(HCN)-C-12 and (HCN)-C-13 excitation analysis in the circumstellar outflow of R Sculptoris

Journal article, 2017

The (CO)-C-12/(CO)-C-13 isotopologue ratio in the circumstellar envelope (CSE) of asymptotic giant branch (AGB) stars has been extensively used as the tracer of the photospheric C-12/C-13 ratio. However, spatially-resolved ALMA observations of R Scl, a carbon rich AGB star, have shown that the (CO)-C-12/(CO)-C-13 ratio is not consistent over the entire CSE. Hence, it can not necessarily be used as a tracer of the C-12/C-13 ratio. The most likely hypothesis to explain the observed discrepancy between the (CO)-C-12/(CO)-C-13 and C-12/C-13 ratios is CO isotopologue selective photodissociation by UV radiation. Unlike the CO isotopologue ratio, the HCN isotopologue ratio is not a ff ected 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 (HCN)-C-13(4-3) and APEX observations of (HCN)-C-12(2-1), (HCN)-C-13(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 (HCN)-C-12(J = 1-0, 2-1, 3-2, 4-3) and (HCN)-C-13(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 (HCN)-C-13(4-3) ALMA observations. Results. We find fractional abundances of (HCN)-C-12/H2 = (5.0 +/- 2.0) x 10(-5) and (HCN)-C-13/H-2 = (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 (HCN)-C-12/(HCN)-C-13 = 26.3 +/- 11.9 is consistent with the photospheric ratio of C-12/C-13 similar to 19 +/- 6. Conclusions. We show that the circumstellar (HCN)-C-12/(HCN)-C-13 ratio traces the photospheric C-12/C-13 ratio. Hence, contrary to the (CO)-C-12/(CO)-C-13 ratio, the (HCN)-C-12/(HCN)-C-13 ratio is not a ff ected 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 (CO)-C-12/(CO)-C-13 and (CO)-C-12/(CO)-C-13 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.