A JWST/MIRI analysis of the ice distribution and polycyclic aromatic hydrocarbon emission in the protoplanetary disk HH 48 NE

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
² Institute of Astronomy, Department of Physics, National Tsing Hua University, Hsinchu, Taiwan
³ Department of Chemistry, University of California, Berkeley, CA 94720-1460, USA
⁴ Institut des Sciences Moléculaires d’Orsay, CNRS, Univ. Paris-Saclay, 91405 Orsay, France
⁵ Institute for Astronomy, University of Hawai’i at Manoa, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
⁶ Astrochemistry Laboratory, NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA
⁷ Department of Physics, Catholic University of America, Washington, DC 20064, USA
⁸ Physikalisch-Meteorologisches Observatorium Davos und Weltstrahlungszentrum (PMOD/WRC), Dorfstrasse 33, 7260, Davos Dorf, Switzerland
⁹ Centre for Interstellar Catalysis, Department of Physics and Astronomy, Aarhus University, 8000 Aarhus, Denmark
¹⁰ Department of Astronomy, University of Virginia, Charlottesville, VA 22904, USA
¹¹ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
¹² Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
¹³ National Radio Astronomy Observatory, Charlottesville, VA 22903, USA
¹⁴ Center for Astrophysics I Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
¹⁵ Physique des Interactions Ioniques et Moléculaires, CNRS, Aix Marseille Univ., 13397 Marseille, France
¹⁶ INAF – Osservatorio Astrofisico di Catania, via Santa Sofia 78, 95123 Catania, Italy
¹⁷ Department of Physics, University of Central Florida, Orlando, FL 32816, USA
¹⁸ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
¹⁹ Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
²⁰ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße 1, 85748 Garching bei München, Germany

Received: 24 May 2024
Accepted: 11 July 2024

Abstract

Context. Ice-coated dust grains provide the main reservoir of volatiles that play an important role in planet formation processes and may become incorporated into planetary atmospheres. However, due to observational challenges, the ice abundance distribution in protoplanetary disks is not well constrained. With the advent of the James Webb Space Telescope (JWST), we are in a unique position to observe these ices in the near- to mid-infrared and constrain their properties in Class II protoplanetary disks.

Aims. We present JWST Mid-InfraRed Imager (MIRI) observations of the edge-on disk HH 48 NE carried out as part of the Direc- tor’s Discretionary Early Release Science program Ice Age, completing the ice inventory of HH 48 NE by combining the MIRI data (5–28 μm) with those of NIRSpec (2.7–5 μm).

Methods. We used radiative transfer models tailored to the system, including silicates, ices, and polycyclic aromatic hydrocarbons (PAHs) to reproduce the observed spectrum of HH 48 NE with a parameterized model. The model was then used to identify ice species and constrain spatial information about the ices in the disk.

Results. The mid-infrared spectrum of HH 48 NE is relatively flat, with weak ice absorption features. We detect CO₂, NH₃, H₂O, and tentatively CH₄ and NH₄⁺. Radiative transfer models suggest that ice absorption features are produced predominantly in the 50–100 au region of the disk. The CO₂ feature at 15 μm probes a region closer to the midplane (z/r = 0.1–0.15) than the corresponding feature at 4.3 μm (z/r = 0.2–0.6), but all observations trace regions significantly above the midplane reservoirs where we expect the bulk of the ice mass to be located. Ices must reach a high scale height (z/r ~ 0.6; corresponding to a modeled dust extinction A_v ~ 0.1), in order to be consistent with the observed vertical distribution of the peak ice optical depths. The weakness of the CO₂ feature at 15 μm relative to the 4.3 μm feature and the red emission wing of the 4.3 μm CO₂ feature are both consistent with ices being located at a high elevation in the disk. The retrieved NH₃ abundance and the upper limit on the CH₃OH abundance relative to H₂O are significantly lower than those in the interstellar medium, but consistent with cometary observations. The contrast of the PAH emission features with the continuum is stronger than for similar face-on protoplanetary disks, which is likely a result of the edge-on system geometry. Modeling based on the relative strength of the emission features suggests that the PAH emission originates in the disk surface layer rather than the ice absorbing layer.

Conclusions. Full wavelength coverage is required to properly study the abundance distribution of ices in disks. To explain the pres- ence of ices at high disk altitudes, we propose two possible scenarios: a disk wind that entrains sufficient amounts of dust, and thus blocks part of the stellar UV radiation, or vertical mixing that cycles enough ices into the upper disk layers to balance ice photodesorption from the grains.