The spin-orbit angles of the transiting exoplanets WASP-1b, WASP-24b, WASP-38b and HAT-P-8b from Rossiter-McLaughlin observations

TL;DR

This study measures the spin-orbit angles of four transiting exoplanets using Rossiter-McLaughlin observations, revealing mostly aligned orbits with one notable misaligned system, contributing to understanding planetary system architectures.

Contribution

First detailed Rossiter-McLaughlin measurements for these four exoplanets, highlighting diverse orbital alignments and challenging existing correlations between star properties and orbital obliquity.

Findings

WASP-24b, WASP-38b, HAT-P-8b are well aligned with their host stars.

WASP-1b exhibits a highly misaligned, nearly polar orbit.

Host star temperature correlates with low obliquity in some cases.

Abstract

We present observations of the Rossiter-McLaughlin effect for the transiting exoplanet systems WASP-1, WASP-24, WASP-38 and HAT-P-8, and deduce the orientations of the planetary orbits with respect to the host stars' rotation axes. The planets WASP-24b, WASP-38b and HAT-P-8b appear to move in prograde orbits and be well aligned, having sky-projected spin orbit angles consistent with zero: {\lambda} = -4.7 \pm 4.0{\deg}, {\lambda} = 15 + 33{\deg}/-43{\deg} and {\lambda} = -9.7 +9.0{\deg}/-7.7{\deg}, respectively. The host stars have Teff < 6250 K and conform with the trend of cooler stars having low obliquities. WASP-38b is a massive planet on a moderately long period, eccentric orbit so may be expected to have a misaligned orbit given the high obliquities measured in similar systems. However, we find no evidence for a large spin-orbit angle. By contrast, WASP-1b joins the growing number of misaligned systems and has an almost polar orbit, {\lambda} = -79 +4.5{\deg}/-4.3{\deg}. It is neither very massive, eccentric nor orbiting a hot host star, and therefore does not share the properties of many other misaligned systems.