Detection of possible p-mode oscillations on procyon

In the course of a search for solar-like oscillations in bright late-type stars, we have observed Doppler variability in the F5 subgiant Procyon. The variations have frequencies within a 1.1 mHz range centered at 0.9 mHz, and a total rms amplitude within that range of 2.5 m s^-1. Observations of Arcturus and scattered sunlight made with the same equipment during the same time interval show no such variation, indicating that the variations seen on Procyon are of stellar origin. The Doppler signal seen is entirely consistent with solar-like p-modes on Procyon, with maximum mode amplitudes of about 50 cm s^-1 and periods around 20 minutes. Several statistical tests support the identification of the signal with narrow-band oscillations, but none does so conclusively. Assuming that the signal does arise from p-modes, there is evidence that the frequency splitting v₀ is 71 μHz. We emphasize, however, that the data do not permit a definitive estimate of this quantity, and other values of v₀ fit the observations about equally well. In order to clarify the sources of ambiguity in this and similar observations, we describe in detail the data acquisition, reduction, and interpretation.