A phenomenological study of the evolution of shock-induced O i emission lines in the spectrum of nova V2891 Cygni

Abstract

ABSTRACT The eruption of Nova V2891 Cygni in 2019 offers a rare opportunity to explore the shock-induced processes in novae ejecta. The spectral evolution shows noticeable differences in the evolution of various oxygen emission lines such as O i 7773 Å, O i 8446 Å, O i 1.1286 μm, O i 1.3164 μm, etc. Here, we use spectral synthesis code cloudy to study the temporal evolution of these oxygen emission lines. Our photoionization model requires the introduction of a component with a very high density ($n \sim 10^{11}$ cm$^{-3}$) and an enhanced oxygen abundance (O/H $\sim$ 28) to produce the O i 7773 Å emission line, suggesting a stratification of material with high oxygen abundance within the ejecta. An important outcome is the behaviour of the O i 1.3164 μm line, which could only be generated by invoking the collisional ionization models in cloudy. Our phenomenological analysis suggests that O i 1.3164 μm emission originates from a thin, dense shell characterized by a high density of about $10^{12.5}\!-\!10^{12.8}$ cm$^{-3}$, which is most likely formed due to the strong internal collisions. If such is the case, the O i 1.3164 μm emission presents itself as a tracer of shock-induced dust formation in V2891 Cyg. The collisional ionization models have also been successful in creating the high-temperature conditions ($\sim 7.07\!-\!7.49 \times 10^5$ K) required to reproduce the observed high ionization potential coronal lines, which coincide with the epoch of dust formation and evolution of the O i 1.3164 μm emission line.