Theoretical Light-curve Models of the Symbiotic Nova CN Cha-Optical Flat Peak for 3 Yr

CN Cha is a slow symbiotic nova characterized by a 3 yr long optical flat peak followed by a rapid decline. We present theoretical light curves for CN Cha, based on hydrostatic approximation, and estimate the white-dwarf (WD) mass to be ~0.6 M _⊙ for a low metal abundance of Z = 0.004. These kinds of flat-peak novae are border objects between classical novae having a sharp optical peak and extremely slow novae, the evolutions of which are too slow to be recognized as nova outbursts on a human timescale. Theoretically, there are two types of nova envelope solutions-static and optically thick wind-in low-mass WDs (≲0.7 M _⊙). Such a nova outburst begins first in a hydrostatic manner, and later it could change to an optically thick wind evolution, due to perturbation by the companion star in the nova envelope. Multiple peaks are a reflection of the relaxation process of the transition. CN Cha supports our explanation of the difference between long-lasting flat-peak novae like CN Cha and multiple-peak novae like V723 Cas, because the companion star is located far outside, and does not perturb, the nova envelope in CN Cha.