The spin-orbit alignment of the transiting exoplanet WASP-3b from Rossiter-McLaughlin observations

Abstract

We present an observation of the Rossiter–McLaughlin effect for the planetary system WASP-3. Radial velocity measurements were made during transit using the SOPHIE spectrograph at the 1.93-m telescope at Haute-Provence Observatory. The shape of the effect shows that the sky-projected angle between the stellar rotation axis and planetary orbital axis (λ) is small and consistent with zero within ⁠. WASP-3b joins the ∼two-thirds of planets with measured spin–orbit angles that are well aligned and are thought to have undergone a dynamically gentle migration process such as planet–disc interactions. We find a systematic effect which leads to an anomalously high determination of the projected stellar rotational velocity (v sin i= 19.6+2.2−2.1 km s−1) compared to the value found from spectroscopic line broadening (v sin i= 13.4 ± 1.5 km s−1). This is thought to be caused by a discrepancy in the assumptions made in the extraction and modelling of the data. Using a model developed by Hirano et al. designed to address this issue, we find v sin i to be consistent with the value obtained from spectroscopic broadening measurements (v sin i= 15.7+1.4−1.3 km s−1).