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Astron. Astrophys. 329, 1156-1169 (1998)

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Observations of 13C isotopomers of HC3N and HC5N in TMC-1: evidence for isotopic fractionation

Shuro Takano 1, Akimasa Masuda 2, Yasuhiro Hirahara 3, Hiroko Suzuki 4 , * , **, Masatoshi Ohishi 4 , **, Shin-ichi Ishikawa 4 , **, Norio Kaifu 5, Yasuko Kasai 6, Kentarou Kawaguchi 4 , ** and T.L. Wilson 7

1 I. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, D-50937 Köln, Germany
2 Department of Chemistry, University of Electro-Communications, Chofugaoka, Chofu, Tokyo 182, Japan
3 Department of Earth and Planetary Sciences, School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-01, Japan
4 Nobeyama Radio Observatory (NRO), Minamimaki-mura, Minamisaku-gun, Nagano 384-13, Japan
5 National Astronomical Observatory, Osawa, Mitaka, Tokyo 181, Japan
6 The Institute of Physical and Chemical Research (RIKEN), Wako, Saitama 351-01, Japan
7 Max Planck Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

Received 4 February 1997 / Accepted 15 July 1997

Abstract

The 13 C substitutions of HC3 N (H13 CCCN, HC13 CCN, and HCC13 CN) were observed in TMC-1 using the [FORMULA] and 5-4 rotational transitions at 18, 36, and 45 GHz, respectively. The spectral lines of HCC13 CN are stronger than those of HC13 CCN in all observed transitions, while the spectral lines of HC13 CCN and H13 CCCN show similar intensity. These differences in the intensities are most probably due to 13 C isotopic fractionation in the formation process of HC3 N. The abundance ratios are 1.0:1.0:1.4 for [H13 CCCN] : [HC13 CCN] : [HCC13 CN] at the cyanopolyyne peak in TMC-1: the 13 C isotope is concentrated in a carbon atom adjacent to the nitrogen atom. Based on these observational results, the production mechanism of HC3 N was discussed. As a result, the formation reactions between a hydrocarbon molecule with two carbon atoms (e.g. C2 H2) and a molecule with a 13 C enriched CN group can explain 13 C isotopic fractionation: a neutral-neutral reaction between C2 H2 and CN is probably most important. The ratio of the contributions of two types of the HC3 N formation reactions which can and cannot produce 13 C isotopic fractionation is discussed. In addition, the 13 C isotopic species of HC5 N (HC13 CCCCN and HCCCC13 CN) were also observed at the cyanopolyyne peak in TMC-1 using the [FORMULA] rotational transitions at 23.7 GHz. The intensity of HCCCC13 CN is marginally stronger than that of HC13 CCCCN. Furthermore, the [FORMULA] emission of NH3 was simultaneously observed with the 13 C isotopic species of HC5 N. With the additional observation of the [FORMULA] 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.

Key words: molecular processes – ISM: abundances – ISM: molecules – ISM: individual objects: TMC-1 – Radio lines: ISM

* Deceased in November 1987.
** NRO, a branch of the National Astronomical Observatory, an inter-university research institute operated by the Ministry of Education, Science, and Culture. Part of this work was carried out under the common use observation program at NRO.

Send offprint requests to: S. Takano

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© European Southern Observatory (ESO) 1998

Online publication: December 16, 1997
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