Learning mid-IR emission spectra of polycyclic aromatic hydrocarbon populations from observations

Abstract

Context. The James Webb Space Telescope (JWST) will deliver an unprecedented quantity of high-quality spectral data over the 0.6-28 µm range. It will combine sensitivity, spectral resolution, and spatial resolution. Specific tools are required to provide efficient scientific analysis of such large data sets. Aims. Our aim is to illustrate the potential of unsupervised learning methods to get insights into chemical variations in the populations that carry the aromatic infrared bands (AIBs), more specifically polycyclic aromatic hydrocarbon (PAH) species and carbonaceous very small grains (VSGs). Methods. We present a method based on linear fitting and blind signal separation for extracting representative spectra for a spectral data set. The method is fast and robust, which ensures its applicability to JWST spectral cubes. We tested this method on a sample of ISO-SWS data, which resemble most closely the JWST spectra in terms of spectral resolution and coverage. Results. Four representative spectra were extracted. Their main characteristics appear consistent with previous studies with populations dominated by cationic PAHs, neutral PAHs, evaporating VSGs, and large ionized PAHs, known as the PAH x population. In addition, the 3 µm range, which is considered here for the first time in a blind signal separation (BSS) method, reveals the presence of aliphatics connected to neutral PAHs. Each representative spectrum is found to carry second-order spectral signatures (e.g., small bands), which are connected with the underlying chemical diversity of populations. However, the precise attribution of theses signatures remains limited by the combined small size and heterogeneity of the sample of astronomical spectra available in this study. Conclusions. The upcoming JWST data will allow us to overcome this limitation. The large data sets of hyperspectral images provided by JWST analysed with the proposed method, which is fast and robust, will open promising perspectives for our understanding of the chemical evolution of the AIB carriers.

A radiolytic origin of organic matter in primitive chondrites and trans-neptunian objects? New clues from ion irradiation experiments

“…no significant contribution of grains formed in the presolar disk, we assume those grains were partly constituted by the carrier of the socalled Aromatic Interstellar Bands (AIBs), i.e. Polycyclic Aromatic Hydrocarbon molecular entities made of 20-100 atoms (Foschino et al, 2019). These species are larger than the polyaromatic units detected in chondritic IOM (1-4 rings, ~6-20 atoms) (Cody et al, 2002;Derenne and Robert, 2010) and they are devoid of oxygen, nitrogen or sulfur.…”

PDRs4All: A JWST Early Release Science Program on Radiative Feedback from Massive Stars

“…The spectra are available in numerical format from [link to be inserted upon publication]. The four spectra have been computed individually for each region and each component using (i) the Cloudy code for the ionized gas in the H II region and the ionization front (Ferland et al 2017); (ii) the Meudon PDR code for the contribution from the atomic and molecular lines in the neutral PDR gas (Le Petit et al 2006); (iii) the AIB spectra extracted by Foschino et al (2019); and (iv) the DustEM tool and SOC radiative transfer code for the contribution from the dust continuum and scattered light (Compiègne et al 2011;Juvela 2019;Schirmer et al 2021). Below we briefly describe the parameters and calculation requirements used for each model and region.…”

“…PAH-related species, namely neutral PAHs (PAH 0 ), cationic PAHs (PAH + ), evaporating very small grains (eVSGs), and large ionized PAHs (PAH x ). The description of these species is given in Pilleri et al (2012Pilleri et al ( , 2015 and Foschino et al (2019).…”

Simulated JWST Data Sets for Multispectral and Hyperspectral Image Fusion

“…This matrixH is composed of k=4 spectra corresponding to four regions of a PDR as depicted in Figure 1: the H II region, the ionization front, the dissociation front, and the molecular cloud. These spectra have been computed individually for each region, using the Meudon PDR code for the contribution from molecular and atomic lines (Le Petit et al 2006), the CLOUDY code for the ionized gas (Ferland et al 1998), the PAHTAT model (Pilleri et al 2012), and results from the decomposition of Foschino et al (2019) for the PAH emission and the DUSTEM model for the contribution from the dust continuum (Compiègne et al 2011). The physical parameters used for these models correspond to those of the Orion Bar, which is a well-studied region.…”