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

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 (lambda) is small and consistent with zero within 2 sigma; lambda = 15(-9 degrees)(+10 degrees) WASP-3b joins the similar to 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 (nu sin i = 19.6(-2.1)(+2.2) km s(-1)) compared to the value found from spectroscopic line broadening (nu 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 (nu sin i = 15.7(-1.3)(+1.4) km s(-1)).