Transit Lightcurves of Extrasolar Planets Orbiting Rapidly-Rotating Stars

TL;DR

This paper discusses how transits of extrasolar planets across rapidly rotating, oblate stars with gravity darkening produce unique lightcurves that can reveal the planets' spin-orbit alignment, aiding understanding of planet formation.

Contribution

It introduces the use of transit lightcurves of planets around rapidly rotating stars to determine spin-orbit alignment, a novel method compared to traditional spectroscopic techniques.

Findings

Transit lightcurves show distinctive signatures of gravity darkening.

Lightcurve analysis can determine spin-orbit alignment without spectroscopic measurements.

Kepler data will enable the study of planet formation around high-mass stars.

Abstract

Main-sequence stars earlier than spectral type ~F6 or so are expected to rotate rapidly due to their radiative exteriors. This rapid rotation leads to an oblate stellar figure. It also induces the photosphere to be hotter (by up to several thousand Kelvin) at the pole than at the equator as a result of a process called gravity darkening that was first predicted by von Zeipel (1924). Transits of extrasolar planets across such a non-uniform, oblate disk yield unusual and distinctive lightcurves that can be used to determine the relative alignment of the stellar rotation pole and the planet orbit normal. This spin-orbit alignment can be used to constrain models of planet formation and evolution. Orderly planet formation and migration within a disk that is coplanar with the stellar equator will result in spin-orbit alignment. More violent planet-planet scattering events should yield spin-orbit misaligned planets. Rossiter-McLaughlin measurements of transits of lower-mass stars show that some planets are spin-orbit aligned, and some are not. Since Rossiter-McLaughlin measurements are difficult around rapid rotators, lightcurve photometry may be the best way to determine the spin-orbit alignment of planets around massive stars. The Kepler mission will monitor ~10^4 of these stars within its sample. The lightcurves of any detected planets will allow us to probe the planet formation process around high-mass stars for the first time.