Searching for planets in the Hyades. II. Some implications of stellar magnetic activity

The Hyades constitute a homogeneous sample of stars ideal for investigating the dependence of planet formation on the mass of the central star. Because of their youth, Hyades members are much more chromospherically active than stars traditionally surveyed for planets using high-precision radial velocity techniques. Therefore, we have conducted a detailed investigation of whether magnetic activity of our Hyades target stars will interfere with our ability to make precise radial velocity (v(rad)) searches for substellar companions. We measure chromospheric activity (which we take as a proxy for magnetic activity) by computing the equivalent of the R'(HK) activity index (which is corrected for photospheric contributions) from the Ca II K line. The value of [R'(HK)] is not constant in the Hyades: we confirm that it decreases with increasing temperature in the F stars and also find it decreases for stars cooler than mid K. We examine correlations between simultaneously measured R'(HK) and radial velocities using both a classical statistical test and a Bayesian odds ratio test. We find that there is a significant correlation between R'(HK) and the radial velocity in only five of the 82 stars in this sample. Thus, simple R'(HK)-v(rad) correlations will generally not be effective in correcting the measured vrad values for the effects of magnetic activity in the Hyades. We argue that this implies long-timescale activity variations (of order a few years; i.e., magnetic cycles or growth and decay of plage regions) will not significantly hinder our search for planets in the Hyades if the stars are closely monitored for chromospheric activity. The trends in the radial velocity scatter (sigma'(v)) with [R'(HK)], v sin i, and P-rot for our stars is generally consistent with those found in field stars in the Lick planet search data, with the notable exception of a shallower dependence of sigma'(v) on [R'(HK)] for F stars.