MOLECULAR ABSORPTION AND ITS TIME VARIATIONS IN CENTAURUS-A

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 (NGC 5128) are presented. In addition the absorption profile of the J=1<--0 line of H-13 CO+ has been detected for the first time, revealing that most absorption features have low opacities. The new HCO+ spectrum al lows a comparison with results obtained more than 7 years earlier No s ignificant change in the spectrum is found. From this remarkable resul t, constraints can be put on the minimum size of the radio source in t he millimeter range (>500 AU). A comparison of abundance ratios betwee n different absorption components suggests that the absorbing gas is e ssentially low density gas, with both low excitation and kinetic tempe ratures. Relative molecular abundances are compatible with those of th e Milky Way. A parametrization of the high signal-to-noise spectrum of HCO+ is presented for future reference when looking for spectral chan ges.