Investigating the origin of hot gas lines in Herbig Ae/Be Stars

TL;DR

This study investigates the origin of hot gas lines in Herbig Ae/Be stars using high-resolution UV spectra, finding that these lines likely originate from stellar winds rather than accretion flows, contrasting with classical T Tauri stars.

Contribution

It provides new insights into the formation of hot gas lines in Herbig Ae/Be stars, suggesting they are formed in stellar winds rather than accretion flows, unlike in T Tauri stars.

Findings

Hot gas lines are formed in weak, optically thin stellar winds.

No evidence of hot, optically thick winds in most HAEBE stars.

The circumstellar environment of HAEBES differs from scaled-up T Tauri star environments.

Abstract

We analyze high-resolution UV spectra of a small sample of Herbig Ae/Be stars (HAEBES) in order to explore the origin of the $T\sim10^5$ K gas in these stars. The C IV 1548,1550 \AA\ line luminosities are compared to non--simultaneous accretion rate estimates for the objects showing C IV emission. We show that the correlation between $L_{CIV}$ and $\dot{M}$ previously established for classical T Tauri stars (CTTSs) seems to extend into the HAEBE mass regime, although the large spread in literature $\dot{M}$ and $A_V$ values makes the actual relationship highly uncertain. With the exception of DX Cha, we find no evidence for hot, optically thick winds in our HAEBE sample. All other objects showing clear doublet emission in C IV can be well described by a two component (i.e., a single component for each doublet member) or four component (i.e., two components for each doublet member) Gaussian emission line fit. The morphologies and peak-flux velocities of these lines suggest they are formed in weak, optically thin stellar winds and not in an accretion flow, as is the case for the hot lines observed in CTTSs. The lack of strong outflow signatures and lack of evidence for line formation in accretion flows is consistent with the conclusion presented in our recent optical and He I 10830 \AA\ studies that the immediate circumstellar environments of HAEBES, in general, are not scaled-up analogs of the immediate environments around CTTSs. The conclusions presented here for hot gas lines around HAEBES should be verified with a larger sample of objects.