# HD 17156 spin-orbit alignment

## HD 17156 spin-orbit alignment

The spin-orbit alignment if HD 17156 has been determined to be 9.4 degrees, compared to ~60 degrees from previous studies. Hmm.

The Spin-Orbit Alignment of the HD17156 Transiting Eccentric Planetary System
http://arxiv.org/abs/0806.4142

abstract wrote:We present high precision radial velocity observations of HD17156 during a transit of its eccentric Jovian planet. In these data, we detect the Rossiter-McLaughlin effect, which is an apparent perturbation in the velocity of the star due to the progressive occultation of part of the rotating stellar photosphere by the transiting planet. This system had previously been reported by Narita et al. (2008) to exhibit a lambda = 62 +/- 25 degree misalignment of the projected planetary orbital axis and the stellar rotation axis. We model our data, along with the Narita et al. data, and obtain lambda = 9.4 +/- 9.3 degrees for the combined data set. We thus conclude that the planetary orbital axis is actually very well aligned with the stellar rotation axis.

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Sirius_Alpha

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## Re: HD 17156 spin-orbit alignment

Improved Measurement of the Rossiter-McLaughlin Effect in the Exoplanetary System HD 17156
http://arxiv.org/abs/0905.4727

Abstract wrote:We present an improved measurement of the Rossiter-McLaughlin effect for the exoplanetary system HD 17156, based on radial-velocity data gathered with the Subaru 8.2m telescope throughout the planetary transit of UT 2008 November 7. The data allow for a precise and independent determination of the projected spin-orbit angle of this system: $\lambda = 10.0^{\circ} \pm 5.1^{\circ}$. This result supersedes the previous claim of $\lambda = 62^{\circ} \pm 25^{\circ}$ by Narita et al., which was based on lower-precision data with poor statistics. Thus the stellar spin and planetary orbital axes of the HD 17156 system are likely to be well-aligned, despite the planet's large orbital eccentricity suggesting a history of strong dynamical interactions.

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## Re: HD 17156 spin-orbit alignment

Which leaves just XO-3b and possibly HD 80606b as examples of misalignment.

Lazarus
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## Re: HD 17156 spin-orbit alignment

Concerning HD 80606, I thought that they didn't have a precise enough measurement of the Rossiter-McLaughlin effect to make that determination? If I recall correctly, they were just able to determine that the orbit is prograde. Would you happen to have a measurement of the spin-orbit alignment handy?

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## Re: HD 17156 spin-orbit alignment

Moutou et al. (2009), "Photometric and spectroscopic detection of the primary transit of the 111-day-period planet HD 80606 b"

From the abstract (emphasis mine):
Radial velocity measurements show the detection of a prograde Rossiter-McLaughlin effect, and provide a hint for a spin-orbit misalignment. If confirmed, this misalignment would corroborate the hypothesis that HD 80606 b owes its unusual orbital configuration to Kozai migration.

Lazarus
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## Re: HD 17156 spin-orbit alignment

Ah, alright. Thanks.

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