ASASSN-16eg: New candidate of long-period WZ Sge-type dwarf nova

TL;DR

This study reports on the 2016 superoutburst of ASASSN-16eg, a long-period WZ Sge-type dwarf nova, revealing unusual properties like a long orbital period and high mass ratio, challenging existing resonance models.

Contribution

It presents the first detailed photometric analysis of ASASSN-16eg, highlighting its unique long orbital period and high mass ratio among WZ Sge-type dwarf novae, and discusses implications for resonance conditions.

Findings

Orbital period of 0.075478 days, exceptionally long for WZ Sge-type.

Mass ratio estimated at 0.166, unusually high for this class.

Long supercycles suggest low mass-transfer rates.

Abstract

We report on our photometric observations of the 2016 superoutburst of ASASSN-16eg. This object showed a WZ Sge-type superoutburst with prominent early superhumps with a period of 0.075478(8) d and a post-superoutburst rebrightening. During the superoutburst plateau, it showed ordinary superhumps with a period of 0.077880(3) d and a period derivative of 10.6(1.1) $\times$ 10$^{-5}$ in stage B. The orbital period ($P_{\rm orb}$), which is almost identical with the period of early superhumps, is exceptionally long for a WZ Sge-type dwarf nova. The mass ratio ($q$ = $M_2/M_1$) estimated from the period of developing (stage A) superhumps is 0.166(2), which is also very large for a WZ Sge-type dwarf nova. This suggests that the 2:1 resonance can be reached in such high-$q$ systems, contrary to our expectation. Such conditions are considered to be achieved if the mass-transfer rate is much lower than those in typical SU UMa-type dwarf novae that have comparable orbital periods to ASASSN-16eg and a resultant accumulation of a large amount of matter on the disk is realized at the onset of an outburst. We examined other candidates of long-period WZ Sge-type dwarf novae for their supercycles, which are considered to reflect the mass-transfer rate, and found that V1251 Cyg and RZ Leo have longer supercycles than those of other WZ Sge-type dwarf novae. This result indicates that these long-period objects including ASASSN-16eg have a low mass-transfer rate in comparison to other WZ Sge-type dwarf novae.