We have studied molecular hydrogen emission in a sample of 21 YSOs using spectra obtained with the Infrared Space Observatory (ISO). H2 emission was detected in 12 sources and can be explained as arising in either a shock, caused by the interaction of an outflow from an embedded YSO with the surrounding molecular cloud, or in a PDR surrounding an exposed young earlytype star. The distinction between these two mechanisms can not always be made from the pure rotational H2 lines alone. Other tracers, such as PAH emission or [SI] 25.25 µm emission, are needed to identify the H2 heating mechanism. No deviations from a 3:1 ortho/para ratio of H2 were found. Both shocks and PDRs show a warm and a hot component in H2, which we explain as thermal emission from warm molecular gas (warm component), or UV-pumped infrared fluorescence in the case of PDRs and the re-formation of H2 for shocks (hot component).

Introduction

Molecular hydrogen is expected to be ubiquitous in the circumstellar environment of Young Stellar Objects (YSOs). It is the main constituent of the molecular cloud from which the young star has formed and is also expected to be the main component of the circumstellar disk. Most of this material will be at temperatures of 20–30 K and difficult to observe. However, some regions may be heated to temperatures of a few hundred K and produce observable H2 emission.