Detection of C8H- and Comparison with C8H toward IRC+10216

We report the detection of new transitions of octatetraynyl (C8H) toward the circumstellar envelope IRC +10 216 using data taken with the 100 m Green Bank Telescope (GBT). In addition, we report five features from the Ku, K, and Q bands that have been identified as transitions of the octatetraynyl anion (C8H-). From a rotational temperature diagram and an assumed source size of 30", we find a total C8H column density of 8(3)×10^12 cm-2 and a rotational temperature of ~13 K. From the five detected transitions of C8H-, we find a total C8H- column density of ~2.1×10^12 cm-2 consistent with a rotational temperature of ~34 K for a total C8H/C8H- column density ratio of ~3.8. This observed C8H/C8H- column density ratio is similar to the theoretical prediction of 3.6, while the observed column densities were lower than that predicted by a factor of ~30. This prompted us to reinvestigate the initial conditions of the circumstellar envelope (CSE) model. The new model results are presented, and they more closely match the C8H and C8H- abundances observed with the GBT. Finally, we use the new CSE model results to predict the abundance of decapentaynyl (C10H), and we compare them with the measured upper limit found from the GBT observations.

Density-Enhanced Gas and Dust Shells in a New Chemical Model for IRC+10216

A new chemical model is presented for the carbon-rich circumstellar envelope (CSE) of the asymptotic giant branch star IRC+10216. The model includes shells of matter with densities that are enhanced relative to the surrounding circumstellar medium. The chemical model uses an updated reaction network including reactions from the RATE06 database and a more detailed anion chemistry. In particular, new mechanisms are considered for the formation of CN-, C3N-, and C2H-, and for the reactions of hydrocarbon anions with atomic nitrogen and with the most abundant cations in the CSE. New reactions involving H- are included which result in the production of significant amounts of C2H- and CN- in the inner envelope. The calculated radial molecular abundance profiles for the hydrocarbons C2H, C4H, and C6H and the cyanopolyynes HC3N and HC5N show narrow peaks which are in better agreement with observations than previous models. Thus, the narrow rings observed in molecular microwave emission surrounding IRC+10216 are interpreted as arising in regions of the envelope where the gas and dust densities are greater than the surrounding circumstellar medium. Our models show that CN- and C2H- may be detectable in IRC+10216 despite the very low theorized radiative electron attachment rates of their parent neutral species. We also show that magnesium isocyanide (MgNC) can be formed in the outer envelope through radiative association involving Mg+ and the cyanopolyyne species.

Large molecules in the envelope surrounding IRC+10216

A new chemical model of the circumstellar envelope surrounding the carbon-rich star IRC+10216 is developed that includes carbon-containing molecules with up to 23 carbon atoms. The model consists of 3851 reactions involving 407 gas-phase species. Sizeable abundances of a variety of large molecules - including carbon clusters, unsaturated hydrocarbons and cyanopolyynes - have been calculated. Negative molecular ions of chemical formulae C-n(-) and CnH- (7 less than or equal to n less than or equal to 23) exist in considerable abundance, with peak concentrations at distances from the central star somewhat greater than their neutral counterparts. The negative ions might be detected in radio emission, or even in the optical absorption of background field stars. The calculated radial distributions of the carbon-chain CnH radicals are looked at carefully and compared with interferometric observations.

A NEW CHEMICAL-MODEL OF THE CIRCUMSTELLAR ENVELOPE SURROUNDING IRC+10216

A new chemical model of the circumstellar envelope surrounding the carbon-rich star IRC+10216 has been developed. This model incorporates a variety of newly measured rapid neutral-neutral reactions between carbon atoms and hydrocarbons and between the radical CN and a variety of stable neutral molecules. In addition, other neutral-neutral reactions in the above two classes or involving atoms such as N or radicals such as C(2n)H have been included with large rate coefficients although they have not yet been studied in the laboratory. Unlike the interstellar case, where the inclusion of these neutral-neutral reactions destroys molecular complexity, our model results for IRC+10216 show that sufficient abundances of large hydrocarbon radicals and cyanpolyynes can be produced to explain observations. We also discuss the formation of H2CN and NH2CN, two potentially observable molecules in IRC+10216.