A near-IR imaging survey of intermediate- and high-mass young stellar outflow candidates

Abstract

We have carried out a near-infrared imaging survey of luminous young stellar\noutflow candidates using the United Kingdom Infrared Telescope. Observations\nwere obtained in the broad band K (2.2 mu) and through narrow band filters at\nthe wavelengths of H_2 v=1--0 S(1) (2.1218 mu) and Br gamma (2.166 mu) lines.\nFifty regions were imaged with a field of view of 2.2 X 2.2 arcmin^2. Several\nyoung embedded clusters are unveiled in our near-infrared images. 76% of the\nobjects exhibit H_2 emission and 50% or more of the objects exhibit aligned H_2\nemission features suggesting collimated outflows, many of which are new\ndetections. These observations suggest that disk accretion is probably the\nleading mechanism in the formation of stars, at least up to late O spectral\ntypes. The young stellar objects responsible for many of these outflows are\npositively identified in our images based on their locations with respect to\nthe outflow lobes, 2MASS colours and association with MSX, IRAS, millimetre and\nradio sources. The close association of molecular outflows detected in CO with\nthe H_2 emission features produced by shock excitation by jets from the young\nstellar objects suggests that the outflows from these objects are jet-driven.\nTowards strong radio emitting sources, H_2 jets were either not detected or\nwere weak when detected, implying that most of the accretion happens in the\npre-UCHII phase; accretion and outflows are probably weak when the YSO has\nadvanced to its UCHII stage.\n