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-¹³CCCH₂ (C_2v symmetry) and c-CC¹³CH₂ (C_s 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-¹³CCCH₂, and c-CC¹³CH₂, 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-¹³CCCH₂] ratio is determined to be 310 ± 80, while the [c-C₃H₂]/[c-CC¹³CH₂] ratio is determined to be 61 ± 11. The [c-C₃H₂]/[c-¹³CCCH₂] and [c-C₃H₂]/[c-CC¹³CH₂] 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: ¹³C⁺ + CO → ¹³CO + C⁺. Moreover, the abundances of the two ¹³C isotopic species are different from each other. The ratio of c-¹³CCCH₂ species relative to c-CC¹³CH₂ is determined to be 0.20 ± 0.05. If ¹³C were randomly substituted for the three carbon atoms, the [c-¹³CCCH₂]/[c-CC¹³CH₂] ratio would be 0.5. Hence, the observed ratio indicates that c-CC¹³CH₂ exists more favorably. Possible origins of the different abundances are discussed.