Optical Mass Flow Diagnostics in Herbig Ae/Be Stars

TL;DR

This study uses high-resolution optical spectra to analyze mass flow diagnostics in Herbig Ae/Be stars, revealing differences from T Tauri stars and suggesting alternative accretion mechanisms due to smaller magnetospheres.

Contribution

It provides new insights into the accretion and wind signatures of Herbig Ae/Be stars, highlighting the role of smaller magnetospheres and boundary layer accretion, based on extensive spectral analysis.

Findings

Herbig Ae/Be stars show fewer outflow and infall signatures than T Tauri stars.

Red-shifted absorption velocities are smaller fractions of escape velocities, indicating deeper accretion flows.

Herbig Be stars may accrete via boundary layers rather than magnetic channels.

Abstract

We examine a broad range of mass flow diagnostics in a large sample of Herbig Ae/Be stars (HAEBES) using high resolution optical spectra. The H-beta and He I 5876 angstrom lines show the highest incidence of P-Cygni (30%) and inverse P-Cygni (14%) morphologies, respectively. The Fe II 4924 angstrom line also shows a large incidence of P-Cygni profiles (11%). We find support for many of the conclusions reached in a study based on the analysis of the He I 10830 angstrom line in a large sample of HAEBES. Namely, HAEBES exhibit smaller fractions of both blue-shifted absorption (i.e. mass outflow) and red-shifted absorption (i.e. mass infall or accretion) than their lower mass cousins, the classical T Tauri stars (CTTSs). In particular, the optical data supports the conclusion that HAEBES displaying red-shifted absorption, in general, show maximum red-shifted absorption velocities that are smaller fractions of their stellar escape velocities than is found for CTTSs. This suggests that HAEBE accretion flows are originating deeper in the gravitational potentials of their stars than in CTTS systems. In addition, we find a lack of inner disk wind signatures in the blue-shifted absorption objects; only stellar wind signatures are clearly observed. These findings, along with the lack of detected magnetic fields around HAEBES, support the idea that large magnetospheres are not prevalent around HAEBES and that accretion flows are instead mediated by significantly smaller magnetospheres with relatively smaller truncation radii (e.g. 1-2 stellar radii). Red-shifted absorption is much more common around Herbig Ae stars than Be stars, suggesting that Herbig Be stars may accrete via a boundary layer rather than along magnetic field lines.