2017A&A...599A..63S

Context. The ¹²CO/¹³CO isotopologue 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.
Aims. We aim to present ALMA observations of H¹³CN (4-3) and APEX observations of H¹²CN (2-1), H¹³CN (2-1, 3-2) towards R Scl. These new data, combined with previously published observations, are used to determine abundances, ratio, and the sizes of line-emitting regions of the aforementioned HCN isotopologues.
Methods. We have performed a detailed non-LTE excitation analysis of circumstellar H¹²CN (J=1-0, 2-1, 3-2, 4-3) and H¹³CN (J=2-1, 3-2, 4-3) line emission around R Scl using a radiative transfer code based on the accelerated lambda iteration (ALI) method. The spatial extent of the molecular distribution for both isotopologues is constrained based on the spatially resolved H¹³CN (4-3) ALMA observations.
Results. We find fractional abundances of H¹²CN/H₂=(5.0±2.0)x10^–5 and H¹³CN/H₂=(1.9±0.4)x10^–6 in the inner wind (r≤(2.0±0.25) x10¹⁵cm) 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±6.
Conclusions. We show that the circumstellar H¹²CN/H¹³CN ratio traces the photospheric ¹²C/¹³C ratio. Hence, contrary to the ¹²CO/¹³CO ratio, the H¹²CN/H¹³CN ratio is not affected by UV radiation. 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. This study shows how important is to have high-resolution data on molecular line brightness distribution in order to perform a proper radiative transfer modelling.