ASASSN-24hd; a dwarf nova bridging WZ Sge-type and SU UMa-type superoutbursts

Abstract

WZ Sge-type dwarf novae (DNe) form a subclass in cataclysmic variables, characterized by short-period variations called superhumps during an outburst. Here we present optical ground-based and TESS observations of ASASSN-24hd in its 2024-2025 outburst. ASASSN-24hd is the first reported WZ Sge-type DN outburst fully covered by TESS, providing a great opportunity to study the evolution of superhumps. Our observations establish its early and stage-A ordinary superhumps as 0.05711(4) and 0.05919(5) d, respectively, resulting in its mass ratio of 0.098(4). The TESS observations confirm that the evolution of its superhump period, amplitude, and profile after the appearance of ordinary superhumps is generally consistent with those of SU UMa-type DNe observed with Kepler and TESS. Furthermore, we find that ASASSN-24hd in outburst shares a great similarity to the 2010 superoutburst of an SU UMa-type DN V585 Lyr, observed by Kepler, particularly including the superhump evolution and the long waiting time ($\gtrsimeq$ 5 d) before the stage A--B transition of ordinary superhumps. The shorter superoutburst cycles and smaller outburst amplitude in V585 Lyr than those of ASASSN-24hd disfavor the interpretation that V585 Lyr is, in fact, a face-on WZ Sge-type DN where early superhumps are undetectable. Instead, one possibility of their critical differences is either low quiescence viscosity or inner disk truncation, which has been invoked to explain the extreme nature of WZ Sge-type DNe, but future observations in quiescence are vital to conclude. These findings emphasize the borderline between SU UMa-type and WZ Sge-type DNe.