A periodic variation in the radial velocity of arcturus
We have detected radial velocity variations in Arcturus (K1 IIIb) spanning a total range of at least 160 m/s, more than 8 times the nightly standard deviation of measurements made during the same season (often on the same nights) on the star Pollux. The velocities of Arcturus tend to alternate between two values separated by 60–100 m/s from night to night. A periodogram of 32 nightly velocity averages spanning 100 days shows significant power (false alarm probability less than 1%) for a period of 2.18 days, and its alias of 1.84 days. Although these periods are close to the Nyquist period of 2.00 days, there is no preference for periods of exactly 2 solar or 2 sidereal days. Another consequence of the aliasing is that periods of 1/3 these values cannot be ruled out. Exhaustive checks indicate no evidence that these variations are terrestrial or instrumental in origin.
Epoch folding of the data onto a phase diagram reveals that the shape of the velocity curve is skewed and that the sense of the skewness is opposite for the two aliased peaks. The time scale of the variation appears superficially to be consistent with the “2H” or “3H” modes of radial oscillation by a star with the properties of Arcturus. The sense of the skewness associated with the peak at 1.84 days corresponds to a short-lived outward acceleration followed by a more prolonged deceleration by the stellar atmosphere.
KeywordsRadial Velocity False Alarm Probability Phase Coherence Stellar Atmosphere Angular Diameter
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