The asymmetric inner disk of the Herbig Ae star HD 163296 in the eyes of VLTI/MATISSE: evidence for a vortex?^★

¹ Leiden Observatory, Leiden University, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands
e-mail: varga@strw.leidenuniv.nl
² Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Konkoly Thege Miklós út 15-17, 1121 Budapest, Hungary
³ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1090 GE Amsterdam, The Netherlands
⁴ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁵ Laboratoire Lagrange, Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Boulevard de l’Observatoire, CS 34229, 06304 Nice Cedex 4, France
⁶ Institute for Mathematics, Astrophysics and Particle Physics, Radboud University, PO Box 9010, MC 62 6500 GL Nijmegen, The Netherlands
⁷ SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
⁸ AIM, CEA, CNRS, Université Paris-Saclay, Université Paris Diderot, Sorbonne Paris Cité, 91191 Gif-sur-Yvette, France
⁹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹⁰ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹¹ NOVA Optical IR Instrumentation Group at ASTRON, Dwingeloo, Netherlands
¹² European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
¹³ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
¹⁴ Institut für Theoretische Physik und Astrophysik, Christian-Albrechts-Universität zu Kiel, Leibnizstraße 15, 24118, Kiel, Germany
¹⁵ Institute of Physics, ELTE Eötvös Loránd University, Pázmány Péter sétány 1/A, 1117 Budapest, Hungary
¹⁶ NASA Goddard Space Flight Center, Astrophysics Division, Greenbelt, MD 20771, USA
¹⁷ Departamento de Astronomía, Universidad de Concepción, Casilla 160-C, Concepción, Chile
¹⁸ Nicolaus Copernicus Astronomical Centre, Polish Academy of Sciences, Bartycka 18, 00-716 Warszawa, Poland
¹⁹ Unidad Mixta Internacional Franco-Chilena de Astronomía (CNRS UMI 3386), Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
²⁰ Department of Astrophysics, University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
²¹ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
²² Zselic Park of Stars, 064/2 hrsz., 7477 Zselickisfalud, Hungary
²³ Sydney Institute for Astronomy, School of Physics, A28, The University of Sydney, NSW 2006, Australia

Received: 11 September 2020
Accepted: 4 December 2020

Abstract

Context. A complex environment exists in the inner few astronomical units of planet-forming disks. High-angular-resolution observations play a key role in our understanding of the disk structure and the dynamical processes at work.

Aims. In this study we aim to characterize the mid-infrared brightness distribution of the inner disk of the young intermediate-mass star HD 163296 from early VLTI/MATISSE observations taken in the L- and N-bands. We put special emphasis on the detection of potential disk asymmetries.

Methods. We use simple geometric models to fit the interferometric visibilities and closure phases. Our models include a smoothed ring, a flat disk with an inner cavity, and a 2D Gaussian. The models can account for disk inclination and for azimuthal asymmetries as well. We also perform numerical hydrodynamical simulations of the inner edge of the disk.

Results. Our modeling reveals a significant brightness asymmetry in the L-band disk emission. The brightness maximum of the asymmetry is located at the NW part of the disk image, nearly at the position angle of the semimajor axis. The surface brightness ratio in the azimuthal variation is 3.5 ± 0.2. Comparing our result on the location of the asymmetry with other interferometric measurements, we confirm that the morphology of the r < 0.3 au disk region is time-variable. We propose that this asymmetric structure, located in or near the inner rim of the dusty disk, orbits the star. To find the physical origin of the asymmetry, we tested a hypothesis where a vortex is created by Rossby wave instability, and we find that a unique large-scale vortex may be compatible with our data. The half-light radius of the L-band-emitting region is 0.33 ±0.01 au, the inclination is 52°_−7°^+5°, and the position angle is 143° ± 3°. Our models predict that a non-negligible fraction of the L-band disk emission originates inside the dust sublimation radius for μm-sized grains. Refractory grains or large (≳10 μm-sized) grains could be the origin of this emission. N-band observations may also support a lack of small silicate grains in the innermost disk (r ≲ 0.6 au), in agreement with our findings from L-band data.