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 _ { 2 } H ^ { + } , HC _ { 3 } N , H _ { 2 } CS , and CH _ { 3 } OH ) , and their isotopic species ( C ^ { 17 } O , ^ { 13 } C ^ { 18 } O , and C ^ { 34 } S ) .
Line profiles consist of a weak broad ( \Delta v \sim 30 km s ^ { -1 } ) and/or a strong narrow ( \Delta v \lesssim 5 km s ^ { -1 } ) components , depending on molecular species .
Strong time variations of H _ { 2 } O 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 \times 10 ^ { -4 } M _ { \odot } yr ^ { -1 } 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 ( \sim 20 M _ { \odot } ) dark cloud .