Detection of SiN in IRC + 10216

Abstract

The first extraterrestrial silicon-nitrogen molecule, SiN, has been detected in the outer circumstellar envelope IRC + 10216, by means of five transitions in the 1 and 3 mm spectral regions. The column density is N(SiN) = 3.8 x 10 exp 13/sq cm, and, together with recent redetermination of the SiC2 abundance, means that SiN/SiC2 is about equal to 0.05 NH3/C2H2, assuming the latter ratio is similar in the outer envelope to the value found by IR observations in the inner core. This result is probably consistent with predictions of the 1989 ion-molecule model of Herbst et al., with Si(+) the progenitor of both SiN and SiC2. The source of Si(+) is proposed to be SiH4, the carrier of gaseous Si to the outer envelope. SiH4 photodissociates first to silylene (SiH2), then to Si(+) and Si. The fraction of all silicon lost to the interstellar medium in gaseous form is between 1 x 10 exp -4 and 2 x 10 exp -3. Thus, the traditional idea that the interstellar gas-to-dust ratio of about 100 represents the condensation of all refractory elements in mass-losing evolved stars needs no revision.