EDP Sciences
Free access
Volume 413, Number 1, January I 2004
Page(s) 241 - 249
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20031487

A&A 413, 241-249 (2004)
DOI: 10.1051/0004-6361:20031487

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

T. Wong1, F. L. Schöier2, 3, M. Lindqvist4 and H. Olofsson3

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

(Received 9 May 2003 / Accepted 19 September 2003 )

We present radio-interferometric observations of HCN $J=1\rightarrow0$ 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, ${\sim} 2\, \times\, 10^{-7}~M_\odot$ 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 H 2 fHCN, of  ${\sim}10^{-5}$ 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 $J=1\rightarrow0$ 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, $f_{\mathrm{HCN}}\la 2\times 10^{-7}$, is consistent with the rapid photodissociation of HCN into CN as it expands away from the star.

Key words: circumstellar matter -- stars: carbon -- stars: AGB and post-AGB -- stars: mass-loss

Offprint request: T. Wong, Tony.Wong@csiro.au

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