Clerkin, Eoin James (2004):
A 2MASS study of candidate precursors to UCHII regions.
Portugal, Universidade do Porto, [Master Thesis]

Abstract

There has been two major efforts in the past 7 years to study UCHII regions (Molinari et al. 1996; Sridharan et al., 2002). They compiled a list of 217 best candidates of the precursors to UCHII regions. We conducted a near IR study into these sources using the 2MASS all sky survey. A method of Nyquist bining stellar counts around these sources revealed the presence of embedded clusters in at least 63 (30%) of the sample. More than 50 of these were previously unknown groupings, 29 are classified as large clusters, 18 as small and the rest as stellar groups. This technique of detection provided stellar density contour maps which allowed us to discuss the morphology of the groupings. We estimated that the ratio of Hierarchical type to central condensed type cluster (H/C = 0.9 \approx 1) which possibly implies both a basic unity of gravity and turbulence but also maybe the decay of the turbulence with time. The positions of the candidate (pre) UCHII regions show a striking preference for the very centre of the clusters.

We statistically calculate the number of cluster members detectable by the 2MASS telescope, and we extend this estimate and the mass to almost the deuterium burning limit with the use of an universal IMF from Muench et al. (2002). We observe a correlation between the relative age of the (pre) UCHII regions and the number of members in the clusters. With the use of cc-diagrams, we obtained the average dust extinction and we discussed the colours of massive protostars. The mass of the dust cloud was available for 15 clusters from Beuther et al. (2002) and so, we estimated the local star formation efficiency.

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