Photometric Amplitude Distribution of the Stellar Rotation of Kepler KOIs: An Indication of Spin-Orbit Alignment in Cool Stars

摘要

The observed amplitude of the rotational photometric modulation of a star with spots depends on the inclination of its rotational axis relative to our line of sight. Therefore, the distribution of observed rotation amplitudes of a large sample of stars depends on the distribution of their projected axes of rotation. Thus, a comparison of the stellar rotational amplitudes of the Kepler KOIs with those of single Kepler stars might provide a measure to indirectly infer the properties of the spin-orbit inclination of Kepler KOIs. We apply this technique to a large sample of 993 KOIs and 33,614 single stars in the temperature range 3500-6500K. We find that the amplitudes of cool KOIs are slightly (on the order of 10%) larger than those of the single stars, suggesting cool systems tend to be aligned with their planetary orbits. In contrast, the amplitudes of hot KOIs are systematically lower by a factor of ~0.6 than those of the hot single stars. Because the samples are large, both findings are highly significant. The border line between the relatively larger and smaller amplitudes, relative to the amplitudes of the single stars, depends on the planet size, occurring at about 6250 K for the larger planets (> 0.14R_(Jup)) and at 5750 K for the smaller planets. The amplitudes of the hot KOIs with small planets are about a factor of 0.5 smaller than those of the single stars. The hot KOI amplitudes cannot be accounted for by the geometrical spin-orbit distribution alone, and call for an additional explanation.