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

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.

Explaining Prolonged Fluctuations in Light Curves of Classical Novae via Modeling

“…First detected during the rise in July 2020 (Sokolovsky et al 2020a), followup observations detecting oscillations (Sokolovsky et al 2020b,c) and then extensively observed over roughly the following six months (Dubovský et al 2021). During this time the nova produced multiple peaks with amplitudes of about 2-3 magnitudes that lasted a few days each (Dubovský et al 2021;Fujii et al 2021). Schmidt (2021) have calculated a remarkably accurate periodicity of 3.8036 ± 0.0005 hours over ∼ 100 days of observation.…”