Observations of ^(13)C isotopomers of HC_(3)N and HC_(5)N in TMC-1: evidence for isotopic fractionation

Details Observations of ^(13)C isotopomers of HC_(3)N and HC_(5)N in TMC-1: evidence for isotopic fractionation Observations of ^(13)C isotopomers of HC_(3)N and HC_(5)N in TMC-1: evidence for isotopic fractionation

Jan 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998a%26a...329.1156t&link_type=abstract

Astronomy and Astrophysics, v.329, p.1156-1169 (1998)

Astronomy and Astrophysics

Astrophysics

28

Molecular Processes, Ism: Abundances, Ism: Molecules, Ism: Individual Objects: Tmc-1, Radio Lines: Ism

Scientific paper

The 13C substitutions of HC3N (H13CCCN, HC13CCN, and HCC13CN) were observed in TMC-1 using the J=2-1,; 4-3, and 5-4 rotational transitions at 18, 36, and 45GHz, respectively. The spectral lines of HCC13CN are stronger than those of HC13CCN in all observed transitions, while the spectral lines of HC13CCN and H13CCCN show similar intensity. These differences in the intensities are most probably due to 13C isotopic fractionation in the formation process of HC3N. The abundance ratios are 1.0:1.0:1.4 for [H13CCCN] : [HC13CCN] : [HCC13CN] at the cyanopolyyne peak in TMC-1: the 13C isotope is concentrated in a carbon atom adjacent to the nitrogen atom. Based on these observational results, the production mechanism of HC3N was discussed. As a result, the formation reactions between a hydrocarbon molecule with two carbon atoms (e.g. C2H2) and a molecule with a 13C enriched CN group can explain 13C isotopic fractionation: a neutral-neutral reaction between C2H2 and CN is probably most important. The ratio of the contributions of two types of the HC3N formation reactions which can and cannot produce 13C isotopic fractionation is discussed. In addition, the 13C isotopic species of HC5N (HC13CCCCN and HCCCC13CN) were also observed at the cyanopolyyne peak in TMC-1 using the J=9-8 rotational transitions at 23.7GHz. The intensity of HCCCC13CN is marginally stronger than that of HC13CCCCN. Furthermore, the (J,K)=(2,2) emission of NH3 was simultaneously observed with the 13C isotopic species of HC5N. With the additional observation of the (J,K)=(1,1) emission of NH3, the rotational temperature between the (1,1) and (2,2) levels, and the column density of NH3 are determined for the cyanopolyyne peak in TMC-1.

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