科研之友
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摘要
We develop a detailed methodology of determining three-dimensionally the angle between the stellar spin and the planetary orbit axis vectors, psi, for transiting planetary systems. The determination of psi requires the independent estimates of the inclination angles of the stellar spin axis and of the planetary orbital axis with respect to the line of sight, i(star) and i(orb), and the projection of the spin-orbit angle on to the plane of the sky, lambda. These are mainly derived from asteroseismology, transit lightcurve, and the Rossiter-McLaughlin effect, respectively. The detailed joint analysis of those three datasets enables an accurate and precise determination of the numerous parameters characterizing the planetary system, in addition to psi. We demonstrate the power of the joint analysis for the two specific systems HAT-P-7 and Kepler-25. HAT-P-7b is the first exoplanet suspected to be a retrograde (or polar) planet because of the significant misalignment lambda approximate to 180 degrees. Our joint analysis indicates i(star) approximate to 30 degrees and psi approximate to 120 degrees, suggesting that the planetary orbit is closer to polar rather than retrograde. Kepler-25 is one of the few multi-transiting planetary systems with measured lambda, and hosts two short-period transiting planets and one outer non-transiting planet. The projected spin-orbit angle of the larger transiting planet, Kepler-25c, has been measured to be lambda approximate to 0 degrees, implying that the system is well aligned. With the help of the tight constraint from asteroseismology, however, we obtain i(star) = 65 degrees.4(-6 degrees.4)(+10 degrees.6) and psi = 26 degrees.9(-9 degrees.2)(+7 degrees.0), and thus find that the system is actually mildly misaligned. This is the first detection of the spin-orbit misalignment for the multiple planetary system with a main-sequence host star, and points to mechanisms that tilt a stellar spin axis relative to its protoplanetary disk.
- 出版日期2014-10
