Dust properties across the CO snowline in the HD 163296 disk from ALMA and VLA observations

¹ INAF–Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
e-mail: guidi@arcetri.astro.it
² Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, 50121 Firenze, Italy
³ ESO, Karl Schwarzschild str. 2, 85748 Garching bei München, Germany
⁴ Excellence Cluster “Universe”, Boltzmannstr. 2, 85748 Garching bei Muenchen, Germany
⁵ Joint ALMA Observatory (JAO), Alonso de Cordova 3107 Vitacura – Santiago de Chile, Chile
⁶ National Radio Astronomy Observatory, Socorro, NM 87801, USA
⁷ Department of Physics and Astronomy, Rice University, 6100 Main Street, Houston, TX 77005, USA
⁸ School of Cosmic Physics, Dublin Institute for Advanced Studies, 31 Fitzwilliams Place, 2 Dublin, Ireland
⁹ Dipartimento di Fisica e Astronomia, Università di Padova, 35122 Padova, Italy
¹⁰ INAF–Osservatorio Astronomico di Padova, Vicolo dell Osservatorio 5, 35122 Padova, Italy
¹¹ Max Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹² Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
¹³ Department of Astronomy, California Institute of Technology, MC 249-17, Pasadena, CA 91125, USA
¹⁴ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
¹⁵ Department of Astronomy, University of Michigan, 830 Dennison Building, 500 Church Street, Ann Arbor, MI 48109, USA
¹⁶ Institute for Theoretical Astrophysics, Heidelberg University, Albert-Ueberle-Strasse 2, 69120 Heidelberg, Germany
¹⁷ SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, KY16 9 SS Scotland, UK
¹⁸ Jet Propulsion Laboratory, California Institute of Technology Pasadena, CA 91109, USA
¹⁹ The Netherlands Institute for Radio Astronomy (ASTRON), 7991 Dwingeloo, The Netherlands
²⁰ Korea Astronomy and Space Science Institute, 776 Daedeok-daero, Yuseong-gu, 34055 Daejeon, Republic of Korea

Received: 6 October 2015
Accepted: 27 January 2016

Abstract

Context. To characterize the mechanisms of planet formation it is crucial to investigate the properties and evolution of protoplanetary disks around young stars, where the initial conditions for the growth of planets are set. The high spatial resolution of Atacama Large Millimeter/submillimeter Array (ALMA) and Karl G. Jansky Very Large Array (VLA) observations now allows the study of radial variations of dust properties in nearby resolved disks and the investigation of the early stages of grain growth in disk midplanes.

Aims. Our goal is to study grain growth in the well-studied disk of the young, intermediate-mass star HD 163296 where dust processing has already been observed and to look for evidence of growth by ice condensation across the CO snowline, which has already been identified in this disk with ALMA.

Methods. Under the hypothesis of optically thin emission, we compare images at different wavelengths from ALMA and VLA to measure the opacity spectral index across the disk and thus the maximum grain size. We also use a Bayesian tool based on a two-layer disk model to fit the observations and constrain the dust surface density.

Results. The measurements of the opacity spectral index indicate the presence of large grains and pebbles (≥1 cm) in the inner regions of the disk (inside ~50 AU) and smaller grains, consistent with ISM sizes, in the outer disk (beyond 150 AU). Re-analyzing ALMA Band 7 science verification data, we find (radially) unresolved excess continuum emission centered near the location of the CO snowline at ~90 AU.

Conclusions. Our analysis suggests a grain size distribution consistent with an enhanced production of large grains at the CO snowline and consequent transport to the inner regions. Our results combined with the excess in infrared scattered light suggests there is a structure at 90 AU involving the whole vertical extent of the disk. This could be evidence of small scale processing of dust at the CO snowline.