Analysis of forbidden neon emission lines in HAeBe stars using Spitzer IRS spectra

Abstract

We analysed high-resolution mid-infrared spectra of 78 well-known Herbig Ae/Be (HAeBe) stars using Spitzer InfraRedSpectrograph data, focusing on the detection of [Ne II] and [Ne III] emission lines as indicators of ionized outflows or disc winds. Emission from [Ne II] at 12.81 μm or [Ne III] at 15.55 μm 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 II] in sources with lower relative accretion luminosity (Lacc/L∗ < 0.1), suggesting a connection to the disc dispersal phase. We examined correlations between neon lines and various spectral features and investigated [Ne III]-to-[Ne II] line flux ratios to explore potential emission mechanisms. Neon emission is predominantly observed in Group I sources (75 per cent), where their flared disc geometry likely contributes to the observed emission, potentially originating from the irradiated disc atmosphere. Interestingly, we also find that Group II sources exhibit a higher median relative [Ne II] line luminosity (L[Ne II]/L∗), suggesting enhanced photoevaporation rates possibly associated with their more settled disc structures. However, larger samples and higher-resolution spectra are required to confirm this trend definitively. The high detection rate of the [Fe II] and [S III] lines, commonly associated with EUV-dominated regions, alongside a [Ne III]-to-[Ne II] emission ratio greater than 0.1 in sources where both lines detected, suggests that EUVradiation is the primary driver of neon emission in our sample.