Optical and Near-Infrared Photometry of Nova V2362 Cyg: Rebrightening Event and Dust Formation

Abstract

Abstract We present optical and near-infrared (NIR) photometry of a classical nova, V2362 Cyg ($=$ Nova Cygni 2006). V2362 Cyg experienced a peculiar rebrightening during the period between 100 and 240 d after the maximum of the nova. Our multicolor observation indicates the emergence of a pseudophotosphere with an effective temperature of $\sim\ $9000 K at the rebrightening maximum. After the rebrightening maximum, the object showed a slow fading homogeneously in all of the used bands for one week. This implies that the fading soon after the rebrightening maximum ($\lesssim\ $1 week) was caused by a slowly shrinking pseudophotosphere. Then, the NIR flux drastically increased, while the optical flux steeply declined. The optical and NIR fluxes were consistent with blackbody radiation with a temperature of $\sim\ $1500 K during this NIR rising phase. These facts are likely to be explained by dust formation in the nova ejecta. Assuming an optically thin case, we estimated the dust mass to be 10$^{-10}$–10$^{-8}\ M_{\odot}$, which is less than those in typical dust-forming novae. These results support the senario that a second, long-lasting outflow which caused the rebrightening interacted with a fraction of the initial outflow and formed dust grains.