The Aligned Orbit of the Eccentric Warm Jupiter K2-232b

TL;DR

This study measures the spin-orbit alignment of the warm Jupiter K2-232b, revealing it is well-aligned, and discusses how obliquity patterns relate to orbital distance and stellar temperature, informing planet formation theories.

Contribution

First measurement of the Rossiter-McLaughlin effect for K2-232b, providing new data on warm Jupiter obliquities and their relation to orbital and stellar properties.

Findings

K2-232b's orbit is well-aligned with its star's spin axis.

High obliquities are linked to planets with larger orbital distances or hotter host stars.

Tidal effects likely dampen obliquities in close-in, cooler star systems.

Abstract

Measuring the obliquity distribution of stars hosting warm Jupiters may help us to understand the formation of close-orbiting gas giants. Few such measurements have been performed due to practical difficulties in scheduling observations of the relatively infrequent and long-duration transits of warm Jupiters. Here, we report a measurement of the Rossiter-McLaughlin effect for K2-232b, a warm Jupiter (M_P=0.39 M_Jup) on an 11.17-day orbit with an eccentricity of 0.26. The data were obtained with the Automated Planet Finder during two separate transits. The planet's orbit appears to be well-aligned with the spin axis of the host star, with a projected spin-orbit angle of lambda = -11.1+/-6.6 deg. Combined with the other available data, we find that high obliquities are almost exclusively associated with planets that either have an orbital separation greater than 10 stellar radii or orbit stars with effective temperatures hotter than 6,000K. This pattern suggests that the obliquities of the closest-orbiting giant planets around cooler stars have been damped by tidal effects.