2011ApJ...728...76N

The IRAS source, 19312+1950, exhibits SiO maser emission, which is predominantly detected in evolved stars enshrouded by a cold molecular envelope. In fact, the majority of the observational properties of IRAS 19312+1950 are consistent with the nature of an asymptotic giant branch (AGB) star or post-AGB star. Interestingly, however, some of the observational properties cannot be readily explained within the standard scheme of stellar evolution, and those are rather reminiscent of young stellar objects. In the present research, we considered the evolutionary status of IRAS 19312+1950 as revealed by the very long baseline interferometry and MERLIN observations in SiO, H₂O, and OH maser lines. The double-peaked profile of the 22 GHz H₂ O maser line is clearly detected, with the emission regions of its redshifted and blueshifted components separately located, leaving a space of about 10.9 mas between them. The kinematic properties of the H₂ O maser emission region appear to be more consistent with a bipolar flow rather than other interpretations such as the Keplerian rotation of a disk. The redshifted component of the SiO maser emission, which exhibits a double-peak profile in previous single-dish observations, is clearly detected in the present interferometry, while the 1612 MHz OH maser line exhibits a complicated line profile consisting of a single strong peak and many weak, high-velocity spikes. The structure of the OH maser emission region is partially resolved, and the kinematic properties of the OH maser emission region are observations reminiscent of a spherically expanding shell, even though the evidence is scant. Collectively, the maser observations described here provide additional support for the evolved star hypothesis for IRAS 19312+1950.