Observational constraints on the abundance and evolution of " XCN" in interstellar grain mantles

The 4.62 mum "XCN" absorption feature, attributed to CN-bearing molecules i n solids, is potentially an important diagnostic of the evolution of organi c matter in the interstellar medium and the envelopes of newly formed stars . We report quantitative limits on the strength of this feature in the diff use interstellar medium toward the reddened B-type star Cyg OB2 No. 12 and in the dark cloud toward the young stellar object R CrA IRS 2. On the basis of an assumed band strength for the carrier species, we estimate that less than 0.3% and less than 0.1% of the elemental nitrogen is in CN bonds alon g these lines of sight, respectively; if they are typical of diffuse and de nse environments, it follows that the carrier of XCN is no more than a trac e constituent of either organic-refractory or icy interstellar grain mantle s. Appreciable XCN abundances seem to occur only in the envelopes of certai n young stellar objects (YSOs), most notably the high-mass objects W33A and AFGL 7009S. We confirm the presence of XCN in the spectrum of the low-mass YSO R CrA IRS 7. The strengths of the XCN absorptions in R CrA IRS 7 and o ther low-mass YSOs indicate mean XCN concentrations relative to H2O in the ices of similar to1%, comparable with the abundance of CN-bearing species i n comets.