SIMBAD references

Class I methanol masers appear to probe very early stages of star formation. An observational survey of the 44 and 36 GHz methanol lines toward several star-forming regions was conducted using the Haystack Observatory 37 m telescope. Examining the intensities of the 36 GHz Class I maser line as compared to the 44 GHz maser line, it is seen that the 36 GHz line is enhanced toward sources where there is no apparent sign of star formation. Sources where the 36 GHz emission is absent, but the 44 GHz emission is strong, appear to be those where ultracompact H II regions and millimeter continuum sources are present. Existing models for the excitation of Class I methanol masers show strong temperature and density dependences for the presence or lack of certain methanol transitions. The 36 GHz masers appear in regimes where the temperatures are low – below 100 K. The 44 GHz masers are excited in a wider range of gas temperatures (80-200 K), supporting the hypothesis that these transitions are still masing even when the 36 GHz masers are quenched.