A low-amplitude periodicity in the radial velocity and chromospheric emission of Beta Geminorum

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Abstract

Using high-quality spectra obtained with the hydrogren fluoride technique at the Canada-France-Hawaii 3.6-meter telescope and Dominion Astrophysical Observatory 1.22-meter telescope, we have detected a low-amplitude periodicity in both the radial velocity and chromospheric emission of Beta Geminorum. In addition, we have detected a long-term change (time scale > 12 years) in thi star's chromospheric emission which is reminiscent of solar-type magnetic cycles believed present in giants of similar spectral type. The radial-velocity variations can be fitted by a sinusoid with amplitude and period of K = 46.23 ± 3.9 m s-1, P=584.65 ± 3.3 days. A similar, statistically significant (false-alarm probability of 1%) period is found in the residuals from the long-term trend in the chromospheric emission: P = 587.7 ± 12 days. However, because of the weakness of the signal, K = 0.583 ± 19 mA, this detection needs confirmation. Although these independently determined periods are in agreement with each other, they are not consistent with the maximum rotation period of 178 days derived from this star's v sin i = 2.5 km s-1 (Gray 1982). Either the observed v sin i value of Beta Geminorum needs to be revised below 0.76 km s-1 or some alternative to rotational modulation of surface phenomena must be found to explain this star's periodicity.

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