# The Rossiter-McLaughlin effect reloaded: Probing the 3D spin-orbit geometry, differential stellar rotation, and the spatially-resolved stellar spectrum of star-planet systems

**Authors:** H. M. Cegla, C. Lovis, V. Bourrier, B. Beeck, C. A. Watson, F. Pepe  
**arXiv:** [1602.00322](https://arxiv.org/abs/1602.00322)  
**Date:** 2016-01-31  
**Summary page:** https://www.summarizepaper.com/en/arxiv-id/1602.00322v1/

## Key points

- The Rossiter-McLaughlin (RM) effect occurs when a planet transits its host star, causing a distortion of the spectral lines and a change in the line-of-sight velocities.
- A new RM modelling technique has been developed that directly measures the spatially-resolved stellar spectrum behind the planet.
- This approach does not assume any shape for intrinsic local profiles and allows for differential stellar rotation and centre-to-limb variations in convective blueshift.
- The new technique was applied to HD189733 and compared to 3D magnetohydrodynamic (MHD) simulations.
- Rigid body rotation was rejected with high confidence (>99% probability), allowing for determination of occulted stellar latitudes and measurement of stellar inclination.
- The sky-projected (lambda ~ -0.4 +/- 0.2 degrees) and true 3D obliquity (psi ~ 7^+12_-4 degrees) were also determined, showing good agreement with MHD simulations without significant centre-to-limb variations detectable in local profiles.
- This breakthrough offers an exciting opportunity to gain insight into star-planet systems' dynamics and evolution while providing valuable information about stars' physical properties beyond what was previously possible using traditional methods.
- This new powerful tool can probe stellar photospheres, differential rotation, determine 3D obliquities, and remove sky-projection biases in planet migration theories.
- It can be implemented with existing instrumentation but will become even more potent with next generation high precision radial velocity spectrographs.

## Summary

The Rossiter-McLaughlin (RM) effect is a phenomenon that occurs when a planet transits its host star, causing a distortion of the spectral lines and a change in the line-of-sight velocities. This effect is sensitive to the star-planet alignment, which provides information on the system's dynamical history. A new RM modelling technique has been developed that directly measures the spatially-resolved stellar spectrum behind the planet. The technique involves scaling the continuum flux of HARPS spectra by the transit light curve and subtracting the in- from out-of-transit spectra to isolate the starlight behind the planet. Unlike previous techniques, this approach does not assume any shape for intrinsic local profiles and allows for differential stellar rotation and centre-to-limb variations in convective blueshift. The new technique was applied to HD189733 and compared to 3D magnetohydrodynamic (MHD) simulations. Rigid body rotation was rejected with high confidence (>99% probability), allowing for determination of occulted stellar latitudes and measurement of stellar inclination. The sky-projected (lambda ~ -0.4 +/- 0.2 degrees) and true 3D obliquity (psi ~ 7^+12_-4 degrees) were also determined, showing good agreement with MHD simulations without significant centre-to-limb variations detectable in local profiles. This breakthrough offers an exciting opportunity to gain insight into star-planet systems' dynamics and evolution while providing valuable information about stars' physical properties beyond what was previously possible using traditional methods. This new powerful tool can probe stellar photospheres, differential rotation, determine 3D obliquities, and remove sky-projection biases in planet migration theories. It can be implemented with existing instrumentation but will become even more potent with next generation high precision radial velocity spectrographs. Overall, this breakthrough offers an exciting opportunity to gain deep insights into star planet systems' dynamics and evolution while providing valuable information about stars' physical properties beyond what was previously achievable through traditional methods.