IY UMa: A new deeply eclipsing SU UMa-type dwarf nova


On January 13, P. Schmeer detected an outburst of the poorly studied variable star TmzV85 whose discovery as a new variable was reported by K. Takamizawa on 1998 November (vsnet-alert 4027, vsnet-obs 18078). M. Uemura and T. Kato performed a time-series observations at Kyoto on January 18 and confirmed the outburst (vsnet-alert 4035). And the next day, after reduction of the data, they found both clear superhumps and eclipses in the light curve and confirmed TmzV85 belongs to a rare class of deeply eclipsing SU UMa-type dwarf novae (vsnet-alert 4037). A. Henden presented V-band image of the field before outburst at: ftp://ftp.nofs.navy.mil/pub/outgoing/aah/temp/tmzv85v.gif (vsnet-alert 4041, 4044). B. Sumner presented a sequence at: vsnet-alert 4053.

According to IAUC 7353, TmzV85 was given a permanent GCVS name "IY UMa"(vsnet-alert 4099).

Observations of superhumps and eclipses during January superoutburst

(Superhump and eclipses, observations by the VSNET Collaboration team: Makoto Uemura, Taichi Kato, Katsura Matsumoto, Lasse Teist Jensen, Rudolf Novak, Denis Buczynski, Tonny Vanmunster, Marko Moilanen, Arto Oksanen, Timo Kinnunen, Jochen Pietz, Brian Martin, Lew Cook)

The object gradually faded until January 26-27 when it entered a rapid decline stage (vsnet-alert 4077, 4080). The VSNET collaboration team received a number of data sets during this superoutburst (vsnet-alert 4098), and its discovery and the first results during the outburst are summed up in M. Uemura et al. (2000) PASJ, 52, L9. According to M. Uemura et al., the outburst amplitude is about 5.4mag, the superhump and orbital period are 0.07588 day and 0.0739132 day, respectively. The superhump amplitude of 0.5mag in the intermediate stage becomes smaller (0.3mag) at the late stage of the superoutburst. On the other hand, the eclipses become deeper with time; typical depth of the eclipse are about 1.3mag and become deeper to about 1.8mag, which suggests that the brightness of the outer part of the disk gradually fades and/or the disk itself shrinks during the advanced stage. The profile of the eclipse is quite asynmmetric, suggesting the presence of a strongly asymmetric accretion disk. Around the rapid fading phase, J. Patterson reported that there were still periodic waves, but their nature (superhumps? orbital?) is still unknown as they were greatly dissimilar to the waves seen even 1 day earlier. Furthermore, Very deep eclipses (>2 mag deep) occurred throughout, and the light was dominated by a small hot object at the center of the disk. This is likely to be the white dwarf - anyway, it's very small and very hot (>40000 K if it's actually the white dwarf) (vsnet-alert 4083). T. Kato reported, from the data on January 27-28 in rapidly fading phase taken by VSNET collaboration team, that pre-eclipse humps have been greatly enhanced (vsnet-alert 4092). The rapid fading phase was interrupted by the again gradual phase lasting for a few days (vsnet-alert 4093).

April normal outburst

P. Schmeer detected the next outburst (14.1mag) on April 20 (vsnet-alert 4646) which confirmed by M. Uemura and T. Kato at Kyoto (vsnet-alert 4650). The Kyoto observation revealed deep eclipses with a depth of 1.7mag (vsnet-alert 4656, 4658). R. Novak reported rapid decline from the peak, and suggested it is a normal outburst (vsnet-alert 4660).

September - October superoutburst

As reported by T. Kinnunen in [vsnet-alert 5291], IY UMa experienced a bright outburst on September 29. M. Uemura confirmed the outburst and, from the observation at Kyoto on September 29, reported the presence of a possible eclipse and hump (vsnet-campaign 517). In J. Pieatz's observation, an eclipse and a part of hump were clearly detected (vsnet-campaign 520). The supercycle is hence around 270 days (vsnet-campaign 519). The object entered a rapid decline phase from October 10 - 11 (vsnet-campaign-dn 198).

December normal outburst

P. Dubovsky reported the next outburst of IY UMa at 14.6mag on December 7 (vsnet-campaign 576, vsnet-campaign-dn 291). M. Uemura and Kyoto team confirmed the outburst on December 8 and their observation yields four eclipses and no humps (vsnet-campaign 578). The depth of eclipse was about 2.5mag (vsnet-campaign-dn 296). He also reported the mean magnitude on December 7.75 was Rc=14.7 (vsnet-campaign-dn 297) and on December 8.71 was Rc=15.6, which indicates this outburst seems to be a normal one (vsnet-campaign-dn 298).

Light Curve and General Information of This Object