DISCOVERY OF A 1.6 YEAR MAGNETIC ACTIVITY CYCLE IN THE EXOPLANET HOST STAR ι HOROLOGII

Abstract

The Mount Wilson Ca HK survey revealed magnetic activity variations in a\nlarge sample of solar-type stars with timescales ranging from 2.5 to 25 years.\nThis broad range of cycle periods is thought to reflect differences in the\nrotational properties and the depths of the surface convection zones for stars\nwith various masses and ages. In 2007 we initiated a long-term monitoring\ncampaign of Ca II H and K emission for a sample of 57 southern solar-type stars\nto measure their magnetic activity cycles and their rotational properties when\npossible. We report the discovery of a 1.6-year magnetic activity cycle in the\nexoplanet host star iota Horologii, and we obtain an estimate of the rotation\nperiod that is consistent with Hyades membership. This is the shortest activity\ncycle so far measured for a solar-type star, and may be related to the\nshort-timescale magnetic variations recently identified in the Sun and HD49933\nfrom helio- and asteroseismic measurements. Future asteroseismic observations\ncan be compared to those obtained near the magnetic minimum in 2006 to search\nfor cycle-induced shifts in the oscillation frequencies. If such short activity\ncycles are common in F stars, then NASA's Kepler mission should observe their\neffects in many of its long-term asteroseismic targets.\n