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Low Stellar Obliquities in Compact Multiplanet Systems

Albrecht, Simon and Winn, Joshua N. and Marcy, Geoffrey W. and Howard, Andrew W. and Isaacson, Howard and Johnson, John A. (2013) Low Stellar Obliquities in Compact Multiplanet Systems. Astrophysical Journal, 771 (1). Art. No. 11. ISSN 0004-637X.

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We measure the sky-projected stellar obliquities (λ) in the multiple-transiting planetary systems KOI-94 and Kepler-25, using the Rossiter-McLaughlin effect. In both cases, the host stars are well aligned with the orbital planes of the planets. For KOI-94 we find λ = –11° ± 11°, confirming a recent result by Hirano and coworkers. Kepler-25 was a more challenging case, because the transit depth is unusually small (0.13%). To obtain the obliquity, it was necessary to use prior knowledge of the star's projected rotation rate and apply two different analysis methods to independent wavelength regions of the spectra. The two methods gave consistent results, λ = 7° ± 8° and –0°.5 ± 5°.7. There are now a total of five obliquity measurements for host stars of systems of multiple-transiting planets, all of which are consistent with spin-orbit alignment. This alignment is unlikely to be the result of tidal interactions because of the relatively large orbital distances and low planetary masses in the systems. In this respect, the multiplanet host stars differ from hot-Jupiter host stars, which commonly have large spin-orbit misalignments whenever tidal interactions are weak. In particular, the weak-tide subset of hot-Jupiter hosts has obliquities consistent with an isotropic distribution (p = 0.6), but the multiplanet hosts are incompatible with such a distribution (p ~ 10^(–6)). This suggests that high obliquities are confined to hot-Jupiter systems, and provides further evidence that hot-Jupiter formation involves processes that tilt the planetary orbit.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Winn, Joshua N.0000-0002-4265-047X
Marcy, Geoffrey W.0000-0002-2909-0113
Howard, Andrew W.0000-0001-8638-0320
Isaacson, Howard0000-0002-0531-1073
Johnson, John A.0000-0001-9808-7172
Additional Information:© 2013 American Astronomical Society. Received 2013 February 18; accepted 2013 May 12; published 2013 June 10. The authors warmly thank Guillaume Hébrard for supporting the observing campaign of KOI-94. The authors are grateful to Bill Cochran, Debra Fischer, and Amaury Triaud for helpful discussions, to Lauren Weiss for sharing an early version of her group’s manuscript about KOI-94, John Brewer for pointing out an error in an earlier version of this manuscript, and the Kepler team for creating an extraordinary tool for discovery. Particular thanks are due to the Kepler follow-up observing program for organizing the characterization of the host stars. Work by S.A., J.N.W., and J.A.J. was supported by NASA Origins award NNX09AB33G, and the work by S.A. and J.N.W. was supported by NSF grant No. 1108595. This research has made use of the following web resources:,, The W. M. Keck Observatory is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration, and was made possible by the generous financial support of the W. M. Keck Foundation. We extend special thanks to those of Hawaiian ancestry on whose sacred mountain of Mauna Kea we are privileged to be guests. The data presented herein were collected with the Keck I telescope at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration.
Funding AgencyGrant Number
W. M. Keck FoundationUNSPECIFIED
Subject Keywords:planetary systems; planets and satellites: formation; planet–star interactions; stars: individual (Kepler-25, KOI-94); stars: rotation; techniques: spectroscopic
Issue or Number:1
Record Number:CaltechAUTHORS:20130820-091735328
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Official Citation:Low Stellar Obliquities in Compact Multiplanet Systems Simon Albrecht et al. 2013 ApJ 771 11
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:40728
Deposited By: Jason Perez
Deposited On:21 Aug 2013 15:59
Last Modified:09 Mar 2020 13:18

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