Discovery of Negative Superhumps during a Superoutburst of January 2011 in ER Ursae Majoris

TL;DR

This paper reports the first observation of negative superhumps during a superoutburst in ER UMa, indicating retrograde disk precession and suggesting a disk tilt as a cause for unusual outburst properties.

Contribution

It provides the first evidence of negative superhumps during a superoutburst in ER UMa, revealing retrograde precession and proposing disk tilt as a key factor.

Findings

Detection of negative superhumps with a period of 0.062242 days

Superoutburst duration was shorter than usual

Normal outburst intervals were longer than typical

Abstract

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.