Study of the Bipolar Nebula IRAS 19312+1950. I. Mapping Observations

IRAS 19312+1950 is an SiO maser source that exhibits a prominent bipolar nebulosity. Mapping observations of this object were made in the CO J=1--0, 13CO J=1--0, C18O J=1--0, CS J=2--1, and HCN J=1--0 lines and in the 150 GHz continuum band. Near-infrared imaging observations were also made in the J, H, and K-bands. The line profiles of the 12CO and HCN spectra consist of a weak broad component with a line width of about 50 km/s and a strong narrow component of the width of about 3 km/s. The profiles of the 13CO, C18O, and CS lines have only the narrow component. Both of the components have an intensity peak at the IRAS position. The narrow component was clearly resolved with a 15'' telescope beam. The spectral energy distribution of this object exhibits a doubly peaked profile between 1 and 25 micron. The 150 GHz continuum flux density was found to be 0.07 Jy, which is consistent with the flux density predicted by the expanding envelope model with a mass loss rate of ~10^{-4} M_sun/y at a distance of 2.5 kpc. We argue that the broad component originates from the expanding envelope of this object, and that the hot dust cloud, which is the source of the narrow component, is also physically associated with this object. Though the present observations do not preclude the possibility of a young stellar object, we argue that it is less plausible. We conclude that IRAS 19312+1950 is an AGB/post-AGB star that is evolved from a massive progenitor.