6.7 GHz methanol masers at sites of star formation

A blind survey of the Galactic plane between 20° 40° and 052

Toruń Centre for Astronomy, Nicolaus Copernicus University, Gagarina 11, 87-100 Toruń, Poland

Corresponding author: M. Szymczak, msz@astro.uni.torun.pl

Received: 30 April 2002
Accepted: 6 June 2002

Abstract

We report the results of an unbiased survey for 6.7 GHz methanol maser emission of a ~21 deg² strip of the Galactic plane carried out with the 32 m Toruń radio telescope. An area at 20° 40°, 052 was surveyed in an equilateral triangular grid with a sensitivity limit of about 1.6 Jy. We detected a total of 100 sources, 26 of which are new detections. All the new sources are of moderate intensity and their peak flux densities have median value of 6.5 Jy, i.e. about half that of previously known sources in the sample. About 80% of maser sources have IR counterpart candidates within a radius but not all the IRAS counterparts of methanol masers have colours typical of ultracompact HII regions. An excess of masers unassociated with IR sources occurs at because of incompleteness of IR catalogues due to strong confusion near the tangential region of the spiral arm. Our unbiased survey doubled the number of detections as compared to IRAS-based observations. Within the positional uncertainty of about one third of the methanol sources have radio continuum counterparts at 5 GHz of a flux density greater than 2.5–10 mJy. The distribution of methanol sources appears to be consistent with a clustered mode of formation of massive stars.