Refined architecture of the WASP-8 system: A cautionary tale for traditional Rossiter-McLaughlin analysis

Observatoire de l’Université de Genève, 51 chemin des Maillettes, 1290 Sauverny, Switzerland
e-mail: vincent.bourrier@unige.ch

Received: 28 October 2016
Accepted: 22 November 2016

Abstract

Probing the trajectory of a transiting planet across the disk of its star through the analysis of its Rossiter-McLaughlin effect can be used to measure the differential rotation of the host star and true obliquity of the system. Highly misaligned systems could be particularly conducive to these measurements, which is why we reanalyzed the HARPS transit spectra of WASP-8b using the Rossiter-McLaughlin effect reloaded (reloaded RM) technique. This approach allows us to isolate the local stellar CCF emitted by the planet-occulted regions. As a result we identified a ~35% variation in the local CCF contrast along the transit chord, which might trace a deepening of the stellar lines from the equator to the poles. Whatever its origin, such an effect cannot be detected when analyzing the RV centroids of the disk-integrated CCFs through a traditional velocimetric analysis of the RM effect. Consequently this effect injected a significant bias into the results obtained by Queloz et al. (2010) for the projected rotational velocity v_eqsini_⋆ (1.59^+0.08_-0.09 km s^-1) and the sky-projected obliquity λ (–123.0^+3.4_-4.4°). Using our technique, we measured these values to be v_eqsini_⋆ = 1.90 ± 0.05 km s^-1 and λ = –143.0^+1.6_-1.5°. We found no compelling evidence for differential rotation of the star, although there are hints that WASP-8 is pointing away from us with the stellar poles rotating about 25% slower than the equator. Measurements at higher accuracy during ingress and egress will be required to confirm this result. In contrast to the traditional analysis of the RM effect, the reloaded RM technique directly extracts the local stellar CCFs, allowing us to analyze their shape and to measure their RV centroids, unbiased by variations in their contrast or FWHM.