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Astron. Astrophys. 324, 51-64 (1997)

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6. Summary

We have obtained new high signal-to-noise molecular absorption spectra towards the Centaurus A radio core. This has allowed the identification of up to 17 components in the [FORMULA] spectrum. The components associated with the systemic velocity (LV) correspond to intervening material in the accreted gaseous disk of Cen A, well outside the circumnuclear ring in non-circular motions. It is not possible at present to derive their exact distances from the center. The high velocity (HV) redshifted components are interpreted in terms of a nuclear ring, at about 100 pc distance, with non-circular motions. The [FORMULA] spectrum indicate that only the central component has relatively moderate opacities, while the HV components have low opacities. From the comparison between the [FORMULA], HCN, HNC and CS spectra, we deduce that the absorbing gas is diffuse and cold on average. Abundances are compatible with Galactic values.

Comparison with the [FORMULA] spectrum obtained 7 years ago by Eckart et al. (1990) revealed no time variations at a level [FORMULA] 10%. Changes were expected from similar Galactic experiments. We therefore suggest that this constrains the apparent size of the mm continuum source to be larger than 500 AU (or [FORMULA] 0.2mas), unless the absorbing gas is made up of a very large number of very small (a few tens of AU) clumps. In the latter case variations tend to average out over time. The core at 8.4 GHz appears as a point source ([FORMULA] 2mas) in VLBI experiments (Jones et al. 1996). The source size is therefore constrained between 0.2 and 2 mas. The larger size might be only apparent, enlarged through interstellar scattering by the ionised gas in Cen A itself.

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© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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