Australia Telescope Compact Array imaging of circumstellar HCN line emission from R Scl

We present radio-interferometric observations of HCN J = 1 → 0 line emission from the carbon star R Scl, obtained with the interim 3-mm receivers of the Australia Telescope Compact Array. The emission is resolved into a central source with a Gaussian FWHM of ∼1″, which we identify as the present mass loss envelope. Using a simple photodissociation model and constraints from single-dish HCN spectra, we argue that the present mass-loss rate is low, ∼2 × 10^-7 M_⊙ yr^-1, supporting the idea that R Scl had to experience a brief episode of intense mass loss in order to produce the detached CO shell at ∼10″ radius inferred from single-dish observations. Detailed radiative transfer modelling yields an abundance of HCN relative to H₂, f_HCN, of ∼10^-5 in the present-day wind. There appears to be a discrepancy between model results obtained with higher transition single-dish data included and those from the J = 1 → 0 interferometer data alone, in that the interferometer data suggest a smaller envelope size and larger HCN abundance than the single-dish data. The lack of HCN in the detached shell, f _HCN ≲ 2 × 10^-7, is consistent with the rapid photodissociation of HCN into CN as it expands away from the star.