Observational Constraints on the Abundance and Evolution of “XCN” in Interstellar Grain Mantles

Abstract

The 4.62 μm "XCN" absorption feature, attributed to CN-bearing molecules in solids, is potentially an important diagnostic of the evolution of organic matter in the interstellar medium and the envelopes of newly formed stars. We report quantitative limits on the strength of this feature in the diffuse 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 along these lines of sight, respectively; if they are typical of diffuse and dense environments, it follows that the carrier of XCN is no more than a trace constituent of either organic-refractory or icy interstellar grain mantles. Appreciable XCN abundances seem to occur only in the envelopes of certain 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 other low-mass YSOs indicate mean XCN concentrations relative to H2O in the ices of ~1%, comparable with the abundance of CN-bearing species in comets.