OAO/MITSuME Photometry of Dwarf Novae. II. HV Virginis and OT J012059.6+325545

TL;DR

This study presents multicolor photometry of two WZ Sge-type dwarf novae during superoutbursts, revealing wavelength-dependent amplitudes of early superhumps and insights into the geometry and physical mechanisms of the accretion disk.

Contribution

It provides detailed observations of early and ordinary superhumps, demonstrating wavelength dependence and linking color variations to physical processes in the accretion disk.

Findings

Early superhump brightness minima align with bluest peaks in color.

Amplitude of early superhumps increases with wavelength, indicating a vertically extended outer disk origin.

Superhump color variations suggest pressure effects dominate during stage B.

Abstract

We report on multicolor photometry of WZ Sge-type dwarf novae, HV Vir and OT J012059.6+325545 during superoutbursts. These systems show early superhumps with the mean periods of 0.057093(45) d for HV Vir and 0.057147(15) d for OT J012059.6+325545, respectively. The observed early superhumps showed a common feature that the brightness minima correspond to the bluest peaks in color variations, which may be a ubiquitous phenomenon among early superhumps of WZ Sge-type dwarf novae. We confirmed that amplitudes of early superhumps depend on wavelength: amplitudes with longer bandpass filters show larger values. This indicates that the light source of early superhumps is generated at the outer region of the vertically-extended accretion disk. On the other hand, amplitudes of ordinary superhumps are likely to be independent of wavelength. This implies that the superhump light source is geometrically thin. We also examined color variations of ordinary superhumps and found that the bluest peaks in $g'-I_{\rm c}$ tend to coincide with the brightness minima, particularily in stage B superhumps. This may reflect that the pressure effect plays a dominant role during stage B superhumps.