Objects earlier than precursors of UC HII regions: Inflow–signpost for a common way of star formation

Abstract

Searching for objects in the earliest phases of star formation, e.g sources at the beginning of a gravitational collapse, are essential to our understanding of massive star formation. Today a number of precursors of ultra compact HII regions (PUCHs) have been found. Embedded in dense gas and dust, these PUCHs have a high bolometric luminosity but little or no 6 cm radio continuum emission (Molinari, et al. 2000; Beuther, et al. 2002). Evidence for Collapse was found in ultra compact (UC) HII regions and 12 water maser sources (Zhang, et al. 1998; Wu & Evans II 2003). This paper presents the identification of massive cores with no detectable infrared and radio sources. These kinds of cores usually have strong sub-mm emission. A special case is the SCUBA core JCMT 18354-0649S which has both infall and outflow motions as indicated by the profiles of high excitation molecular lines. This core is at a stage earlier than PUCHs. Blue profiles are also found in UC HII region, which indicates that material is still infalling in this phase. Our observations suggest that infall exists in different evolutionary stages for high mass star formation, similar to the low mass cases.