Observational Evidence of the Formation of Cyanopolyynes in CRL 618 through the Polymerization of HCN
Résumé
The abundance ratio of consecutive members of the cyanopolyyne family has been explored in CRL 618 using data acquired in a complete line survey covering the frequency range 81-356 GHz. The Jup range explored for the different molecules is 1-4 for HCN and HNC, 9-39 for HC3N, 31-133 for HC5N, and 72-85 for HC7N (not detected beyond Jup=85). The lowest vibrationally excited state of HC7N (nu15 at 62 cm-1) has been tentatively detected. Data analysis has been performed by extending our previous geometric and radiative transfer model of the slowly expanding envelope (SEE) surrounding the compact central continuum source of CRL 618, which was established from the study of rotational lines in several vibrationally excited states of HC3N. The new lines analyzed here require modeling of the high-velocity wind (HVW) component and the colder circumstellar gas, a remnant of the AGB phase of CRL 618. The derived HC3N/HC5N and HC5N/HC7N abundance ratios from this set of uniformly calibrated lines are between ~3 and 6 in the different regions, similar to standard values in the circumstellar and interstellar media and consistent with previous estimates obtained from ISO observations and chemical models. However, the abundance ratios of HC3N, HC5N, and HC7N with respect to HCN are at least 2 orders of magnitude larger than those typical for AGB C-rich stars, such as IRC +10216. This fact indicates that in the short transition toward the planetary nebula phase, HCN is quickly reprocessed into longer cyanopolyyne chains. A similar behavior was previously found in this object for the polyacetylenic chains (C2nH2).