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| Paper: |
Evidence for Solar-like Oscillations in Arcturus |
| Volume: |
135, A Half Century of Stellar Pulsation Interpretations: a Tribute to Arthur N. Cox |
| Page: |
208 |
| Authors: |
Merline, W. J. |
| Abstract: |
High accuracy measurements of variations in the radial velocity of the K1 giant star Arcturus have been obtained. The observations span 5 years and have a point-to-point repeatability of 5 m/s and night-to-night stability of better than 20 m/s. Velocity oscillations of Arcturus were discovered during the course of this work in 1986. Subsequent, extensive additional data, indicate that Arcturus is exhibiting global nonradial acoustic oscillations with characteristics similar to those occurring in the Sun. All observations were done using a radial velocity spectrometer, designed to search for extrasolar planets, at a dedicated facility of the University of Arizona on Kitt Peak. 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. Preliminary pulsation models were performed in collaboration with Art Cox at Los Alamos National Lab. 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 spectrum of evenly spaced modes is apparent, yielding a value for Dn0 = ~1.2 5Hz. The average power spectrum peaks near 3 days. 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. Recent observations of the G5 IV star h Boo by Brown et al. 1997 (Ap. J. 475, 322) have failed to confirm the detection of p-mode oscillations reported by Kjeldsen et al. 1995 (A.J. 109, 1313). Thus, Arcturus may be one of the first stars known to exhibit solar-like oscillations. If other K- giant variables can be shown to exhibit similar oscillations, Arcturus may represent the prototype for a new class of variable stars. We know some other K-giants are variable (Hatzes and Cochran 1994, Ap. J. 432, 763), but many are not (Horner 1996, Ap.J. 302, 757). Furthermore, the recent suspicion that nonradial oscillations are mimicking a purported planet around 51 Peg (Gray 1997, Nature 385, 795) means that to understand planetary signatures, we must understand solar-like oscillations in other stars. |
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