Discovery of Negative Superhumps during a Superoutburst of 2011 January 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 its orbital period by 2.2%. No evidence of a positive superhump 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 interval of its normal outbursts was longer than those of ordinary normal outbursts of ER UMa. We suggest the possibility that such unusual outburst properties are likely to be a result of a 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. This 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 supports the hypothesis that hydrodynamical lift is the cause of the disk tilt.