Abundance Anomaly of the $^{13}$C Isotopic Species of c-C$_3$H$_2$ in
  the Low-Mass Star Formation Region L1527
  The rotational spectral lines of c-C$_3$H$_2$ and two kinds of the $^{13}$C
isotopic species, c-$^{13}$CCCH$_2$ ($C_{2v}$ symmetry) and c-CC$^{13}$CH$_2$
($C_s$ 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-C$_3$H$_2$,
c-$^{13}$CCCH$_2$ , and c-CC$^{13}$CH$_2$, respectively, with the Nobeyama 45 m
telescope, and 34, 6, and 13 lines, respectively, with the IRAM 30 m telescope,
where 7, 2, and 2 transitions, respectively, are observed with the both
telescopes. With these data, we have evaluated the column densities of the
normal and $^{13}$C isotopic species. The [c-C$_3$H$_2$]/[c-$^{13}$CCCH$_2$]
ratio is determined to be $310\pm80$, while the
[c-C$_3$H$_2$]/[c-CC$^{13}$CH$_2$] ratio is determined to be $61\pm11$. The
[c-C$_3$H$_2$]/[c-$^{13}$CCCH$_2$] and [c-C$_3$H$_2$]/[c-CC$^{13}$CH$_2$]
ratios expected from the elemental $^{12}$C/$^{13}$C 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-C$_3$H$_2$. Hence, this observation
further confirms the dilution of the $^{13}$C species in carbon-chain molecules
and their related molecules, which are thought to originate from the dilution
of $^{13}$C$^+$ in the gas-phase C$^+$ due to the isotope exchange reaction:
$\mathrm{^{13}C^++CO\rightarrow{}^{13}CO+C^+}$. Moreover, the abundances of the
two $^{13}$C isotopic species are different from each other. The ratio of
c-$\mathrm{^{13}CCCH_2}$ species relative to c-$\mathrm{CC^{13}CH_2}$ is
determined to be $0.20\pm0.05$. If $^{13}$C were randomly substituted for the
three carbon atoms, the [c-$\mathrm{^{13}CCCH_2}$]/[c-$\mathrm{CC^{13}CH_2}$]
ratio would be 0.5. Hence, the observed ratio indicates that
c-$\mathrm{CC^{13}CH_2}$ exists more favorably. Possible origins of the
different abundances are discussed.
