The edge-on protoplanetary disk HH 48 NE

II. Modeling ices and silicates

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: sturm@strw.leidenuniv.nl
² Department of Chemistry, University of California, Berkeley, CA 94720-1460, USA
³ Institute of Astronomy, Department of Physics, National Tsing Hua University, Hsinchu, Taiwan
⁴ Institut des Sciences Moléculaires d’Orsay, CNRS, Univ. Paris-Saclay, 91405 Orsay, France
⁵ Center for Space and Habitability, Universität Bern, Gesellschaftsstrasse 6, 3012, Bern, Switzerland
⁶ Center for Interstellar Catalysis, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, Aarhus C 8000, Denmark
⁷ Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
⁸ INAF–Osservatorio Astrofisico di Catania, via Santa Sofia 78, 95123 Catania, Italy
⁹ Department of Physics, University of Central Florida, Orlando, FL 32816, USA
¹⁰ Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹¹ TMT International Observatory, 100 W Walnut St, Suite 300, Pasadena, CA USA
¹² National Astronomical Observatory of Japan, National Institutes of Natural Sciences (NINS), 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan

Received: 1 February 2023
Accepted: 29 April 2023

Abstract

Context. The abundance and distribution of ice in protoplanetary disks is critical for an understanding of the link between the composition of circumstellar matter and the composition of exoplanets. Edge-on protoplanetary disks are a useful tool for constraining this ice composition and its location in the disk because the spectral signatures of the ice can be observed in absorption against the continuum emission that arises from the warmer regions in the central disk.

Aims. The aim of this work is to model ice absorption features in protoplanetary disks and to determine how well the abundance of the main ice species throughout the disk can be determined within the uncertainty of the physical parameter space. The edge-on proto-planetary disk around HH 48 NE, a target of the James Webb Space Telescope Early Release program Ice Age, is used as a reference system.

Methods. We used the full anisotropic scattering capabilities of the radiative transfer code RADMC-3D to ray-trace the mid-infrared continuum. Using a constant parameterized ice abundance, we added ice opacities to the dust opacity in regions in which the disk was cold enough for the main carbon, oxygen, and nitrogen carriers to freeze out.

Results. The global abundance relative to the dust content of the main ice carriers in HH 48 NE can be determined within a factor of 3 when the uncertainty of the physical parameters is taken into account. Ice features in protoplanetary disks can be saturated at an optical depth of ≲1 due to local saturation. Ices are observed at various heights in the disk model, but in this model, spatial information is lost for features at wavelengths >7 µm when observing with James Webb Space Telescope because the angular resolution decreases towards longer wavelengths. Spatially observed ice optical depths cannot be directly related to column densities, as would be the case for direct absorption against a bright continuum source, because of radiative transfer effects. Vertical snowlines will not be a clear transition because the height of the snow surface increases radially, but their location may be constrained from observations using radiative transfer modeling. Radial snowlines are not really accessible. Not only the ice abundance, but also the inclination, the settling, the grain size distribution, and the disk mass have a strong impact on the observed ice absorption features in disks. Relative changes in the ice abundance can only be inferred from observations if the source structure is well constrained.