A search for methanol masers at 9.978 GHz and 10.058 GHz

We performed NLTE-radiative transfer calculations for interstellar A-type methanol maser lines in order to find out under which physical conditions the level populations show strong inversion. In our model we considered collisional as well as radiative pumping and accounted for line overlap as well as for torsional excitation of the methanol molecules. The radiation field was calculated using a simple on-the-spot approximation. Our results show that torsional excitation can be essential for the maser pumping mechanism while line overlap appears to be of minor importance. In addition to the well known Class I and Class II maser lines, we found two new transitions which possibly may show up as strong masers. These are the 43–52A− and A+ transitions at 10.058GHz and 9.978GHz respectively. These masers are radiatively pumped by IR radiation, i.e. they should be found near IR sources similar to the classical Class II methanol masers. 88 galactic CH3OH maser sources were searched for emission in these two lines, but no detection was made. Upper limits are given and discussed.