  Abridged. The 12CO/13CO ratio in the circumstellar envelope (CSE) of
asymptotic giant branch (AGB) stars has been extensively used as the tracer of
the photospheric 12C/13C ratio. However, spatially-resolved ALMA observations
of R Scl, a carbon rich AGB star, have shown that the 12CO/13CO ratio is not
consistent over the entire CSE. Hence, it can not necessarily be used as a
tracer of the 12C/13C ratio. The most likely hypothesis to explain the observed
discrepancy between the 12CO/13CO and 12C/13C ratios is CO isotopologue
selective photodissociation by UV radiation. Unlike the CO isotopologue ratio,
the HCN isotopologue ratio is not affected by UV radiation. Therefore, HCN
isotopologue ratios can be used as the tracer of the atomic C ratio in UV
irradiated regions. We have performed a detailed non-LTE excitation analysis of
circumstellar H12CN and H13CN line emission around R Scl, observed with ALMA
and APEX, using a radiative transfer code, ALI. The spatial extent of the
molecular distribution for both isotopologues is constrained based on the
spatially resolved H13CN(4-3) ALMA observations. We find fractional abundances
of H12CN/H2 = (5.0 +\- 2.0) x 10^{-5} and H13CN/H2 = (1.9 +\- 0.4) x 10^{-6} in
the inner wind (r < (2.0 +\- 0.25) x 10^{15} cm) of R Scl. The derived
circumstellar isotopologue ratio of H12CN/H13CN = 26.3 +\- 11.9 is consistent
with the photospheric ratio of 12C/13C ~ 19 \pm 6. We show that the
circumstellar H12CN/H13CN ratio traces the photospheric 12C/13C ratio. These
results support the previously proposed explanation that CO isotopologue
selective-shielding is the main factor responsible for the observed discrepancy
between 12C/13C and 12CO/13CO ratios in the inner CSE of R Scl. This indicates
that UV radiation impacts on the CO isotopologue ratio.
