Letter to the Editor

The near-infrared silhouette of the Flying Saucer edge-on disc revealed by the JWST JEDIce program

¹ Institut des Sciences Moléculaires d’Orsay, CNRS, Univ. Paris-Saclay 91405 Orsay, France
² Physique des Interactions Ioniques et Moléculaires, CNRS, Aix Marseille Université Marseille, France
³ Dept. Chemistry, University of California Berkeley CA, USA
⁴ Jet Propulsion Laboratory, California Institute of Technology 4800 Oak Grove Drive Pasadena CA 91109, USA
⁵ Astronomy Department, University of Virginia 530 McCormick Rd. Charlottesville VA 22903, USA
⁶ Institute of Astronomy, Department of Physics, National Tsing Hua University Hsinchu, Taiwan
⁷ Leiden Observatory, Leiden University Leiden, The Netherlands
⁸ Center for Astrophysics, Harvard & Smithsonian 60 Garden St. Cambridge MA 02138, USA
⁹ Astrophysics & Space Institute, Schmidt Sciences New York NY 10011, USA
¹⁰ Physikalisch-Meteorologisches Observatorium Davos und Weltstrahlungszentrum Dorfstrasse 33 7260 Davos Dorf, Switzerland
¹¹ Univ. Grenoble Alpes, CNRS, IPAG 38000 Grenoble, France
¹² Star and Planet Formation Laboratory 2-1 Hirosawa Wako Saitama 351-0198, Japan

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 9 September 2025
Accepted: 15 December 2025

Abstract

Aims. Edge-on discs offer a unique opportunity to probe radial and vertical dust and gas distributions in the protoplanetary phase. This study aims to investigate the distribution of micron-sized dust particles in the Flying Saucer in Rho Ophiuchi by leveraging the unique observational conditions of a bright infrared background that enables the edge-on disc to be seen in both silhouette and scattered light at specific wavelengths.

Methods. We used NIRSpec IFU observations from the JWST Edge-on Disc Ice program (JEDIce) of the Flying Saucer serendipitously observed against a Polycyclic Aromatic Hydrocarbons-emitting background to constrain the dust distribution and grain sizes via radiative transfer modelling.

Results. The observation of the Flying Saucer in silhouette at 3.29 μm reveals that the midplane radial extent of small dust grains is ∼235 au, i.e. larger than the large-grain disc extent previously determined to be 190 au from millimetre data. The scattered light observed in emission probes micron-sized icy grains at large vertical distances above the midplane. The vertical extent of the disc silhouette is similar at visible, near-IR, and mid-IR wavelengths, corroborating the conclusion that dust settling is inefficient for grains as large as tens of microns, both vertically and radially.