A Multi-Epoch, Simultaneous Water and Methanol Maser Survey toward
  Intermediate-Mass Young Stellar Objects
  We report a multi-epoch, simultaneous 22 GHz H2O and 44 GHz class I CH3OH
maser line survey towards 180 intermediate-mass young stellar objects,
including 14 Class 0, 19 Class I objects, and 147 Herbig Ae/Be stars. We
detected H2O and CH3OH maser emission towards 16 (9%) and 10 (6%) sources with
one new H2O and six new CH3OH 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. H2O maser velocities deviate
on average 9 km s^-1 from the ambient gas velocities whereas CH3OH maser
velocities match quite well with the ambient gas velocities. For both maser
emissions, large velocity differences (|v_{H2O} - v_{sys} | > 10 km s^-1 and
|v_{CH3OH} - v_{sys}| > 1 km s^-1) are mostly confined to Class 0 objects. The
formation and disappearance of H2O masers is frequent and their integrated
intensities change by up to two orders of magnitude. In contrast, CH3OH maser
lines usually show no significant change in intensity, shape, or velocity. This
is consistent with the previous suggestion that H2O maser emission originates
from the base of an outflow while 44 GHz class I CH3OH 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 L_{H2O} = 1.71 * 10^{-9}
(L_{bol})^{0.97} and L_{CH3OH} = 1.71 * 10^{-10} (L_{bol})^{1.22}.
