A sensitive search for methanol line emission toward evolved stars
  We present a sensitive search for methanol line emission in evolved stars at
1 cm, aiming to detect, for the first time, methanol masers in this type of
objects. Our sample comprised post-AGB stars and young planetary nebulae (PNe),
whose mass-loss processes and circumstellar structures resemble those of young
stellar objects (YSOs), where methanol masers are detected. Class I masers were
searched for in 73 objects, whereas Class II ones were searched in 16. No
detection was obtained. The non-detection of Class I methanol masers indicated
that methanol production in dust grains and/or the enhancement of its gas-phase
abundance in the shocked regions of evolved objects are not as efficient as in
YSOs. We suggest that relatively more evolved PNe might have a better
probability of harboring Class II masers.
