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

TL;DR

This study measures the spin-orbit alignment of exoplanet WASP-3b using Rossiter-McLaughlin observations, finding it to be well aligned, which suggests a gentle migration history, and addresses systematic measurement discrepancies.

Contribution

It provides the first Rossiter-McLaughlin measurement for WASP-3b, confirming its aligned orbit and introduces a model to correct for systematic errors in stellar rotational velocity estimation.

Findings

WASP-3b has a small sky-projected spin-orbit angle, consistent with alignment.

A systematic effect affects the measurement of stellar rotational velocity.

Applying a correction model yields a stellar rotational velocity consistent with spectroscopic data.

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 1.93m 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 +10/-9 deg. 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 (vsini = 19.6 +2.2/-2.1 km/s) compared to the value found from spectroscopic line broadening (vsini = 13.4 +/- 1.5 km/s). 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. (2009) designed to address this issue, we find vsini to be consistent with the value obtained from spectroscopic broadening measurements (vsini = 15.7 +1.4/-1.3 km/s).