Two upper limits on the Rossiter-MCLAUGHLIN effect, with differing implications: WASP-1 has a high obliquity and WASP-2 is indeterminate

Simon Albrecht, Joshua N. Winn, John Asher Johnson, R. Paul Butler, Jeffrey D. Crane, Stephen A. Shectman, Ian B. Thompson, Norio Narita, Bun'Ei Sato, Teruyuki Hirano, Keigo Enya, Debra Fischer

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Abstract

We present precise radial-velocity (RV) measurements of WASP-1 and WASP-2 throughout transits of their giant planets. Our goal was to detect the Rossiter-McLaughlin (RM) effect, the anomalous RV observed during eclipses of rotating stars, which can be used to study the obliquities of planet-hosting stars. For WASP-1, a weak signal of a prograde orbit was detected with 2σ confidence, and for WASP-2 no signal was detected. The resulting upper bounds on the RM amplitude have different implications for these two systems because of the contrasting transit geometries and the stellar types. Because WASP-1 is an F7V star, and such stars are typically rapid rotators, the most probable reason for the suppression of the RM effect is that the star is viewed nearly pole-on. This implies that the WASP-1 star has a high obliquity with respect to the edge-on planetary orbit. Because WASP-2 is a K1V star, and is expected to be a slow rotator, no firm conclusion can be drawn about the stellar obliquity. Our data and our analysis contradict an earlier claim that WASP-2b has a retrograde orbit, thereby revoking this system's status as an exception to the pattern that cool stars have low obliquities.

Original languageEnglish (US)
Article number50
JournalAstrophysical Journal
Volume738
Issue number1
DOIs
StatePublished - Sep 1 2011

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

Keywords

  • planet-star interactions
  • planetary systems
  • planets and satellites: formation
  • stars: rotation
  • techniques: spectroscopic

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    Albrecht, S., Winn, J. N., Johnson, J. A., Paul Butler, R., Crane, J. D., Shectman, S. A., Thompson, I. B., Narita, N., Sato, BE., Hirano, T., Enya, K., & Fischer, D. (2011). Two upper limits on the Rossiter-MCLAUGHLIN effect, with differing implications: WASP-1 has a high obliquity and WASP-2 is indeterminate. Astrophysical Journal, 738(1), [50]. https://doi.org/10.1088/0004-637X/738/1/50