On the orbit of the short-period exoplanet WASP-19b

TL;DR

This paper investigates the orbit of the ultra-short period exoplanet WASP-19b, revealing its aligned orbit, low eccentricity, and discussing its likely inward orbital evolution due to tidal decay.

Contribution

It provides the first detailed measurement of WASP-19b's orbital alignment and eccentricity, and discusses its unique evolutionary history among hot Jupiters.

Findings

WASP-19b's orbit is aligned with the stellar equator.

The orbit's eccentricity is less than 0.02.

Hot Jupiters with such short periods are much rarer than those with 3-4 day periods.

Abstract

WASP-19b has the shortest orbital period of any known exoplanet, orbiting at only 1.2 times the Roche tidal radius. By observing the Rossiter-McLaughlin effect we show that WASP-19b's orbit is aligned, with lambda = 4.6 +/- 5.2 degrees. Using, in addition, a spectroscopic vsini and the observed rotation period we conclude that the obliquity, psi, is less than 20 degrees. Further, the eccentricity of the orbit is less than 0.02. We argue that hot Jupiters with orbital periods as short as that of WASP-19b are two orders of magnitude less common than hot Jupiters at the 3-4-d `pileup'. We discuss the evolution of WASP-19b's orbit and argue that most likely it was first moved to near twice the Roche limit by third-body interactions, and has since spiralled inwards to its current location under tidal decay. This is compatible with a stellar tidal-dissipation quality factor, Qs, of order 10^7.