THE RADIAL-VELOCITY VARIABILITY OF THE K-GIANT BETA-OPHIUCHI .1. THE DETECTION OF LOW-AMPLITUDE, SHORT-PERIOD PULSATIONS

We present precise radial velocity measurements (sigma similar to 20 m s(-1)) of the K giant beta Oph taken over 8 consecutive nights in 1992 June and 2 nights in 1989 July. An analysis of the 1992 June data revealed the presence of a 0.255 +/- 0.005 day period. The 1989 July data also show short-term variability; however, aliasing is more severe for these data, making it difficult to determine a period reliably. A Scargle-type periodogram yields a period of 0.813 +/- 0.007 days, whereas a CLEAN analysis results in a period of 0.455 +/- 0.007 days for the 1989 July data. Subtracting the nightly means from the 1989 July data results in a period of 0.237 +/- 0.007 days. These short-period radial velocity variations can only result from stellar pulsations. Use of the empirical Q equation of Cox, King, and Stellingwerf indicates that a second-overtone mode can account for the 0.255 day period if beta Oph has a mass of 7 M. and a radius of 10 R.. These values result, however, in a log g much higher than published values. If beta Oph possesses a lower mass, then higher (n greater than or equal to 4-6) overtone radial or nonradial modes are needed to account for such a short period. Theoretical work by Ando on nonradial acoustic modes in the envelope of late-type stars yields periods of about 2 hr for high-order acoustic modes (l > 10) in stars having a mass and luminosity near that of beta Oph. Extrapolating these results to low-order (l = 1, 2) modes (that can be detected by radial velocity measurements) yields oscillation periods of 2-16 hr. A detailed pulsational analysis using a stellar model appropriate for beta Oph is needed to identify the pulsation mode of the 0.255 day period. There is some evidence that a period different from 0.255 days was present in the 1989 July data. If so, then beta Oph may be another K giant, like alpha Boo, that is switching pulsation modes, although more observations are needed to confirm this.