Molecular line observations of southern main-sequence stars with dust disks: alpha Ps A, beta Pic, epsilon Eri and HR 4796 A - Does the low gas content of the beta Pic and epsilon Eri disks hint at the presence of planets?

The results of molecular line observations with the 15 m SESTstar Of southe rn Vega-excess stars are presented. The stars alpha Ps A, epsilon Eri and H R 4796 A were observed in the CO (1-0) and (2-1) lines and beta Pic was obs erved in the vibrational ground state of SiO, in the (2-1) and (5-4) transi tions. In spite of considerably more sensitive observations than in previou s attempts, none of these systems was detected with the SEST. We use theoretical models of stellar atmospheres, of the structure and chem istry of interface regions (PDRs) and of molecular excitation in Keplerian disks of gas and dust to analyze these observational results. Among the obs erved objects, the K2 V star epsilon Eri appears particularly suitable and the analysis focusses on this system. A disk model with simple geometry is capable of explaining recent dust continuum observations. Applying this mod el to the associable molecular gas leads to the conclusion that it is most likely that the disk/ring around epsilon Eri is largely devoid of any gas ( m(gas)/m(dust) less than 10(-3) of the interstellar value), presumably due to consumption during planetary system formation. We propose that e Eri sho uld be a prime candidate for searches for extrasolar planets. In the beta Pic disk, the gas content may be as low, or even lower, as for epsilon Eri which could be taken as indirect support of the suggested exist ence of a planetary system associated with this star.