A search for methanol masers at 9.978 GHz and 10.058 GHz

We performed NLTE-radiative transfer calculations for interstellar A-type m ethanol maser lines in order to find out under which physical conditions th e level populations show strong inversion. In our model we considered colli sional 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 t hat torsional excitation can be essential for the maser pumping mechanism w hile line overlap appears to be of minor importance. In addition to the wel l known Class I and Class II maser lines, we found two new transitions whic h possibly may show up as strong masers. These are the 4(3)-5(2) A(-) and A (+) transitions at 10.058 GHz and 9.978 GHz respectively. These masers are radiatively pumped by IR radiation, i.e. they should be found near LR sourc es similar to the classical Class LI methanol masers. 88 galactic CH3OH mas er sources were searched for emission in these two lines, but no detection was made. Upper limits are given and discussed.