Orbital alignment of HD 332231 b

TL;DR

This study measures the stellar obliquity of HD 332231 b, revealing a well-aligned orbit and suggesting a gentle formation history without high-eccentricity migration, based on spectroscopic and photometric data.

Contribution

First measurement of the projected stellar obliquity for HD 332231 b using multiple analysis methods, confirming a well-aligned orbit and providing insights into its formation history.

Findings

Projected obliquity of -2+/-6 degrees indicating alignment

Evidence for transit timing variations suggesting a third body

Low orbital eccentricity implying gentle migration history

Abstract

Contrary to the orthodox picture of planet formation resulting in a neatly ordered Solar System, exoplanet systems exhibit highly diverse orbits: short and long periods, circular and eccentric, well- and misaligned, and even retrograde orbits. In order to understand this diversity it is essential to probe key orbital parameters. Spin-orbit alignment is such a parameter and can provide information about the formation and migration history of the system. However, tidal circularisation and alignment might hamper interpretations of orbital eccentricity and obliquities in the context of planet formation and evolution for planets on orbits shorter than about 10 days. Here we aim to measure the projected stellar obliquity in the HD 332231 system in which a warm (period~18.7 days) giant planet orbits a bright F star on a circular orbit. We observed the system during a transit with the HARPS-N spectrograph and obtained data on the Rossiter-McLaughlin effect. We analysed the spectroscopic transit data together with new TESS photometry employing three different analysis methods. The results from the different approaches are fully consistent. We find a projected obliquity of -2+/-6 deg, indicating the stellar spin axis to be well-aligned with the orbit of the planet. We furthermore find evidence for transit timing variations suggesting the presence of an additional third body in the system. Together with the low orbital eccentricity, the good alignment suggests that this warm giant planet has not undergone high-eccentricity migration.