Detection of MgNC in CRL 618: Tracing Metal Chemistry with Asymptotic Giant Branch Evolution

Abstract

The MgNC radical has been detected toward the circumstellar shell of CRL 618, the first metal-bearing molecule (in the chemist's sense) observed in this proto-planetary nebula. Five rotational transitions of this 2Σ species were measured using the Arizona Radio Observatory (ARO) 12 m and IRAM 30 m telescopes, several which clearly display the characteristic spin-rotation structure. Searches for NaCN, NaCl, AlF, and AlCl in CRL 618, in contrast, proved negative. The line profiles of MgNC in this source, measured at the 30 m, were somewhat U-shaped and showed no evidence of high-velocity wings, indicating that this radical arises from the remnant asymptotic giant branch (AGB) wind. The column density of MgNC in CRL 618 is Ntot = 2.4 × 1012 cm-2, corresponding to a fractional abundance, relative to H2, of f ~ 5.3 × 10-9. The upper limits for the sodium and aluminum species are typically f < 10-9 to 10-8. The MgNC abundance in CRL 618 is comparable to those measured in CRL 2688 and IRC +10216; the upper limits for the sodium and aluminum compounds are at least a factor of 10 lower in CRL 618 relative to abundances in IRC +10216, but similar to those in CRL 2688. These data suggest that metal-bearing molecules produced by LTE chemistry (NaCl, AlCl, AlF) are destroyed by the events associated with the second stage of AGB mass loss, while radicals such as MgNC survive in the outer envelope for a significant time in the proto-planetary nebula phase.