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The signature of orbital motion from the dayside of the planet τ Boötis b

Nature volume 486, pages 502–504 (2012)Cite this article

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Abstract

The giant planet orbiting τ Boötis (named τ Boötis b) was amongst the first extrasolar planets to be discovered1. It is one of the brightest exoplanets and one of the nearest to us, with an orbital period of just a few days. Over the course of more than a decade, measurements of its orbital inclination have been announced2 and refuted3, and have hitherto remained elusive4,5,6,7,8. Here we report the detection of carbon monoxide absorption in the thermal dayside spectrum of τ Boötis b. At a spectral resolution of ∼100,000, we trace the change in the radial velocity of the planet over a large range in phase, determining an orbital inclination of 44.5° ± 1.5° and a mass 5.95 ± 0.28 times that of Jupiter, demonstrating that atmospheric characterization is possible for non-transiting planets. The strong absorption signal points to an atmosphere with a temperature that is decreasing towards higher altitudes, in contrast to the temperature inversion inferred for other highly irradiated planets9,10. This supports the hypothesis that the absorbing compounds believed to cause such atmospheric inversions are destroyed in τ Boötis b by the ultraviolet emission from the active host star11.

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Figure 1: CO signal in the dayside spectrum of the extrasolar planet τ Boötis b.
Figure 2: Comparison of in-trail and out-of-trail cross-correlation values.
Figure 3: The orbital trail of CO absorption.

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Acknowledgements

We thank the ESO support staff of the Paranal Observatory for their help during the observations. This work is based on observations collected at the European Southern Observatory (186.C-0289). S.A. acknowledges support by a Rubicon fellowship from the Netherlands Organisation for Scientific Research (NWO), and by NSF grant no. 1108595.

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Authors and Affiliations

  1. Leiden Observatory, Leiden University, Postbus 9513, 2300RA Leiden, The Netherlands ,

    Matteo Brogi, Ignas A. G. Snellen, Jayne Birkby & Ernst J. W. de Mooij

  2. SRON, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands ,

    Remco J. de Kok

  3. Department of Physics, and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, 02139, Massachusetts, USA

    Simon Albrecht

  4. Department of Astronomy and Astrophysics, University of Toronto, 50 St George Street, Toronto, Ontario M5S 3H4, Canada,

    Ernst J. W. de Mooij

Authors
  1. Matteo Brogi
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  2. Ignas A. G. Snellen
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  3. Remco J. de Kok
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  4. Simon Albrecht
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  5. Jayne Birkby
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  6. Ernst J. W. de Mooij
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Contributions

M.B. led the observations and data analysis, and co-wrote the manuscript. I.A.G.S. conceived the project, contributed to the analysis and co-wrote the manuscript. R.J.d.K. constructed the planet atmosphere models. S.A. conducted the MCMC orbital analysis. J.B., E.J.W.d.M., R.J.d.K. and S.A. discussed the analyses, results and commented on the manuscript.

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Correspondence to Matteo Brogi.

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The authors declare no competing financial interests.

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Brogi, M., Snellen, I., de Kok, R. et al. The signature of orbital motion from the dayside of the planet τ Boötis b. Nature 486, 502–504 (2012). https://doi.org/10.1038/nature11161

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