The occurrence of H2O masers in HII regions

Details The occurrence of H2O masers in HII regions The occurrence of H2O masers in HII regions

Nov 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...291..261c&link_type=abstract

Astronomy and Astrophysics (ISSN 0004-6361), vol. 291, no. 1, p. 261-270

Astronomy and Astrophysics

Astronomy

34

Emission Spectra, H Ii Regions, Radio Astronomy, Radio Spectra, Star Clusters, Water Masers, Brightness Distribution, Flux Density, Star Formation

Scientific paper

The results of a survey of 22.2 GHz H2O maser emission towards a large sample of H II regions (427) detected in hydrogen recombination lines (Lockman 1989) are presented. The sample favors classical H II regions with large excitation parameter. We found 68 water maser sources, 17 of which are new detections. Adding the detections found in the literature, the total number of masers associated with the sample is 80. The overall maser percentage is low (19%), much less than what found in samples of ultracompact (UC) H II regions. The percentage changes if we divide our sample into two sub-classes: H II regions known to have ultracomapct components (i.e. where a younger generation of star formation is also present) and the rest. In the first sub-class the maser percentage is 28%, in the second 15%. The total number of masers in H II regions where no UC component has been (so far) reported is 43. This value is greater than that expected from the ratio between the lifetime of the H II region 106 yr and that of the maser phase, even assuming a maser lifetime as long as 105 yr. This implies that maser excitation (and star formation) occurs several times in the lifetime of the same star forming complex. From 410 positions observed, 230 can be associated with a high degree of confidence to an IRAS Point Source (PS). We expect that most of the H II regions not associated with IRAS PS are diffuse regions in the last phases of their evolution. The percentage of water masers in this sub-class is very low: 8%. The number of masers with no counterpart in the Lockman sample (and above delta = 30 deg) is very large: 334. This number most probably reflects the extreme high density conditions known to exist in the earliest phases of an H II region, which strongly reduce its ratio continuum and line emission. The analysis of a homogeneous sample of classical H II regions allows to confirm in a quantitative fashion the well known result that the maser phenomenon occurs during the earliest phase of the evolution of a massive star and that it as the H II region evolves into the diffuse stage.

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