Observations of small-amplitude oscillations in the radial velocity of Arcturus.

Abstract

High accuracy measurements of variations in the radial velocity of the K 1 giant star Arcturus have been obtained. The observations span 5 years and have a point-to-point repeatability of 5 m s⁻¹ and night-to-night stability of better than 20 m s⁻¹. Velocity oscillations of Arcturus were discovered during the course of this work in 1986. Extensive additional data, presented here, indicate that Arcturus is exhibiting global nonradial acoustic oscillations with characteristics similar to those occurring in the Sun. A Fabry-Perot interferometer, used in transmission, is employed to accurately tag the stellar wavelengths. The light is dispersed by a cross-dispersed echelle. About 750 points in the spectrum are monitored over 4250-4750 Å. All observations were done using the 0.9 m telescope of the University of Arizona on Kitt Peak, which is dedicated half-time for use with this instrument. A dedicated facility was crucial to this work - because of the changing nature of the oscillations, many observing runs, over several years, were required to understand the star's behavior. Continuous data sets as long as 30 days were acquired. The velocity power spectra are complicated and variable. There is substantial evidence that the variations are solar-like p-mode oscillations. At least 10 frequencies have been identified, over the range 8.3 to 1.7 days. A tell-tale spectrum of evenly spaced modes is apparent, yielding a value for Δv₀ ≈ 1.2 μHz. The average power spectrum peaks near 3 days, approximately as expected from the acoustic cut-off frequency. There is a broad envelope of power with a distribution reminiscent of that seen in the Sun. The oscillations do not maintain phase coherence and they show abrupt discontinuities, indicating that something is disrupting them, as in the Sun. Coherence of the modes is estimated to be a few weeks to a few months. Driving is likely to be due to stochastic excitation by turbulent convection. Arcturus may be one of the first analogues of solar-like oscillations and/or the first member of a new class of variable stars. Because Arcturus is an evolved star of approximately solar mass, these oscillations will provide a test for stellar evolution theory, as well as for asteroseismology and the study of driving mechanisms for stellar oscillations.