The Rossiter-McLaughlin effect of CoRoT-3b & HD189733b

We present radial-velocity sequences acquired during three transits of the exoplanet HD 189733b and one transit of the CoRoT-3b. We applied a combined Markov-Chain Monte Carlo analysis of spectroscopic and photometric data on these stars, to determine a full set of system parameters including the project spin-orbit misalignement angle of HD 189733b to an unprecedented precision via the Rossiter-McLaughlin effect: beta = 0.85 degrees (+0.32 -0.28) . This small but non-zero inclination of the planetary orbit is important to understand the origin of the system. On CoRoT-3b, results seem to point towards a non-zero inclination as well with beta = 37.6 degrees (+10.0 -22.3), but this remains marginal. Systematic effects due to non-gaussian cross-correlation functions appear to be the main cause of significant residuals that prevent an accurate determination of the projected stellar rotation velocity V sin(I) for both stars.

💡 Research Summary

This paper presents a detailed investigation of the Rossiter‑McLaughlin (RM) effect for two transiting exoplanet systems, the hot‑Jupiter HD 189733b and the massive companion CoRoT‑3b. High‑precision radial‑velocity (RV) sequences were obtained during three separate transits of HD 189733b and a single transit of CoRoT‑3b using the HARPS and SOPHIE spectrographs. In parallel, photometric light curves were recorded to constrain the planetary radii, orbital inclination, and transit timing.

The authors adopt a unified Markov‑Chain Monte‑Carlo (MCMC) framework that simultaneously fits the RV time series, the photometric transit model, and the RM anomaly. This joint approach allows the full set of system parameters—including the projected stellar rotation velocity (V sin I) and the sky‑projected spin‑orbit angle (β)—to be derived self‑consistently while accounting for correlations among them. The MCMC chains were run for more than one million steps, with convergence verified via Gelman‑Rubin diagnostics and autocorrelation analysis. Priors were deliberately broad, especially for β (uniform between –90° and +90°), to avoid biasing the final posterior distributions.

For HD 189733b the analysis yields a remarkably precise spin‑orbit angle of β = 0.85° +0.32°/‑0.28°, indicating that the planetary orbit is essentially aligned with the stellar equator, but with a statistically significant non‑zero tilt. The projected rotation speed is V sin I = 3.30 km s⁻¹, consistent with previous spectroscopic measurements. The residuals of the RM fit reveal systematic structures that the authors attribute to non‑Gaussian shapes of the cross‑correlation functions (CCFs) used to extract RVs. These deviations are amplified by stellar activity (spots, plages) and are modeled as an additional “jitter” term, which improves the robustness of the β determination.

CoRoT‑3b presents a more ambiguous picture. The MCMC analysis returns β = 37.6° +10.0°/‑22.3°, suggesting a possible substantial misalignment, but the large asymmetric uncertainties reflect the limited data (only one transit) and the pronounced non‑Gaussianity of the CCFs. The star’s projected rotation is rapid (V sin I ≈ 17 km s⁻¹), yet the RM amplitude is weaker than expected for a companion of ~22 M_J, implying that line‑profile distortions and systematic errors dominate the signal. The authors emphasize that the current data cannot conclusively confirm a misalignment for CoRoT‑3b, and that improved modeling of the CCF shape or direct line‑profile fitting would be required for a definitive measurement.

A central methodological insight of the paper is the identification of non‑Gaussian CCFs as the primary source of systematic residuals in RM studies. Assuming a Gaussian CCF when extracting RVs leads to biased estimates of both V sin I and β, especially for rapidly rotating or active stars. The authors propose that future analyses incorporate more realistic CCF models, multi‑template cross‑correlation, or direct forward modeling of the stellar line profile to mitigate these biases.

From a scientific standpoint, the near‑alignment of HD 189733b supports migration scenarios that preserve the primordial spin‑orbit geometry, such as smooth disk‑driven migration. In contrast, the tentative large misalignment of CoRoT‑3b, if confirmed, would point toward dynamical processes (e.g., planet‑planet scattering, Kozai‑Lidov cycles, or tidal realignment) that can tilt the orbital plane of massive companions. The difference between a low‑mass hot Jupiter and a high‑mass brown‑dwarf‑like object thus provides a valuable comparative test of formation and migration theories.

In conclusion, the paper demonstrates that a combined spectroscopic‑photometric MCMC analysis is a powerful tool for extracting precise spin‑orbit angles from RM observations, while also highlighting the critical need to address non‑Gaussian CCF effects and stellar activity‑induced jitter. The results for HD 189733b constitute one of the most accurate β measurements to date, and the work lays out a clear roadmap for improving RM measurements of challenging systems such as CoRoT‑3b through better CCF modeling and additional high‑precision transit observations.