Maser emission in planetary nebulae

Stars at the top of the asymptotic giant branch (AGB) can exhibit maser emission from molecules like SiO, H₂O and OH. These masers appear in general stratified in the envelope, with the SiO masers close to the central star and the OH masers farther out in the envelope. As the star evolves to the planetary nebula (PN) phase, mass-loss stops and ionization of the envelope begins, making the masers disappear progressively. The OH masers in PNe can be present in the envelope for periods of ~1000 years but the H₂O masers can survive only hundreds of years. Then, H₂O maser emission is not expected in PNe and its detection suggests that these objects are in a very particular moment of its evolution in the transition from AGB to PNe. We discuss the unambiguous detection of H₂O maser emission in two planetary nebulae: K 3-35 and IRAS 17347-3139. The water-vapor masers in these PNe are tracing disk-like structures around the core and in the case of K3-35 the masers were also found at the tip of its bipolar lobes. Kinematic modeling of the H₂O masers in both PNe suggest the existence of a rotating and expanding disk. Both PNe exhibit a bipolar morphology and in the particular case of K 3-35 the OH masers are highly polarized close to the core in a disk-like structure. All these observational results are consistent with the models where rotation and magnetic fields have been proposed to explain the asymmetries observed in planetary nebulae.