Excitation mechanism of OI lines in Herbig Ae/Be stars

TL;DR

This study investigates the excitation mechanisms behind OI emission lines in Herbig Ae/Be stars, concluding that Lyman beta fluorescence is the dominant process based on spectral correlations and line ratio analysis.

Contribution

It provides observational evidence identifying Lyman beta fluorescence as the main excitation mechanism for OI lines in Herbig Ae/Be stars, supported by spectral correlation and line ratio analysis.

Findings

Lyman beta fluorescence is the dominant excitation mechanism.

Strong correlation between OI λ8446 and λ11287 line fluxes.

Collisional excitation contributes but is not dominant.

Abstract

We have investigated the role of a few prominent excitation mechanisms viz. collisional excitation, recombination, continuum fluorescence and Lyman beta fluorescence on the OI line spectra in Herbig Ae/Be stars. The aim is to understand which of them is the central mechanism that explains the observed OI line strengths. The study is based on an analysis of the observed optical spectra of 62 Herbig Ae/Be stars and near-infrared spectra of 17 Herbig Ae/Be stars. The strong correlation observed between the line fluxes of OI $\lambda$8446 and OI $\lambda$11287, as well as a high positive correlation between the line strengths of OI $\lambda$8446 and H$\alpha$ suggest that Lyman beta fluorescence is the dominant excitation mechanism for the formation of OI emission lines in Herbig Ae/Be stars. Further, from an analysis of the emission line fluxes of OI $\lambda\lambda$7774, 8446, and comparing the line ratios with those predicted by theoretical models, we assessed the contribution of collisional excitation in the formation of OI emission lines.