Evidence for global pressure oscillations on Procyon

Precise Doppler measurements of the star Procyon (αCMi, HR 2943) have been obtained with the ELODIE fiberfed cross-dispersed echelle spectrograph on the 1.93 m telescope at Observatoire de Haute Provence. Here, we present the analysis of data from 10 days observing run carried out in November 1998. We detect significant excess in the power between 0.5–1.5 mHz in the periodograms of the time series of mean Doppler shifts. Observations of η Cas made with the same instrument during the same time interval and in almost 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 analysis places an upper limit of 0.50– 0.60 ms−1 for the amplitude of oscillations, 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 individual nights, which is consistent with the expected p-mode oscillation properties for Procyon. Several equispaced peaks in frequency are recurrent in the power spectra of two independent segments of 4 and 3 contiguous nights; the most probable frequency spacing seems to be 55μHz. 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 velocity precisions, to detect the oscillation modes of sun-like stars.