Kuiper Belt-Like Hot and Cold Populations of Planetesimal Inclinations in the $\beta$ Pictoris Belt Revealed by ALMA

Abstract

The inclination distribution of the Kuiper belt provides unique constraints on its origin and dynamical evolution, motivating vertically resolved observations of extrasolar planetesimal belts. We present ALMA observations of millimeter emission in the near edge-on planetesimal belt around $\beta$ Pictoris, finding that the vertical distribution is significantly better described by the sum of two Gaussians compared to a single Gaussian. This indicates that, as for the Kuiper belt, the inclination distribution of $\beta$ Pic's belt is better described by the sum of dynamically hot and cold populations rather than a single component. The hot and cold populations have RMS inclinations of 8.9${}_{-0.5}^{+0.7}$ and 1.1${}_{-0.5}^{+0.5}$ degrees. We also report that an axisymmetric belt model provides a good fit to new and archival ALMA visibilities, and confirm that the midplane is misaligned with respect to $\beta$ Pic b's orbital plane. However, we find no significant evidence for either the inner disk tilt observed in scattered light and CO emission or the South-West/North-East (SW/NE) asymmetry previously reported for millimeter emission. Finally, we consider the origin of the belt's inclination distribution. Secular perturbations from $\beta$ Pic b are unlikely to provide sufficient dynamical heating to explain the hot population throughout the belt's radial extent, and viscous stirring from large bodies within the belt alone cannot reproduce the two populations observed. This argues for an alternative or additional scenario, such as planetesimals being born with high inclinations, or the presence of a `$\beta$ Pic c' planet, potentially migrating outwards near the belt's inner edge.

19 pages, 12 figures; Accepted for publication in AJ

References in corpus (9)

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• Solar System Physics for Exoplanet Research
• The Gemini Planet Imager Exoplanet Survey: Dynamical Mass of the Exoplanet beta Pictoris b from Combined Direct Imaging and Astrometry
• Exocomets from a Solar System Perspective
• Constraining planetesimal stirring: how sharp are debris disc edges?
• Dust Populations in the Iconic Vega Planetary System Resolved by ALMA
• Upper limits on protolunar disc masses using ALMA observations of directly-imaged exoplanets
• Extrasolar Kuiper Belts
• Relating grain size distributions in circumstellar discs to the spectral index at millimetre wavelengths
• Debris Disks in Multi-Planet Systems: Are Our Inferences Compromised by Unseen Planets?