Reliable galaxy-wide identification of ultracompact HII regions

Abstract

We examine the problem of reliably identifying ultracompact (UC) H ii regions from Galaxy- wide data bases. It is shown that there is significant contamination in the sample of potential UC H ii regions, selected from the IRAS PSC using the two-colour criterion of Wood & Churchwell, due to cloud cores with lower mass stars. We identify additional criteria to reduce this contamination. First, we use the differences in the radio emission between the cores with embedded high- and low-mass stars to segregate them. Then, through a latitude analysis we further improve the reliability. Effectively, the total number of potential UC H ii regions is brought down by a factor of ~4. This reduction eases the birth rate problem for massive stars, and offers a more reliable list of young massive stars for galactic structure studies. In the process, we have also identified a group of objects which may contain high-mass equivalents of the class 0 objects and clusters of intermediate-mass stars.