A demarcation criterion for hydrogen burning of millinovae

Abstract

Millinovae are a new class of transient supersoft X-ray sources with no clear signature of mass ejection. They show similar triangle shapes of $V/I$ band light curves with thousand times fainter peaks than typical classical novae. Maccarone et al. regarded the prototype millinova, ASASSN-16oh, as a dwarf nova and interpreted the supersoft X-rays to originate from an accretion belt on a white dwarf (WD). Kato et al. proposed a nova model induced by a high-rate mass-accretion during a dwarf nova outburst; the X-rays originate from the photosphere of a hydrogen-burning hot WD whereas the $V/I$ band photons are from the irradiated accretion disk. Because each peak brightness differs largely from millinova to millinova, we suspect that not all the millinova candidates host a hydrogen burning WD. Based on the light curve analysis of the classical nova KT Eri that has a bright disk, we find that the disk is more than two magnitudes brighter when the disk is irradiated by the hydrogen burning WD than when not irradiated. We present the demarcation criterion for hydrogen burning to be $I_{\rm q} - I_{\rm max} > 2.2$, where $I_q$ and $I_{\rm max}$ are the $I$ magnitudes in quiescence and at maximum light, respectively. Among many candidates, this requirement is satisfied with the two millinovae in which soft X-rays were detected.