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

¹ CSIRO Australia Telescope National Facility, PO Box 76, Epping NSW 1710, Australia
² Sterrewacht Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
³ Stockholm Observatory, AlbaNova, 106 91 Stockholm, Sweden
⁴ Onsala Space Observatory, 439 92 Onsala, Sweden

Corresponding author: T. Wong, Tony.Wong@csiro.au

Received: 9 May 2003
Accepted: 19 September 2003

Abstract

We present radio-interferometric observations of HCN 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,  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, , of  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 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, , is consistent with the rapid photodissociation of HCN into CN as it expands away from the star.