Transient Formation of C2 and CN in the Near-maximum Phase of Nova Cas 2020(=V1391 Cas)

Abstract

Abstract Nova Cas 2020 was a second example of a classical nova forming both C2 and CN molecules during its near-maximum phase. The formation C2 and CN is indicative of the carbon-rich (C/O > 1) gas envelope of the nova. Our low-resolution spectroscopic observations in the optical from UT 2020 July 31 to August 19 revealed the appearance and the disappearance of the molecular absorption bands of C2 and CN in Nova Cas 2020. These molecules were present during ∼3 days only. Based on analysis of the C2 Swan band profiles, C2 (and probably CN also) formed at ∼5000 K in the nova envelope (see ∼8000 K for typical novae at their visual brightness maxima). Spectral evolution and the formation conditions of the molecules are similar to those of V2676 Oph, which is the first example of a C2/CN-forming nova. We predict that a late-phase grain formation episode similar to that seen in V2676 Oph will occur in Nova Cas 2020.