Detection of sodium cyanide (NaCN) in IRC 10216

Abstract

The first molecule containing Na has been detected in the outer circumstellar envelope of IRC 10216, by means of four rotational transitions in the 3 and 2mm spectral regions. Observations are well fitted by a low rotation temperature of 11 K, typical of the outer envelope. The column density and fractional abundance depend on the distribution assumed for NaCN, the abundance varying between 6 x 10<SUP>-9</SUP> and 1 x 10<SUP>-7</SUP> for distributions corresponding to HC3N (outer plus intermediate envelope) and HC7N (outer envelope only). The fraction of cosmic Na in the form of NaCN is probably several percent, so the significant amounts of Na escape incorporation into grains, unlike the fraction (less than 0.002) that applies for Si. If IRC 10216 is typical, relative abundance if Na and Si as observed in the diffuse interstellar medium are consistent with rates of ejection in the gas phase from circumstellar envelopes.