T. Wong (ATNF); F. L. Schoeier (Leiden Obseratory, The Netherlands & Stockholm Observatory, Sweden); M. Lindqvist, H. Olofsson (Stockholm Observatory, Sweden)

One of the first targets for the Compact Array 3-mm system was the carbon-rich red giant star R Sculptoris, which is believed to be encircled by a detached shell of molecular gas and dust. For a dying star to be losing mass in a slow stellar wind is quite typical, but the presence of a thin detached shell indicates that at some point in the past there was a sudden increase in the rate of mass loss, perhaps due to the sudden onset of helium shell burning. By observing molecular emission in the shell at high angular resolution, one can determine not only the expansion velocity of the shell, but also the time scale of the mass-loss variation (related to the shell thickness) and the degree of isotropy of the wind (related to the symmetry of the shell). Notwithstanding their scientific interest, circumstellar shells also make ideal test sources for the current 3-mm system because they are relatively simple structures of small angular size (compared, for instance, to molecular clouds).

Although the previous single-dish observations that had indicated the presence of a shell around R Scl were made in a CO emission line with the Swedish-ESO Submillimetre Telescope (SEST), the Compact Array's restricted frequency range led us to observe the HCN (J=1-0) line at 88.6 GHz. We observed R Scl in several compact configurations of three antennas, with baselines ranging from 31-413 m, between June and October 2002. We found that the HCN emission is extremely compact, with a Gaussian full width at half maximum (FWHM) of about one arcsecond, so that even our high-resolution imaging only partially resolves the source (Figure 1). The total Compact Array flux is consistent with the SEST flux within the calibration uncertainties. Thus, it appears that the HCN emission is associated with an attached envelope resulting from recent mass loss rather than the older (and hence larger) detached shell inferred from the CO data. The difference in geometry is corroborated by higher-transition, single-dish HCN spectra, which indicate that the HCN envelope is expanding more slowly than the bulk of the CO emission. The lack of HCN in the detached shell is most likely due to its photodissociation into CN, as has been observed in several other sources.

A detailed radiative transfer code, based on the Monte Carlo method, has been used to jointly model the Compact Array and single-dish HCN data. The main inputs to the model are the current mass-loss rate (in solar masses per year) and the stellar radiation field. With reasonable assumptions for these, we have calculated the best-fit values for the HCN abundance (relative to H₂) and envelope size, using the HCN data (both single-dish and interferometer) as constraints. The best-fit envelope size is about 1,000 AU, whereas modelling the Compact Array data alone yields a size of only 300 AU (comparable to the size of the emission region in our high-resolution maps). Thus a single set of model parameters cannot simultaneously fit both the single-dish and interferometer data. Possibly the high optical depth of the HCN lines makes the emission very sensitive to changes in the physical conditions of the inner envelope (such as clumping in the mass distribution) that are not accounted for in our model.

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