Near-IR Molecular Hydrogen Emission of Protostars: Probing Circumstellar Environments

TL;DR

This study investigates near-infrared molecular hydrogen emission in 18 low-mass protostars, revealing extended emission linked to jets or winds, with excitation mechanisms including shocks and UV/X-ray processes, similar to T Tauri stars.

Contribution

First comprehensive near-IR H2 emission analysis of Class I protostars, highlighting emission origins and excitation mechanisms in circumstellar environments.

Findings

H2 emission is often extended and associated with jets or winds.

Line ratios suggest shock excitation, with some UV/X-ray influence.

Emission properties are similar to those of T Tauri stars.

Abstract

We present new observations of near-infrared molecular hydrogen (H2) line emission in a sample of 18 Class I and flat-spectrum low mass protostars, primarily in the Tau- Aur and {\rho} Oph dark clouds. The line emission is extended by up to several arcseconds (several hundred AU) for most objects, and there is little night-to-night variation in line strength coincident with the continuum point source. Flux ratios of H2 v = 2 - 1 S(1) and v = 1 - 0 S(1) lines are consistent with this emission arising in jets or winds in many objects. However, most objects have only small offsets (under 10 km s-1) between their H2 and photospheric radial velocities. No objects have line ratios which are clearly caused solely by UV excitation, but the H2 emission of several objects may be caused by UV or X-ray excitation in the presence of circumstellar dust. There are several objects in the sample whose observed velocities and line fluxes suggest quiescent, non-mechanical origins for their molecular hydrogen emissions. Overall we find the H2 emission properties of these protostars to be similar to the T Tauri stars studied in previous surveys.