Other
Scientific paper
May 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011iaus..280e..21g&link_type=abstract
The Molecular Universe, Proceedings of the 280th Symposium of the International Astronomical Union held in Toledo, Spain, May 30
Other
Scientific paper
Following the spectral characterization of C-chain anions, in the Harvard microwave laboratory, we have discovered 6 anions in the envelope of the C-rich star IRC+10216 and set limits to the abundance of two others. The anions we studied in this source range from CCH- to C8H- and from CN- to C5N- [1,2,3]. Anion abundances are low and the rotational lines weak, even in favorable cases. Two years ago, we have been able to map C6H- in IRC+10216, with the PdB interferometer, and to show that it is located, like the C6H radical, in a hollow shell. The angular resolution of the observations (5) and the S/N ratio were however insufficient to spatially differentiate the anion from the radical. Taking advantage of the recent installation of a wide-band correlator on PdBI, we have re-observed with a twice higher resolution the 83 GHz line of C6H- , as well as nearby lines of C5N-, C4H-, C2H- and of several C-chain molecules and radicals. Except for CCH-, all those species are detected and we are currently analyzing their distributions and abundances. Chemistry models, based on the electron radiative attachment theory developed by Herbst and collaborators, predict that anions and parent radicals peak at different radii in the expanding CSE[4]. Predicted radial shifts range from a few arcsec, for C6H-, to more than 10 (500 yr) for C4H- and CN-. We know, from previous single-dish observations, that the actual abundances of C4H- and CN- , relative to C4H and CN, differ by orders of magnitude from radiative attachment model predictions, while those of the long anions, C6H- and C8H agree well with predictions. The new interferometric observations, allied to single-dish data, allow us to measure the formation/destruction time of the different species and to set constraints on the rates of electron radiative attachment, rates which, so far, are only crudely evaluated through the statistical theory. The relatively high abundances of CN-, C3N - and C5N - , compared to CCH-, C4H - and C6H-, suggest that other routes, such as the reaction of N atoms with large C-chain anions, may proceed efficiently.
Agúndez Marcelino
Cernicharo Jose
Gottlieb C.
Guélin Michel
McCarthy Martin
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