科研之友
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摘要
PTFO 8-8695b represents the first transiting exoplanet candidate orbiting a pre-main-sequence star (van Eyken et al. 2012, ApJ, 755, 42). We find that the unusual lightcurve shapes of PTFO 8-8695 can be explained by transits of a planet across an oblate, gravity-darkened stellar disk. We develop a theoretical framework for understanding precession of a planetary orbit%26apos;s ascending node for the case when the stellar rotational angular momentum and the planetary orbital angular momentum are comparable in magnitude. We then implement those ideas to simultaneously and self-consistently fit two separate lightcurves observed in 2009 December and 2010 December. Our two self-consistent fits yield M-p = 3.0 M-Jup and M-p = 3.6 M-Jup for assumed stellar masses of M-* = 0.34 M-circle dot and M-* = 0.44 M-circle dot respectively. The two fits have precession periods of 293 days and 581 days. These mass determinations (consistent with previous upper limits) along with the strength of the gravity-darkened precessing model together validate PTFO 8-8695b as just the second hot Jupiter known to orbit an M-dwarf. Our fits show a high degree of spin-orbit misalignment in the PTFO 8-8695 system: 69 degrees +/- 2 degrees or 73 degrees.1 +/- 0 degrees.5, in the two cases. The large misalignment is consistent with the hypothesis that planets become hot Jupiters with random orbital plane alignments early in a system%26apos;s lifetime. We predict that as a result of the highly misaligned, precessing system, the transits should disappear for months at a time over the course of the system%26apos;s precession period. The precessing, gravity-darkened model also predicts other observable effects: changing orbit inclination that could be detected by radial velocity observations, changing stellar inclination that would manifest as varying v sin i, changing projected spin-orbit alignment that could be seen by the Rossiter-McLaughlin effect, changing transit shapes over the course of the precession, and differing lightcurves as a function of wavelength. Our measured planet radii of 1.64 R-Jup and 1.68 R-Jup in each case are consistent with a young, hydrogen-dominated planet that results from a %26quot;hot-start%26quot; formation mechanism.
- 出版日期2013-9-1
