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The Salty Scrambled Egg: Detection of NaCl Toward CRL 2688

2003; IOP Publishing; Volume: 593; Issue: 1 Linguagem: Inglês

10.1086/376446

1538-4357

J. L. Highberger, Kelly Thomson, Patrick Young, David Arnett, L. M. Ziurys,

Astrophysics and Star Formation Studies

NaCl has been detected toward the circumstellar envelope of the post-AGB star CRL 2688 using the IRAM 30 m telescope, the first time this molecule has been identified in a source other than IRC +10216. The J = 7 → 6, 11 → 10, 12 → 11, and 18 → 17 transitions of NaCl at 1, 2, and 3 mm have been observed, as well as the J = 8 → 7 line of the 37Cl isotopomer. The J = 12 → 11 line was also measured at the ARO 12 m telescope. An unsuccessful search was additionally conducted for AlCl toward CRL 2688, although in the process new transitions of NaCN were observed. Both NaCl and NaCN were found to be present in the AGB remnant wind, as suggested by their U-shaped line profiles, indicative of emission arising from an optically thin, extended shell-like source of radius ~10''-12''. These data contrast with past results in IRC +10216, where the distribution of both molecules is confined to within a few arcseconds of the star. A high degree of excitation is required for the transitions observed for NaCl and NaCN; therefore, these two species likely arise in the region where the high-velocity outflow has collided with the remnant wind. Here the effects of shocks and clumping due to Rayleigh-Taylor instabilities have raised the densities and temperatures significantly. The shell source is thus likely to be clumpy and irregular. The chemistry producing the sodium compounds is consequently more complex than simple LTE formation. Abundances of NaCl and NaCN, relative to H2, are f ~ 1.6 × 10-10 and ~5.2 × 10-9, respectively, while the upper limit to AlCl is f < 2 × 10-9. These values differ substantially from those in IRC +10216, where AlCl has an abundance near 10-7. The NaCl observations additionally indicate a chlorine isotope ratio of 35Cl/37Cl = 2.1 ± 0.8 in CRL 2688, suggestive of s-process enhancement of chlorine 37.