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

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, O I 8446, O I 1. 1286 m, O I 1. 3164 m, 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 ~ 10^11 cm^-3) and an enhanced oxygen abundance (O/H 28) to produce the O I 7773 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 m line, which could only be generated by invoking the collisional ionization models in CLOUDY. Our phenomenological analysis suggests that O I 1. 3164 m 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 m 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 (~ 7. 07 - 7. 49 10⁵ 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 m emission line.