H-II REGIONS AND IRAS PSC SOURCES - THE RELIABILITY OF THE ASSOCIATION

The IRAS Point Source Catalogue has been compared with a catalogue of 462 (diffuse) HII regions detected in hydrogen radio recombination lines. To distinguish the true IRAS counterpart of the H II region from chance coincidences the distribution of offset and the position in colour-colour plots as a function of F(60 mum) are investigated. It is found that sources with F(60 mum) < 100 Jy have a high probability of being chance coincidences. The far infrared properties of the H II regions are investigated for a subsample of 252 IRAS PSC sources which have a high (> 80%) probability of being true coincidences. The majority satisfy the Wood and Churchwell colour criteria which identify ultracompact H II regions; therefore, these criteria select ultracompact as well as more diffuse H II regions, or, alternatively, diffuse H II and ultracompact H II regions may often be mixed, as predicted by the sequential star formation theory. The luminosity of a single star required to produce the observed H II line emission correlates very well with the far infrared luminosity and is, on the average, higher than the observed value. Consequently, the stars ionizing the extended HII regions can account for the bulk of the observed FIR emission and no contribution from lower mass stars of the cluster is necessary.