Mathematics – Probability
Scientific paper
Feb 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt.......249c&link_type=abstract
PhD Thesis, Dipartimento di Astronomia e Scienza dello Spazio, Università di Firenze, Italy, 1995.
Mathematics
Probability
Scientific paper
It is well known that 22.2 GHz water maser sources are associated with young star forming regions and that the conditions for the onset of the maser phase are already set in the evolutionary stages in which high density ionized gas can be detected. The main question approached with the present work was to see whether water maser emission continues up to the stage in which a diffuse H ii region becomes observable. In order to study the occurrence of water maser in these evolutionary stages, a sample of 462 classic H ii regions (defined as those which have already entered the diffuse stage) has been chosen to be investigated through a search for maser emission performed with the 32-m radiotelescope of Medicina (Italy). It has been found 68 water maser sources, 17 of which are new detections. The overall maser percentage is low (19%), much less than what found in samples of ultracompact (UC) H ii regions. The percentage changes if the sample is divided into two sub-classes: H ii regions known to have ultracompact 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%. Moreover, in apparent contrast with the common belief that galactic H2O masers are always found associated with H ii regions, if one considers all the H2O masers of star forming type, the majority (80%) are without any known diffuse H ii region. In order to see if these sources represent a homogeneous sub-class in an earlier evolutionary phase in which a diffuse H ii region has not yet formed a search for H89α (3 cm) hydrogen recombination line towards a sub-sample of 60 of these masers has been performed with the NRAO 43-m radiotelescope of Green Bank (USA). The detection rate is very low: only 2 sources (3%) show recombination line emission. In order to explain this low detection rate, possible selection effects have been examined, such as sensitivity of the recombination line observations, or lack of ionized gas because of the low luminosity of the associated star. Although both effects can play some role, by no means can they account for the large number of masers without associated diffuse H ii regions. The most probable answer is that masers form very early, much before the formation of a diffuse H ii region, and spend most of their life in this evolutionary stage. In conclusion, the 20% positional agreement between H ii regions and water masers found in low resolution (arcmin) surveys may simply reflect that more than one star-formation event (i.e. the formation of a new maser) occurs in a stellar cluster and that these are spread over an extended period, longer than that required by the first massive stars of the cluster to develop their own H ii regions. In this thesis, the investigation of the association between H ii regions and IRAS point sources has been also reported. 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 the F(60 microns) have been studied. It has been found that sources with 60 micron flux density less than 100 Jy have a high probability of being chance coincidences. The majority of the IRAS counterparts 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. Morover, another related research effort has been reported in the thesis: a search for water maser emission at 22.2 GHz has been performed with the Medicina antenna towards 160 IRAS sources selected using the Wood & Churchwell colour criteria to identify high-mass star forming regions. The aim of the survey was to verify the existence of a substantial variation of the maser detection rate within the Wood & Churchwell sample, and to estimate its possible contamination due to spurious sources. Out of the whole sample, water maser emission was found in only 11 sources (7%), 2 of which for the first time. There is a strong dependence of the maser occurrence on the IRAS flux density at 60 microns: the rate drops from ~24% for sources brighter than 100 Jy to ~1% for weaker sources. These results, combined with those found in previous surveys, indicate that it is very unlikely that the population of weak IRAS sources with shallow far-infrared continuum spectra is associated with high-mass star forming regions. Since these sources account for about 50% of the total number of the IRAS PS located inside the Wood & Churchwell colour box, the predicted population of OB-type stars may have been overestimated by a factor up to 50%.
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