Observational Evidence of the Formation of Cyanopolyynes in CRL 618 through the Polymerization of HCN

Abstract

The abundance ratio of consecutive members of the cyanopolyynes family has been explored in CRL618 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 the following: 1 to 4 for HCN and HNC, 9 to 39 for HC 3 N, 31 to 133 for HC 5 N, and 72 to 85 for HC 7 N (not detected beyond J up =85). The lowest vibrationally excited state of HC 7 N (ν 15 at 62 cm −1 ) has been tentatively detected. Data analysis has been performed by extending our previous geometrical and radiative transfer model of the slowly expanding envelope (SEE) surrounding the compact central continuum source of CRL 618, that was established from the study of rotational lines in several vibrationally excited states of HC 3 N. The new lines analyzed here require to model the high velocity wind (HVW) component and the colder circumstellar gas, remnant of the AGB phase of CRL618. The derived HC 3 N/HC 5 N and HC 5 N/HC 7 N abundance ratios from this set of uniformly calibrated lines are between ∼ 3 and 6 in the different regions, similar to standard values in the CSM and ISM, and consistent with previous estimates obtained from ISO observations and chemical models. However, the abundance ratios of HC 3 N, HC 5 N and HC 7 N with respect to HCN are at least two 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 H 2 ).

The chemistry in circumstellar envelopes of evolved stars: Following the origin of the elements to the origin of life

“…23, 26, 27, and 47). In CRL 618, the chemistry is dramatically altered by UV radiation, indicated by appearance of ions (HCO ϩ ) and an enhancement of the cyanopolyyne chains (48). These PPN are the only other sources of metal-containing species, with the exception of one O-rich star (see below).…”

A Spectral Line Survey of NGC 7027 at Millimeter Wavelengths

“…HC 3 N HC 3 N is commonly observed in AGB stars and proto-PNs (e.g., Cernicharo et al 2000;Pardo et al 2004). Pardo et al (2005) found that HC 3 N is quickly reprocessed from HCN in PPN stage. Prior to this work, HC 3 N has never been detected in PNs and its absence is often explained as the result of efficient photodissociation due to the strong UV radiation in PNs.…”

Molecular line emission from planetary and protoplanetary nebulae

“…See e.g. Cernicharo et al (2001), Cernicharo (2004), Pardo et al (2005) and references therein; there is also a very interesting poster in this symposium on this source (Pardo et al). Models explain the presence of all these molecules assuming photoinduced chemistry.…”