Proper motions of molecular hydrogen outflows in the ρ Ophiuchi molecular cloud

摘要

Context. Proper motion measurements provide unique and powerful means to identify the driving sources of mass outflows, which are of particular importance in regions with complex star formation activity and deeply embedded protostars. They also provide the necessary kinematic information to study the dynamics of mass outflows, the interaction between outflows and the ambient medium, and the evolution of outflows with the age of the driving sources. Aims. We aim to take a census of molecular hydrogen emission line objects (MHOs) in the ρ Ophiuchi molecular cloud and to make the first systematic proper motion measurements of these objects in this region. The driving sources are identified based on the measured proper motions, and the outflow properties are characterized. The relationship between outflow properties and the evolutionary stages of the driving sources are also investigated. Methods. Deep H2 near-infrared imaging is performed to search for molecular hydrogen emission line objects. Multi-epoch data are used to derive the proper motions of the features of these objects, and the lengths and opening angles of the molecular hydrogen outflows. Results. Our imaging covers an area of ~0.11 deg2 toward the L1688 core in the ρ Ophiuchi molecular cloud. In total, six new MHOs are discovered, 32 previously known MHOs are detected, and the proper motions for 86 features of the MHOs are measured. The proper motions lie in the range of 14 to 247?mas/yr, corresponding to transversal velocities of 8 to 140 km?s-1 with a median velocity of about 35 km?s-1. Based on morphology and proper motion measurements, 27 MHOs are ascribed to 21 driving sources. The molecular hydrogen outflows have a median length of ~0.04 pc and random orientations. We find no obvious correlation between H2 jet length, jet opening angle, and the evolutionary stage of the driving sources as defined by their spectral indices. We find that the fraction of protostars (23%) that drive molecular hydrogen outflows is similar to the one for Class II sources (15%). For most molecular hydrogen outflows, no obvious velocity variation along the outflow has been found. Conclusions. In Ophiuchus the frequency of occurrence of molecular hydrogen outflows has no strong dependency on the evolutionary stage of the driving source during the evolution from the protostellar stage to the classical T Tauri stage.