Heavy-metal chemistry in proto-planetary nebulae: Detection of MgNC, NaCN, and AlF toward CRL 2688

The metal-containing molecules AlF, MgNC, and NaCN have been detected toward CRL 2688, a circumstellar envelope in the proto-planetary nebula (PPN) phase. These measurements are the first detections of such species in a source other than the carbon star IRC + 10216. Three transitions of MgNC were observed, each of which consists of two spin-rotation components, and five lines of NaCN were measured; both detections were made in the 2 mm wavelength region using the NRAO 12 m telescope. Three transitions of AlF were observed as well at 2 and 1.2 mm with the IRAM 30 m antenna. All three species appear to trace the AGB wind, not the high-velocity outflows characteristic of post-AGB mass loss. Rotational temperature analysis suggests that MgNC emission is from cooler gas than AlF and NaCN, indicating an outer shell distribution for this molecule, as is found in IRC +10216. AlF and NaCN appear to be confined to the inner envelope of CRL 2688. The column density obtained for MgNC in this source assuming a shell-like distribution is N-tot similar to 4 x 10(12) cm(-2), corresponding to a fractional abundance, relative to H-2, of f similar to 4 x 10(-9). This abundance is about a factor of 10 less than that in IRC +10216. For NaCN, the column density and fractional abundance in CRL 2688 are N-tot similar to0.7-3 x 10(14) cm(-2) and f similar to 3-5 x 10(-8), comparable to what has been measured for IRC +10216. In the case of AlF, the column density toward CRL 2688 was determined to be N-tot similar to 0.9-3 x 10(13) cm(-2), resulting in f similar to 4-5 x 10(-9). In IRC +10216, f(AlF) is 1-2 x 10(-7). These data suggest that the chemistry in the outer envelope of CRL 2688 has altered abundances for species like MgNC, but inner shell molecules like NaCN may remain intact over a longer timescale. While the abundance of AlF in CRL 2688 is less than that of F-19 in the solar system, in IRC +10216, the AlF abundance exceeds the fluorine solar value. Hence, observations of this molecule suggest that 19F is produced in thermal pulses in the AGB phase.