5 research outputs found
Discovery of Negative Superhumps during a Superoutburst of January 2011 in ER Ursae Majoris
We report on a discovery of "negative" superhumps during the 2011 January
superoutburst of ER UMa. During the superoutburst which started on 2011 January
16, we detected negative superhumps having a period of 0.062242(9) d, shorter
than the orbital period by 2.2%. No evidence of positive superhumps was
detected during this observation. This finding indicates that the disk
exhibited retrograde precession during this superoutburst, contrary to all
other known cases of superoutbursts. The duration of this superoutburst was
shorter than those of ordinary superoutbursts and the intervals of normal
outbursts were longer than ordinary ones. We suggest a possibility that such
unusual outburst properties are likely a result of the disk tilt, which is
supposed to be a cause of negative superhumps: the tilted disk could prevent
the disk from being filled with materials in the outmost region which is
supposed to be responsible for long-duration superoutbursts in ER UMa-type
dwarf novae. The discovery signifies the importance of the classical prograde
precession in sustaining long-duration superoutbursts. Furthermore, the
presence of pronounced negative superhumps in this system with a high
mass-transfer rate favors the hypothesis that hydrodynamical lift is the cause
of the disk tilt.Comment: 8 pages, 3 figures, Accepted for publication in PASJ Lette
Superoutburst of WZSge-type dwarf nova below the period minimum: ASASSN-15po
arXiv:1610.04941v1We report on a superoutburst of a WZ Sge-type dwarf nova (DN), ASASSN-15po. The light curve showed the main superoutburst and multiple rebrightenings. In this outburst, we observed early superhumps and growing (stage A) superhumps with periods of 0.050454(2) and 0.051809(13) d, respectively. We estimated that the mass ratio of secondary to primary (q) is 0.0699(8) by using P and a superhump period PSH of stage A. ASASSN-15po [P ∼ 72.6 min] is the first DN with an orbital period between 67-76 min. Although the theoretical predicted period minimum P of hydrogen-rich cataclysmic variables (CVs) is about 65-70 min, the observational cut-off of the orbital period distribution at 80 min implies that the period minimum is about 82min, and the value is widely accepted. We suggest the following four possibilities: the object is (1) a theoretical period minimum object, (2) a binary with a evolved secondary, (3) a binary with a metal-poor (Popullation II) seconday, or (4) a binary which was born with a brown-dwarf donor below the period minimum.This work was supported by a Grant-in-Aid “Initiative for High- Dimensional Data-Driven Science through Deepening of Sparse Modeling” from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan (25120007). It was also partially supported by grants RFBR No. 15-02-06178, RFBR No. 14-02-0082 (S.S.), VEGA No. 2/0002/13 (S.S.) and RSF No. 14-12-00146 (P.G., for processing observation data from Slovak observatory).Peer Reviewe