Azimuthal C/O variations in a planet-forming disk

Nature Astronomy, vol. 7, pp. 684–693

Abstract

The elemental carbon-to-oxygen ratio (C/O) in the atmosphere of a giant planet is a promising diagnostic of that planet’s formation history in a protoplanetary disk. Alongside efforts in the exoplanet community to measure the C/O ratio in planetary atmospheres, observational and theoretical studies of disks are increasingly focused on understanding how the gas-phase C/O ratio varies both with radial location and between disks. This is mostly tied to the icelines of major volatile carriers such as CO and H2O. Using ALMA observations of CS and SO, we have found evidence for an entirely unexpected type of C/O variation in the protoplanetary disk around HD 100546: an azimuthal variation from a typical, oxygen-dominated ratio (C/O ≈ 0.5) to a carbon-dominated ratio (C/O ≳ 1.0). We show that the spatial distribution and peculiar line kinematics of both CS and SO molecules can be well explained by azimuthal variations in the C/O ratio. We propose a shadowing mechanism that could lead to such a chemical dichotomy. Our results imply that tracing the formation history of giant exoplanets using their atmospheric C/O ratios will need to take into account time-dependent azimuthal C/O variations in a planet’s accretion zone. ALMA observations of the protoplanetary disk around HD 100546 reveal an unexpected C/O variation with azimuth. The carbon-dominated wedge of the disk can be reproduced via a model with a shadowing mechanism.

Authors 11

1. Luke Keyte corresponding

   University College London

   Affiliation as printed

   Department of Physics and Astronomy, University College London, London, UK

2. Mihkel Kama

   Tartu Observatory · University College London

   Affiliation as printed

   Department of Physics and Astronomy, University College London, London, UK

   Tartu Observatory, Tõravere, Estonia

3. Alice S. Booth Aachen Leiden Observatory

   Leiden University · Leiden Observatory

   Affiliation as printed

   Leiden Observatory, Leiden University, Leiden, the Netherlands

4. Edwin A. Bergin

   University of Michigan

   Affiliation as printed

   Department of Astronomy, University of Michigan, Ann Arbor, MI, USA

5. L. Ilsedore Cleeves

   University of Virginia

   Affiliation as printed

   Department of Astronomy, University of Virginia, Charlottesville, VA, USA

6. Ewine F. van Dishoeck Aachen Leiden Observatory

   Leiden University · Max Planck Institute for Extraterrestrial Physics · Leiden Observatory

   Affiliation as printed

   Leiden Observatory, Leiden University, Leiden, the Netherlands

   Max-Planck Institut für Extraterrestrische Physik (MPE), Garching, Germany

7. M. N. Drozdovskaya

   University of Bern · Center for Space and Habitability

   Affiliation as printed

   Center for Space and Habitability, Universität Bern, Bern, Switzerland

8. Kenji Furuya

   National Astronomical Observatory of Japan

   Affiliation as printed

   National Astronomical Observatory of Japan, Mitaka, Japan

9. Jonathan M. C. Rawlings

   University College London

   Affiliation as printed

   Department of Physics and Astronomy, University College London, London, UK

10. Oliver Shorttle

    University of Cambridge

    Affiliation as printed

    Department of Earth Sciences and Institute of Astronomy, University of Cambridge, Cambridge, UK

11. Catherine M. Walsh

    University of Leeds

    Affiliation as printed

    School of Physics and Astronomy, University of Leeds, Leeds, UK