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Astron. Astrophys. 360, 311-318 (2000)

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A survey of the 6.7 GHz methanol maser emission from IRAS sources

II. Statistical analysis

M. Szymczak and A.J. Kus

Toru Centre for Astronomy, Nicolaus Copernicus University, ul. Gagarina 11, 87100 Toru, Poland

Received 9 May 2000 / Accepted 30 May 2000

Abstract

We present an analysis of the 6.7 GHz methanol maser data taken with the 32 m Toru telescope during recently completed survey of IRAS based sample of star formation sites. Methanol masers are associated with 13% of the sources in the sample. The mean detection rate slightly increases with the IRAS flux density quality measurements. It is 62% for sources with the 60 µm flux density, [FORMULA] Jy and decreases to 7% for sources with [FORMULA] Jy. The detection rate in the inner Galaxy region of 18% is a factor of eight higher than that in the anticentre region. The maser velocity range, [FORMULA] km s-1 is observed in sources with the mean linewidth, [FORMULA]FWHM[FORMULA] 0.14 km s-1, whereas the distribution of [FORMULA] in sources with [FORMULA]FWHM[FORMULA] 0.14 km s-1 is flat with only a slight excess of the source number with [FORMULA] km s-1. [FORMULA] slightly increases with the [25-12] colour and [FORMULA] km s-1 is usually not observed among the bluest sources ([25-12][FORMULA]).

Several of the methanol sources found have colours well outside the colour ranges commonly accepted for ultracompact HII regions. The maximum detection rate of methanol masers is observed for specific colour ranges. The maser flux density is weakly correlated with the [FORMULA]m flux density and far-infrared pumping with an efficiency of about 3% is possible. The median photon luminosity of methanol masers in the solar neighbourhood is [FORMULA] s-1, being between the OH and H2O maser luminosities. The inferred lower limit for the linewidth of 0.05 km s-1 implies the kinetic temperature of less than 35 K in the methanol maser regions. It is proposed that the differences in the observed maser properties primarily reflect evolutionary changes of star-forming regions.

Key words: masers – surveys – stars: formation – ISM: molecules – radio lines: ISM – ISM: H ii regions

Send offprint requests to: M. Szymczak (msz@astro.uni.torun.pl)

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© European Southern Observatory (ESO) 2000

Online publication: July 27, 2000
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