Infrared and optical observations of Herbig-Haro objects. .

Abstract

Evidence is presented to support the view that Herbig-Haro (H-H) objects represent reflection nebulae illuminated by optically invisible stars (H-H stars) which are probably no older than a few hundred thousand years. The H-H stars are thought to represent the predecessors of those field T Tauri stars which possess circumstellar nebulae having emission-line characteristics very similar to those of the H-H objects. Several of these H-H stars have been discovered by means of infrared surveys of the dark cloud regions containing H-H objects. In general, they are displaced relative to the optical H-H objects and seem to be deeply embedded in dark cloud material; the H-H stars studied here are probably obscured by between 10 and 30 magnitudes of visual extinction. Several H-H objects have large angular dimensions, reminiscent of variable reflection nebulae such as NGC 6726 and Hubble's variable nebulae NGC 2261. Moreover, these objects are shown to have the relatively large, radial linear polarization characteristic of more ordinary reflection nebulae. The chance illumination of a reflection nebula by a young stellar object provides a fortunate opportunity to study some characteristics of the early phases of stellar evolution which otherwise would be hidden from view. The study of the emission-line radial velocities suggests for the H-H stars a mass outflow similar to that observed in most young emission-line objects. If this interpretation is correct, then mass outflow rather than infall characterizes young stars, even at ages of a few hundred thousand years or less. Finally, if the H-H stars are viewed as the direct predecessors of T Tauri stars or other young emission-line objects, it is possible to make some tentative statements about the evolutionary history of dark interstellar clouds. Subject headings: infrared sources - interstellar matter - nebulae - pre-main-sequence stars - stellar evolution