1 Introduction

Interstellar methanol has been found in numerous transitions and several of them exhibit maser activity. The 5₁ - 6₀ transition of $\rm A^+$ methanol at 6.7 GHz discovered by Menten (1991) is of particular interest. The sources of this line show a variety of spectra, from a single to several blended features over the velocity range of 10 km s^-1 or even exceeding 20 km s^-1, in a few objects. In a number of sources the flux density of several thousand Jy is observed and the linewidths of unblended maser features are 0.3 - 0.5 km s^-1 (Menten 1991; Caswell et al. 1995). The widespread occurrence and high intensity of this maser transition makes it a powerful tool to identify the massive early-type stars still embedded in their parental dense molecular clouds.

In many cases the 6.7 GHz methanol masers are associated with phenomena typical for environment of a newly formed massive star such as thermal and maser lines, radio continuum and far-infrared emission. A close association of the methanol sources with the known star-forming regions traced by OH or H₂O masers and/or ultracompact HII regions was established. Menten (1991) found 80 methanol sources among 123 molecular cloud regions. Caswell et al. (1995) found methanol emission in 184 out of 208 star-forming regions with OH masers and detected methanol masers at 56 sites without OH emission. Search for the 6.7 GHz methanol transition towards the southern 12.2 GHz methanol and 1.6 GHz OH masers resulted in the detection rates of 100% and 67% respectively (MacLeod et al. 1992; MacLeod & Gaylard 1992). Later on, several surveys of IRAS objects which satisfy the colour criteria for ultracompact HII regions (Wood & Churchwell 1989, hereafter WC) have also been successful, providing many new detections mostly in the southern hemisphere (Schutte et al. 1993; van der Walt et al. 1995, 1996; Walsh et al. 1997; Lyder & Galt 1997; MacLeod et al. 1998). Slysh et al. (1999) surveyed a sample of the northern hemisphere IRAS sources supplemented with several known ultracompact HII regions and bipolar outflows.

For each flux density measurement an associated quality flag is given in IRAS catalogue. Flags at four wavelength bands indicate whether the observation is of high or moderate quality, or only an upper limit. A significant number of IRAS sources, searched for methanol emission, have usually high quality flux density measurements. However, there is evidence that a quite large number of maser sources has IRAS counterparts with moderate quality of flux density measurements, or even upper limits (Cohen et al. 1988). It is likely that some of these IRAS objects are methanol sources.

van der Walt (1997) has shown that faint IRAS objects satisfying the WC criteria have a distribution in the galactic latitude that is completely different from that expected for embedded ionizing stars. He argues that the WC criteria can be too simple to identify all the massive stars with an associated methanol maser emission. Blind surveys of methanol masers in the narrow fields along the galactic plane (Caswell 1996; Ellingsen et al. 1996) yielded several new sources not associated with IRAS objects. It is suggested that some methanol masers may be associated with sources that have colours unusual for ultracompact HII regions (Ellingsen et al. 1996). To clarify a relationship between far-infrared sources and the 6.7 GHz methanol masers we have undertaken a search for methanol emission in a large sample of IRAS objects selected with no restrictions imposed on flux density quality and less stringent criteria for colours. As our survey provided nearly two hundred detections it would be useful for statistical studies of maser properties and related phenomena in star-forming regions. This paper contains the observational results only whilst a full analysis will be presented in a subsequent publication.