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Measurement of Spin-orbit Misalignment and Nodal Precession for the Planet around Pre-main-sequence Star PTFO 8-8695 from Gravity Darkening

Barnes, Jason W. and van Eyken, Julian C. and Jackson, Brian K. and Ciardi, David R. and Fortney, Jonathan J. (2013) Measurement of Spin-orbit Misalignment and Nodal Precession for the Planet around Pre-main-sequence Star PTFO 8-8695 from Gravity Darkening. Astrophysical Journal, 774 (1). Art. No. 53. ISSN 0004-637X. https://resolver.caltech.edu/CaltechAUTHORS:20130919-113432051

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Abstract

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'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 Mp = 3.0 M_Jup and Mp = 3.6 M_Jup for assumed stellar masses of M* = 0.34 M_☉ and M* = 0.44 M_☉ 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° ± 2° or 73°.1 ± 0°.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'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'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 vsin 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 "hot-start" formation mechanism.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1088/0004-637X/774/1/53DOIArticle
http://iopscience.iop.org/0004-637X/774/1/53/PublisherArticle
http://arxiv.org/abs/1308.0629arXivDiscussion Paper
ORCID:
AuthorORCID
van Eyken, Julian C.0000-0003-2192-5371
Ciardi, David R.0000-0002-5741-3047
Fortney, Jonathan J.0000-0002-9843-4354
Additional Information:© 2013 American Astronomical Society. Received 2013 April 19; accepted 2013 July 3; published 2013 August 16.
Group:Infrared Processing and Analysis Center (IPAC)
Subject Keywords:eclipses; planetary systems; stars: individual (PTFO 8-8695); techniques: photometric
Issue or Number:1
Record Number:CaltechAUTHORS:20130919-113432051
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20130919-113432051
Official Citation:Measurement of Spin-orbit Misalignment and Nodal Precession for the Planet around Pre-main-sequence Star PTFO 8-8695 from Gravity Darkening Jason W. Barnes et al. 2013 ApJ 774 53
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:41408
Collection:CaltechAUTHORS
Deposited By: Jason Perez
Deposited On:20 Sep 2013 14:42
Last Modified:09 Mar 2020 13:19

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