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

TL;DR

This study investigates the spectral evolution of oxygen emission lines in nova V2891 Cygni, revealing shock-induced processes and stratified ejecta through spectral modeling with CLOUDY, highlighting the role of collisional ionization and dust formation.

Contribution

It introduces a detailed phenomenological model of shock-induced oxygen emission lines in nova ejecta, emphasizing the role of collisional ionization and high-density stratification, which is novel in nova spectral analysis.

Findings

O I 7773 Å line requires high-density, oxygen-rich ejecta.

O I 1.3164 μm line originates from dense, shock-formed shells.

High-temperature conditions coincide with dust formation epoch.

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 {\AA}, O I 8446 {\AA}, O I 1.1286 $\mu$m, O I 1.3164 $\mu$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 $\sim$ 28) to produce the O I 7773 {\AA} 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 $\mu$m line, which could only be generated by invoking the collisional ionization models in CLOUDY. Our phenomenological analysis suggests that O I 1.3164 $\mu$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 $\mu$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 ($~ 7.07 - 7.49 \times 10^5$ 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 $\mu$m emission line.