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

TL;DR

This study analyzes mid-infrared spectra of 78 Herbig Ae/Be stars, detecting neon emission lines that reveal insights into disk evolution, ionized outflows, and the influence of EUV radiation on disk atmospheres.

Contribution

It provides the largest sample of HAeBe stars with neon emission lines detected and explores their connection to disk geometry, accretion, and EUV-driven photoevaporation mechanisms.

Findings

Neon emission detected in 25 sources, mainly in Group I disks.

Higher [Ne II] detection in sources with lower accretion luminosity.

EUV radiation likely drives neon emission, indicated by line ratios and associated lines.

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.