Abundance Anomaly of the 13C Isotopic Species of c-C3H2 in the Low-mass Star Formation Region L1527

The rotational spectral lines of c-C3H2 and two kinds of the 13C isotopic species, c- ( symmetry) and c- (Cs symmetry), have been observed in the 1–3 mm band toward the low-mass star-forming region L1527. We have detected 7, 3, and 6 lines of c-C3H2, c-, and c-, respectively, with the Nobeyama 45 m telescope and 34, 6, and 13 lines, respectively, with the IRAM 30 m telescope, where seven, two, and two transitions, respectively, are observed with both telescopes. With these data, we have evaluated the column densities of the normal and 13C isotopic species. The [c-C3H2]/[c-] ratio is determined to be 310 ± 80, while the [c-C3H2]/[c-] ratio is determined to be 61 ± 11. The [c-C3H2]/[c-] and [c-C3H2]/[c-] ratios expected from the elemental 12C/13C ratio are 60–70 and 30–35, respectively, where the latter takes into account the statistical factor of 2 for the two equivalent carbon atoms in c-C3H2. Hence, this observation further confirms the dilution of the 13C species in carbon-chain molecules and their related molecules, which are thought to originate from the dilution of 13C+ in the gas-phase C+ due to the isotope exchange reaction: . Moreover, the abundances of the two 13C isotopic species are different from each other. The ratio of c- species relative to c- is determined to be 0.20 ± 0.05. If 13C were randomly substituted for the three carbon atoms, the [c-]/[c-] ratio would be 0.5. Hence, the observed ratio indicates that c- exists more favorably. Possible origins of the different abundances are discussed.