A JWST inventory of protoplanetary disk ices

The edge-on protoplanetary disk HH 48 NE, seen with the Ice Age ERS program^★

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: sturm@strw.leidenuniv.nl
² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
³ Institute of Astronomy, Department of Physics, National Tsing Hua University, Hsinchu, Taiwan
⁴ Department of Chemistry, University of California, Berkeley, CA 94720-1460, USA
⁵ Institut des Sciences Moléculaires d’Orsay, CNRS, Univ. Paris-Saclay, 91405 Orsay, France
⁶ Institute for Astronomy, University of Hawai’i at Manoa, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
⁷ Physical Science Department, Diablo Valley College, 321 Golf Club Road, Pleasant Hill, CA 94523, USA
⁸ SETI Institute, 339 N. Bernardo Ave Suite 200, Mountain View, CA 94043, USA
⁹ Astrochemistry Laboratory, NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA
¹⁰ Department of Physics, Catholic University of America, Washington, DC 20064, USA
¹¹ Center for Space and Habitability, Universität Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
¹² Center for Interstellar Catalysis, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, Aarhus C 8000, Denmark
¹³ Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
¹⁴ Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹⁵ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
¹⁶ Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
¹⁷ National Radio Astronomy Observatory, Charlottesville, VA 22903, USA
¹⁸ Physique des Interactions Ioniques et Moléculaires, CNRS, Aix-Marseille Univ., 13397 Marseille, France
¹⁹ INAF – Osservatorio Astrofísico di Catania, via Santa Sofia 78, 95123 Catania, Italy
²⁰ Department of Physics, University of Central Florida, Orlando, FL 32816, USA
²¹ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
²² Southwest Research Institute, San Antonio, TX 78238, USA
²³ TMT International Observatory, 100 W Walnut St., Suite 300, Pasadena, CA, USA
²⁴ National Astronomical Observatory of Japan, National Institutes of Natural Sciences (NINS), 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
²⁵ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße 1, 85748 Garching bei München, Germany

Received: 20 July 2023
Accepted: 14 September 2023

Abstract

Ices are the main carriers of volatiles in protoplanetary disks and are crucial to our understanding of the protoplanetary disk chemistry that ultimately sets the organic composition of planets. The Director’s Discretionary-Early Release Science (DD-ERS) program Ice Age on the James Webb Space Telescope (JWST) follows the ice evolution through all stages of star and planet formation. JWST’s exquisite sensitivity and angular resolution uniquely enable detailed and spatially resolved inventories of ices in protoplanetary disks. JWST/NIRSpec observations of the edge-on Class II protoplanetary disk HH 48 NE reveal spatially resolved absorption features of the major ice components H₂O, CO₂, and CO, and multiple weaker signatures from less abundant ices NH₃, OCN⁻, and OCS. Isotopologue ¹³CO₂ ice has been detected for the first time in a protoplanetary disk. Since multiple complex light paths contribute to the observed flux, the ice absorption features are filled in by ice-free scattered light. This implies that observed optical depths should be interpreted as lower limits to the total ice column in the disk and that abundance ratios cannot be determined directly from the spectrum. The ¹²CO₂/¹³CO₂ integrated absorption ratio of 14 implies that the ¹²CO₂ feature is saturated, without the flux approaching zero, indicative of a very high CO₂ column density on the line of sight, and a corresponding abundance with respect to hydrogen that is higher than interstellar medium values by a factor of at least a few. Observations of rare isotopologues are crucial, as we show that the ¹³CO₂ observation allowed us to determine the column density of CO₂ to be at least 1.6 × 10¹⁸ cm⁻², which is more than an order of magnitude higher than the lower limit directly inferred from the observed optical depth. Spatial variations in the depth of the strong ice features are smaller than a factor of two. Radial variations in ice abundance, for example snowlines, are significantly modified since all observed photons have passed through the full radial extent of the disk. CO ice is observed at perplexing heights in the disk, extending to the top of the CO-emitting gas layer. Although poorly understood radiative transfer effects could contribute to this, we argue that the most likely interpretation is that we observed some CO ice at high temperatures, trapped in less volatile ices such as H₂O and CO₂. Future radiative transfer models will be required to constrain the physical origin of the ice absorption and the implications of these observations for our current understanding of disk physics and chemistry.