Spin-orbit angle measurements for six southern transiting planets

Abstract

Context. Several competing scenarios for planetary-system formation and evolution seek to explain how hot Jupiters came to be so close to their parent stars. Most planetary parameters evolve with time, making it hard to distinguish between models. The obliquity of an orbit with respect to the stellar rotation axis is thought to be more stable than other parameters such as eccentricity. Most planets, to date, appear aligned with the stellar rotation axis; the few misaligned planets so far detected are massive (>2 M J ). Aims. Our goal is to measure the degree of alignment between planetary orbits and stellar spin axes, to search for potential correlations with eccentricity or other planetary parameters and to measure long term radial velocity variability indicating the presence of other bodies in the system. Methods. For transiting planets, the Rossiter-McLaughlin effect allows the measurement of the sky-projected angle β between the stellar rotation axis and a planet's orbital axis. Using the HARPS spectrograph, we observed the Rossiter-McLaughlin effect for six transiting hot Jupiters found by the WASP consortium. We combine these with long term radial velocity measurements obtained with CORALIE. We used a combined analysis of photometry and radial velocities, fitting model parameters with the Markov Chain Monte Carlo method. After obtaining β we attempt to statistically determine the distribution of the real spin-orbit angle ψ. Results. We found that three of our targets have β above 90 • : WASP-2b: β = 153 • +11 −15 , WASP-15b: β = 139.6 • +5.2 −4.3 and WASP17b: β = 148.5 • +5.1 −4.2 ; the other three (WASP-4b, WASP-5b and WASP-18b) have angles compatible with 0 • . We find no dependence between the misaligned angle and planet mass nor with any other planetary parameter. All six orbits are close to circular, with only one firm detection of eccentricity e = 0.00848 +0.00085 −0.00095 in WASP-18b. No long-term radial acceleration was detected for any of the targets. Combining all previous 20 measurements of β and our six and transforming them into a distribution of ψ we find that between about 45 and 85% of hot Jupiters have ψ > 30 • . Conclusions. Most hot Jupiters are misaligned, with a large variety of spin-orbit angles. We find observations and predictions using the Kozai mechanism match well. If these observational facts are confirmed in the future, we may then conclude that most hot Jupiters are formed from a dynamical and tidal origin without the necessity to use type I or II migration. At present, standard disc migration cannot explain the observations without invoking at least another additional process.

Statistical patterns in ground-based transit surveys

“…Even the massive, close-orbiting WASP-18b (Hellier 2009) has an eccentricity which, despite appearing significant at the 3.3-σ level in the discovery data, has a 17% chance of being spurious according to the Lucy-Sweeney test. Subsequent RV observations by Triaud et al (2010), however, reduce the Lucy-Sweeney false-alarm probability to 2.7%, showing WASP-18b to have a small but genuinely significant e sin ω.…”

Turning solar systems into extrasolar planetary systems in stellar clusters

“…In this case, tidal interactions between the planet and the host star may circularize the orbit, leaving the planet on a much tighter orbit, which may also be rather inclined compared to its initial orbital plane (eg Fabrycky & Tremaine 2007;Wu, Murray & Ramshai 2007). Indeed observations of the so-called Rossiter-McLaughlin Effect in transiting planets suggests that at least some systems are highly inclined (eg Triaud et al 2010).…”

The diverse origin of exoplanets' eccentricities & inclinations

“…Recently, spectroscopic observations during transit (i.e., Rossitter-McLaughlin measurements) provide evidence for a substantial population of hot-Jupiters on highly-inclined, nearly-polar or even retrograde orbits (measured relative to the current stellar spis axis; Fabrycky & Winn 2009;Morton & Johnson 2011;Triaud et al 2010;Winn et al 2010). The highly-inclined population includes some planets that remain highly eccentric (e.g., HD 80606; Winn et al 2009) and some that are nearly circular today.…”