2005ApJ...628..275P

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 J_up 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 J_up=85). The lowest vibrationally excited state of HC₇N (ν₁₅at 62/cm) 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 (C_2n_H2_).