• DOI:10.3847/2041-8213/ab9d27
• Corpus ID: 219956049

Spin–Orbit Alignment of the β Pictoris Planetary System

@article{Kraus2020SpinOrbitAO,
  title={Spin–Orbit Alignment of the $\beta$ Pictoris Planetary System},
  author={Stefan Kraus and Jean-Baptiste Le Bouquin and Alexander Kreplin and Claire L. Davies and Edward Hone and John D. Monnier and Tyler Gardner and Grant M. Kennedy and Sasha Hinkley},
  journal={arXiv: Earth and Planetary Astrophysics},
  year={2020}
}

• S. Kraus, J. Le Bouquin, +6 authors S. Hinkley
• Published 18 June 2020
• Physics, Geology
• arXiv: Earth and Planetary Astrophysics

A crucial diagnostic that can tell us about processes involved in the formation and dynamical evolutionof planetary systems is the angle between the rotation axis of a star and a planet's orbital angular momentum vector ("spin-orbit" alignment or "obliquity"). Here we present the first spin-orbit alignment measurement for a wide-separation exoplanetary system, namely on the directly-imaged planet $\beta$ Pictoris b. We use VLTI/GRAVITY spectro-interferometry with an astrometric accuracy of 1 $\mu$as (microarcsecond) in the Br$\gamma$ photospheric absorption line to measure the photocenter displacement associated with the stellar rotation. Taking inclination constraints from astroseismology into account, we constrain the 3-dimensional orientation of the stellar spin axis and find that $\beta$ Pic b orbits its host star on a prograde orbit. The angular momentum vectors of the stellar photosphere, the planet, and the outer debris disk are well-aligned with mutual inclinations $<3\pm5^{\circ}$, which indicates that $\beta$ Pic b formed in a system without significant primordial misalignments. Our results demonstrate the potential of infrared interferometry to measure the spin-orbit alignment for wide-separation planetary systems, probing a highly complementary regime to the parameter space accessible with the Rossiter-McLaughlin effect. If the low obliquity is confirmed by measurements on a larger sample of wide-separation planets, it would lend support to theories that explain the obliquity in Hot Jupiter systems with dynamical scattering and the Kozai-Lidov mechanism.