Analysis of Forbidden Neon Emission lines in HAeBe Stars using Spitzer IRS spectra

Abstract

We analyzed high-resolution mid-infrared spectra of 78 well-known Herbig Ae/Be (HAeBe) stars using Spitzer InfraRed Spectrograph data, focusing on the detection of [Ne {\sc II}] and [Ne {\sc III}] emission lines as indicators of ionized outflows or disk winds. Emission from [Ne {\sc II}] at 12.81 $\mu$m or [Ne {\sc III}] at 15.55 $\mu$m was identified in 25 sources, constituting the largest sample of HAeBe stars with these detected lines. Our analysis revealed a higher detection frequency of [Ne {\sc II}] in sources with lower relative accretion luminosity (L$_{acc}$/L$_*$ $<$ 0.1), suggesting a connection to the disk dispersal phase. We examined correlations between neon lines and various spectral features and investigated [Ne {\sc III}]-to-[Ne {\sc II}] line flux ratios to explore potential emission mechanisms. Neon emission is predominantly observed in Group I sources (75\%), where their flared disk geometry likely contributes to the observed emission, potentially originating from the irradiated disk atmosphere. Interestingly, we also find that Group II sources exhibit a higher median relative [Ne\,\textsc{ii}] line luminosity (L$_\mathrm{[Ne\,II]}$/L$_*$), suggesting enhanced photoevaporation rates possibly associated with their more settled disk structures. However, larger samples and higher-resolution spectra are required to confirm this trend definitively. The high detection rate of the [Fe {\sc II}] and [S {\sc III}] lines, commonly associated with EUV-dominated regions, alongside a [Ne {\sc III}]-to-[Ne {\sc II}] emission ratio greater than 0.1 in sources where both lines detected, suggests that EUV radiation is the primary driver of neon emission in our sample.