PDRs4All. XXIII. JWST-MIRI template spectra of polycyclic aromatic hydrocarbons in the Orion Bar obtained with blind signal separation

The aromatic infrared bands (AIBs) are observed in a wide variety of astrophysical sources, and now in unprecedented detail by the Space Telescope (JWST). These bands are generally attributed to the emission from polycyclic aromatic hydrocarbons (PAHs). Our aim is to extract mid-infrared AIB template spectra from JWST observations of the Orion Bar photo-dissociation region that can be used to derive PAH properties from observations of other objects. We used the JWST MIRI-MRS observations from the PDRs4All program. We first isolated the AIBs from the data using a multi-component fitting approach. We then applied a nonnegative matrix factorization to the isolated AIB spectra and extracted two templates, named James Webb and following previous work. The spatial distribution and spectral characteristics of these two spectra suggest that they originate from two chemically distinct populations. We interpret as emission originating from a PAH population strongly exposed to ultraviolet (UV) radiation and mainly composed of large, compact ionized PAHs. Conversely, is interpreted as originating from a more shielded population of PAHs, which is less exposed to the UV by being more embedded in the cloud. This population consists of small, asymmetric, and mainly neutral PAHs with bonded aliphatic side groups. These templates can serve to analyze PAH emission in a wide variety of objects observed with MIRI-MRS.