Anomalous ~(13)C isotope abundances in C_3S and C _4H observed toward the cold interstellar cloud, taurus molecular cloud-1

摘要

We have studied the abundances of the ~(13)C isotopic species of C_3S and C_4H in the cold molecular cloud, Taurus Molecular Cloud-1 (Cyanopolyyne Peak), by radioastronomical observations of their rotational emission lines. The CCCS/~(13)CCCS and CCCS/C~(13)CCS ratios are determined to be >206 and 48 ± 15, respectively. The CC~(13)CS line is identified with the aid of laboratory microwave spectroscopy, and the range of the CCCS/CC~(13)CS ratio is found to be from 30 to 206. The abundances of at least two ~(13)C isotopic species of C_3S are thus found to be different. Similarly, it is found that the abundances of the four ~(13)C isotopic species of C_4H are not equivalent. The CCCCH/~(13)CCCCH, CCCCH/C~(13)CCCH, CCCCH/CC~(13)CCH, and CCCCH/CCC~(13)CH ratios are evaluated to be 141 ± 44, 97 ± 27, 82 ± 15, and 118 ± 23, respectively. Here the errors denote 3 times the standard deviation. These results will constrain the formation pathways of C_3S and C _4H, if the nonequivalence is caused during the formation processes of these molecules. The exchange reactions after the formation of these two molecules may also contribute to the nonequivalence. In addition, we have confirmed that the ~(12)C/~(13)C ratio of some species are significantly higher than the interstellar elemental ~(12)C/ ~(13)C ratio of 60-70. The observations of the ~(13)C isotopic species provide us with rich information on chemical processes in cold interstellar clouds.