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 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 ~ 10¹¹ 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 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 1012.5 - 1012.8 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 (~ 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 μm emission line.