1997A&A...324...51W

New high quality absorption spectra of J=1←0 HCO⁺, HCN, HNC and the J=2←1 line of CS towards the center of the radio core of Cen A (NGC5128) are presented. In addition the absorption profile of the J=1←0 line of H¹³CO⁺ has been detected for the first time, revealing that most absorption features have low opacities. The new HCO⁺ spectrum allows a comparison with results obtained more than 7 years earlier. No significant change in the spectrum is found. From this remarkable result, constraints can be put on the minimum size of the radio source in the millimeter range (>500AU). A comparison of abundance ratios between different absorption components suggests that the absorbing gas is essentially low density gas, with both low excitation and kinetic temperatures. Relative molecular abundances are compatible with those of the Milky Way. A parametrization of the high signal-to-noise spectrum of HCO⁺ is presented for future reference when looking for spectral changes.