1507.05610) and discussed the statistics. We could detect early superhumps with amplitude larger than 0.02 mag in 63% of the studied WZ Sge-type dwarf novae, which makes early superhumps a useful distinguishing feature for WZ Sge-type dwarf novae. Theoretical light curves of early superhumps generally appear to reproduce the existence of many low-amplitude objects, supporting the geometrical origin of early superhumps. Using the recently developed method of measuring mass ratios using developing phase of superhumps (stage A superhumps), we showed that there is a linear relation between the period variation of superhumps and the mass ratio in WZ Sge-type objects. By using this relation, we were able to draw an evolutionary picture of a large number of WZ Sge-type and identified the type of outburst to be an evolutionary sequence: type D->C->A->B->E, with some outliers for type-B objects. The duration of stage A (evolutionary phase) of superhumps is also well correlated with the estimated mass ratios. By using mass ratios from stage A superhumps and durarion of stage A, we have been able to identify best candidates for the period bouncer.

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RE: WZ Sge-type dwarf nova
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Title: Late Superhumps in WZ Sge-Type Dwarf Novae
Authors: Taichi Kato, Hiroyuki Maehara (Kyoto University), Berto Monard (Bronberg Observatory)

We report on the detection of very stable modulations with periods unexpectedly (~0.5%) longer than superhump periods during the slowly fading stage of WZ Sge-type superoutbursts in three systems, GW Lib, V455 And and WZ Sge. These periods are naturally explained by assuming that these modulations are superhumps arising from matter near the tidal truncation radius. This finding provides an additional support to the hypothetical idea of expansion of the accretion disk well beyond the 3:1 orbital resonance in some low mass-ratio systems. Combined with the effect of 2:1 resonance, we present an explanation of the origin of positive period derivatives in certain short-period SU UMa-type dwarf novae.

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Posts: 131433
Date:
SDSS J102146.44+234926.3
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Title: Discovery of a WZ Sge-Type Dwarf Nova, SDSS J102146.44+234926.3: Unprecedented Infrared Activity during a Rebrightening Phase
Authors: M. Uemura, A. Arai, T. Krajci, E. Pavlenko, S. Yu. Shugarov, N. A. Katysheva, V. P. Goranskij, H. Maehara, A. Imada, T. Kato, D. Nogami, K. Nakajima, T. Ohsugi, T. Yama****a, K. S. Kawabata, O. Nagae, S. Chiyonobu, Y. Fukazawa, T. Mizuno, H. Katagiri, H. Takahashi, A. Ueda, T. Hayashi, K. Okita, M. Yoshida, K. Yanagisawa, S. Sato, M. Kino, K. Sadakane

Several SU UMa-type dwarf novae, in particular, WZ Sge-type stars tend to exhibit rebrightenings after superoutbursts. The rebrightening phenomenon is problematic for the disk instability theory of dwarf novae since it requires a large amount of remnant matter in the disk even after superoutbursts. Here, we report our optical and infrared observations during the first-ever outburst of a new dwarf nova, SDSS J102146.44+234926.3. During the outburst, we detected superhumps with a period of 0.056281 0.000015 d, which is typical for superhump periods in WZ Sge stars. In conjunction with the appearance of a long-lived rebrightening, we conclude that the object is a new member of WZ Sge stars. Our observations, furthermore, revealed infrared behaviours for the first time in the rebrightening phase of WZ Sge stars. We discovered prominent infrared superhumps. We calculate the colour temperature of the infrared superhump source to be 4600-6400 K. These temperatures are too low to be explained with a fully-ionised disk appearing during dwarf nova outbursts. We also found a Ks-band excess over the hot disk component. These unprecedented infrared activities provide evidence for the presence of mass reservoir at the outermost part of the accretion disk. We propose that a moderately high mass-accretion rate at this infrared active region leads to the long-lived rebrightening observed in SDSS J102146.44+234926.3.

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Posts: 131433
Date:
WZ Sge-type dwarf nova
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Title: SDSS J102146.44+234926.3: New WZ Sge-type dwarf nova
Authors: Alex Golovin, Ayani Kazuya, Pavlenko Elena P., Krajci Tom, Kuznyetsova Yuliana, Henden Arne, Krushevska Victoria, Dvorak Shawn, Sokolovsky Kirill, Sergeeva Tatyana P., James Robert, Crawford Tim, Corp Laurent
(version v2)

We report CCD photometry and spectroscopy during 2006 outburst of the dwarf nova SDSS J102146.44+234926.3 (SDSS J1021). The photographic plates from the MAO, SAI and CrAO plate archives, which cover the position of the SDSS J1021, were inspected for the presence of previous outbursts. We also present the BVRcIc photometric calibration of 52 stars in SDSS J1021 vicinity, which have V-magnitude in the range of 11.21-17.23m and can serve as comparison stars. The large amplitude of the SDSS J1021 outburst of 7m, superhumps with a period below the ''period gap'', rebrightening during the declining stage of superoutburst, rarity of outbursts and obtained spectrum allow to classify this object as a WZ Sge type dwarf nova.

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