Study of the bipolar nebula IRAS 19312 + 1950. II. Circumstellar chemistry

Abstract

The bipolar nebula IRAS 19312+1950 is a unique SiO maser source exhibiting both properties of 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). The line profiles consist of weak broad (Δu ∼ 30kms -1) and/or strong narrow (Δu ∼ 30kms -1) components, depending on the molecular species. Strong time variations of H 2O and SiO masers were also observed. Numerical modeling of the envelope with the LVG-code resulted in a good fit of the model with a mass loss rate of 2.6 × 10 -4 M⊙yr -1 to the observed intensities for the broad-component lines. On the other hand, non-O-bearing molecules, which only have narrow profiles, were found to have abundances typical of those in cool dust clouds. No isotopic enrichment was found, indicating little evidence of the narrow cool component being 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⊙) dark cloud.