Speaker
Description
Accretion discs experience various kinds of instabilities, observed as energetic events in our Universe. Compact binaries especially offer a unique opportunity to study these instabilities as transient phenomena. Here we report optical studies of the cataclysmic variable CM Mic, characterized by highly frequent dwarf nova outbursts driven by thermal-viscous instability and occasional standstills in between. Our observations newly established its orbital period and mass ratio as 0.077059(3) d and 0.17(1), respectively. These values agree with standard binary evolution models of cataclysmic variables. Nevertheless, its short outburst cycles imply that its mass-transfer rate must be an order of magnitude higher than those of systems with similar orbital periods. We further find that CM Mic shows positive superhumps, driven by tidal instability, during both its dwarf nova outbursts and standstills. This makes CM Mic a unique laboratory for directly comparing superhump properties in outburst and standstill. The alternation between the standstill and dwarf nova states in CM Mic implies a change in its mass-transfer rate by a factor of 1.05--1.4 near the critical value. Overall, these findings establish CM Mic as an important laboratory for studying a tidally unstable disc around the critical mass-transfer rate.