A Multi-Epoch, Simultaneous Water and Methanol Maser Survey Toward Intermediate-Mass Young Stellar Objects

We report a multi-epoch, simultaneous 22 GHz H 2 O and 44 GHz Class I CH 3 OH maser line survey toward 180 intermediate-mass young stellar objects, including 14 Class 0 and 19 Class I objects, and 147 Herbig Ae/Be stars. We detected H 2 O and CH 3 OH maser emission toward 16 (9%) and 10 (6%) sources with one new H 2 O and six new CH 3 OH maser sources. The detection rates of both masers rapidly decrease as the central (proto)stars evolve, which is contrary to the trends in high-mass star-forming regions. This suggests that the excitations of the two masers are closely related to the evolutionary stage of the central (proto)stars and the circumstellar environments. H 2 O maser velocities deviate on average 9 km s −1 from the ambient gas velocities whereas CH 3 OH maser velocities match quite well with the ambient gas velocities. For both maser emissions, large velocity differences ( and ) are mostly confined to Class 0 objects. The formation and disappearance of H 2 O masers is frequent and their integrated intensities change by up to two orders of magnitude. In contrast, CH 3 OH maser lines usually show no significant change in intensity, shape, or velocity. This is consistent with the previous suggestion that H 2 O maser emission originates from the base of an outflow while 44 GHz Class I CH 3 OH maser emission arises from the interaction region of the outflow with the ambient gas. The isotropic maser luminosities are well correlated with the bolometric luminosities of the central objects. The fitted relations are and .