Multiphase State Transitions and Thermal−Tidal Decoupling in the AM CVn−Type Star V803 Cen

Abstract AM CVn systems are ultracompact, short-period binaries emitting multimessenger signals and showing both normal outbursts and superoutbursts. They serve as ideal laboratories for studying accretion physics under extreme conditions and for testing the disk instability model (DIM) and thermal−tidal instability (TTI) framework. Here we report the detection of 10 new minor outbursts for the AM CVn−type system V803 Cen by TESS. Positive superhumps (PSHs) are discovered across multiple evolutionary phases, providing direct evidence of decoupled tidal and thermal instabilities. Notably, transient PSHs appear during the initial rise of normal outbursts and fade as the outburst progresses—contrary to predictions from both the DIM (which expects the disk radius to increase during rise) and the TTI (which predicts that PSHs should not disappear once the disk has surpassed the 3:1 resonance radius and continues to expand). During the high state, we reveal multiphase transitions through a superoutburst, dip, cycling state (∼13.5–14.5 mag), and stable standstill (∼13.5 mag). This stable standstill is found for the first time, suggesting that the accretion disk variability is more complex than simple oscillations during the standstill-like states, offering new insights into the physics of disk evolution. While superoutburst and supercycle durations remain stable, high-state durations range from tens to hundreds of days, suggesting additional disk replenishment mechanisms beyond supercycle accumulation. We suggest enhanced mass transfer due to variable irradiation of the secondary as the primary driver. The complex variations in V803 Cen likely arise from coupled processes including DIM, TTI in low mass ratio systems, modulated mass transfer fluctuations, and irradiation-enhanced mass transfer.