Study of the Bipolar Nebula IRAS 19312+1950. II. Circumstellar Chemistry

The bipolar nebula, IRAS 19312+1950, is a unique SiO maser source exhibiting both properties as young and evolved objects. To clarify the nature of this object, we made molecular line observations with the Nobeyama 45-m radio telescope. We detected emission from O-bearing (HCO^+, SiO, SO, and SO_2), C- and N-bearing molecules (CN, CS, HCN, HNC, NH_3, N_2H^+, HC_3N, H_2CS, and CH_3OH), and their isotopic species (C^17O, ^13C^18O, and C^34S). Line profiles consist of a weak broad (Dv 30 km/s) and/or a strong narrow (DV< 5 km/s) components, depending on molecular species. Strong time variations of H_2O and SiO masers were also observed. Numerical modeling of the envelope with the LVG-code results in a good fit of the model with a mass loss rate of 2.6 10^-4 M_sun/yr to the observed intensities for the broad-component lines. On the other hand, non-O-bearing molecules, which have only the narrow profiles, were found to have abundances typical to those in cool dust clouds. No isotopic enrichment was found, indicating little evidence of the narrow cool component being an ejecta of the central AGB star or a possible companion. These facts compelled us to conclude that IRAS 19312+1950 is an exotic mass-losing evolved star embedded in a low-mass (~20 M_sun) dark cloud.