BIMA Array Observations of the Highly Unusual SiO Maser Source with a Bipolar Nebulosity IRAS 19312+1950

We report the results of mapping observations of the bipolar nebula with SiO maser emission, IRAS 19312+1950, in the CO (J = 1-0 and 2-1), 13CO (J = 1-0 and 2-1), C18O (J = 1-0), CS (J = 2-1), SO (JK = 32-21), and HCO+ (J = 3-2) lines with the Berkeley-Illinois-Maryland Association array. The evolutional status of this source has been evoking a controversy since its discovery, although SiO maser sources are usually identified as late-type stars with active mass loss. In line profiles, two kinematical components are found, as reported in previous single-dish observations: a broad pedestal component and a narrow component. Spatiokinetic properties of a broad-component region traced by 12CO lines are roughly explained by a simple spherical outflow model with an expanding velocity typical of an AGB star, although some properties of the broad-component region still conflict with properties of a typical AGB spherical outflow. A narrowcomponent region apparently exhibits a bipolar flow. The angular size of the narrow-component region is spatially larger than that of the broad-component region. The intensity distribution of the CS emission avoids the central region of the source, and that of the SO broad-component emission exhibits a small feature peaked exactly at the mapping center. According to the present results, if the broad component really originates in a spherical outflow, an oxygen-rich evolved stellar object seems to be a natural interpretation for the central star of IRAS 19312+1950.