Letter to the Editor

JWST MIRI-MRS observations of the Red Rectangle: Aromatic infrared band class transformation in the outer nebula

¹ Department of Physics & Astronomy, The University of Western Ontario, London, ON N6A 3K7, Canada
² Institute for Earth and Space Exploration, The University of Western Ontario, London, ON N6A 3K7, Canada
³ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH, Amsterdam, The Netherlands
⁴ School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom
⁵ Ritter Astrophysical Research Center, University of Toledo, Toledo, OH 43606, USA
⁶ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁷ SRON Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA, Leiden, The Netherlands
⁸ Institute for Mathematics, Astrophysics and Particle Physics, Radboud University, MC 62, NL-6500 GL, Nijmegen, The Netherlands

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

Received: 23 February 2026
Accepted: 1 April 2026

Abstract

Aims. We characterize the mid-infrared spectrum of the outer regions of the Red Rectangle nebula to probe the carbonaceous dust and molecular content beyond the circumbinary disk.

Methods. We present JWST MIRI-MRS observations of the southwest whisker that were extracted from three distinct environments: the biconical outflow, the whisker itself, and the shadow region outside the outflow. We compare them with an archival ISO-SWS observation of the inner nebula.

Results. The JWST spectra display only classical aromatic infrared band (AIB) emission on a weak dust continuum, with no signatures of the oxygen-rich circumbinary disk mineralogy nor of the rich molecular emission seen at optical wavelengths. The AIBs are predominantly Class 𝒜, which is in marked contrast to the exclusively Class ℬ profiles previously reported for the inner regions; there are systematic differences between the outflow and shadow regions that point to environmentally driven polycyclic aromatic hydrocarbon processing.