Evidence for global pressure oscillations on Procyon

Precise Doppler measurements of the star Procyon (alpha CMi, HR 2943) have been obtained with the ELODIE fiber-fed cross-dispersed echelle spectrograp h on the 1.93 m telescope at Observatoin de Haute Provence. Here, we presen t the analysis of data from 10 days observing run carried out in November 1 998. We detect significant excess in the power between 0.5-1.5 mHz in the p eriodograms of the time series of mean Doppler shifts. Observations of eta Cas made with the same instrument during the same time interval and in almo st identical night conditions show a flat spectrum in this frequency range, indicating that the excess of Doppler signal seen on Procyon is of stellar origin. When data from the whole run are jointly analyzed, a period analys is places an upper limit of 0.500.60 ms(-1) for the amplitude of oscillatio ns, while the frequency cutoff is around 1.5 mHz. The power evidently drops near 0.55 and 1.5 mHz on the average of unfiltered power spectra of indivi dual nights, which is consistent with the expected p-mode oscillation prope rties for Procyon. Several equispaced peaks in frequency are recurrent in t he power spectra of two independent segments of 4 and 3 contiguous nights; the most probable frequency spacing seems to be 55 mu Hz. In conclusion, we now have an instrument set-up which is sufficiently stable and fast to be used for a multi-site campaign involving instruments with comparable veloci ty precisions, to detect the oscillation modes of sun-like stars.