Measurement of Spin‐Orbit Alignment in an Extrasolar Planetary System

TLDR

The Rossiter–McLaughlin effect, caused by a transiting planet occulting a rotating stellar surface, provides a strong detection of stellar spin–orbit alignment. The study aims to determine the stellar, planetary, and orbital properties of HD 209458 by jointly analyzing high‑precision radial velocities, photometry, and secondary‑eclipse timing. A new modeling technique for the Rossiter–McLaughlin effect was applied to these data to measure the sky‑projected spin–orbit angle (λ = –4.4 ± 1.4°) and the star’s projected rotation speed (v sin I⋆ = 4.70 ± 0.16 km s⁻¹). The measurements reduce the uncertainties by an order of magnitude, reveal a small but nonzero misalignment likely primordial, and uniquely isolate rotational line broadening.

Abstract

We determine the stellar, planetary, and orbital properties of the transiting planetary system HD 209458 through a joint analysis of high-precision radial velocities, photometry, and timing of the secondary eclipse. Of primary interest is the strong detection of the Rossiter-McLaughlin effect, the alteration of photospheric line profiles that occurs because the planet occults part of the rotating surface of the star. We develop a new technique for modeling this effect and use it to determine the inclination of the planetary orbit relative to the apparent stellar equator (λ = -4.°4 ± 1.°4), and the line-of-sight rotation speed of the star (v sin I⋆ = 4.70 ± 0.16 km s-1). The uncertainty in these quantities has been reduced by an order of magnitude relative to the pioneering measurements by Queloz and collaborators. The small but nonzero misalignment is probably a relic of the planet formation epoch, because the expected timescale for tidal coplanarization is larger than the age of the star. Our determination of v sin I⋆ is a rare case in which rotational line broadening has been isolated from other broadening mechanisms.