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E. Peeters

Publications and source records attributed to E. Peeters.

At least 19 recordsLinked to original sources
arXiv2026

An extended red wing of the 5797 Å diffuse interstellar band in the Red Rectangle nebula

Diffuse interstellar bands (DIBs) are interstellar molecular absorption features that are ubiquitous in the interstellar medium (ISM). So far, there was only one known sight line where DIBs show an extend tail towards the red (ETR), namely Herschel 36. Such sight lines are essential to illuminate the DIB problem, namely the identification of their carriers, because they show more DIB variation than standard ISM sight lines. To report a new detection of the 5797 Å DIB in the Red Rectangle nebula with an ETR, which adds a new observational constraint for the DIB carrier. We analysed archival GIRAFFE/FLAMES integral field spectroscopy to extract all spectral features between 5776 and 5810 Å at different spatial positions. We used time series spectra to identify stellar and circumstellar features, and data from different instruments to exclude instrumental effects. We report the firm detection of the 5797 Å DIB that is localised in the Red Rectangle nebula, or in the ISM in the sight line towards it. Curiously, while the 5797 Å DIB shows an ETR in the Red Rectangle, the 5780 Å DIB does not. Further, we report an exceptionally high equivalent ratio of EW(5797)/EW(5780) = 1.79 higher than any other literature value.

astro-ph.GA↗
arXiv2026

PDRs4All XXIII. JWST-MIRI template spectra of PAHs 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 James Webb space telescope (JWST). These bands are generally attributed to the emission from polycyclic aromatic hydrocarbon (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 non-negative matrix factorization to the isolated AIB spectra and extracted two templates, named AIBirrad and AIBshield following previous work. The spatial distribution and spectral characteristics of these two spectra suggest that they originate from two chemical distinct populations. We interpret AIBirrad as emission originating from a PAH population strongly exposed to ultra-violet (UV) radiation and mainly composed of large, compact ionized PAHs. Conversely, AIBshield 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.

astro-ph.GA↗
arXiv2026

H2 emission in the Orion Bar, NGC7023 and the Horsehead Nebula observed with the James Webb Space Telescope

Molecular hydrogen (H2), the most abundant molecule in the interstellar medium, is a handy tool to study Photodissociation Regions. The JWST, with its high spatial resolution, sensitivity, and wavelength coverage, provides unique access to the detection and spatial morphology of the H2 rotational and rovibrational lines. Our goal is to analyze H2 line emission detected with JWST in several PDRs (the Orion Bar, NGC7023, and the Horsehead Nebula) to constrain the physical structure of dissociation front, and compare the impact of the ultraviolet field and the gas density across different regions. The analysis of H2 spatial morphology reveals similar filamentary structures across all regions. Spatial shifts between H2 lines (0.5") are observed and linked to differences in excitation mechanisms (thermalized vs FUV-pumped lines) and the temperature gradient. Despite differences in incident irradiation conditions, analysis of H2 rotational excitation yields high gas temperatures that are similar across the three PDRs. This is expected for a certain regime of excited PDRs (such as Orion and NGC7023), where the G0 at the DFs and the density are similar. However, it is surprising for less excited PDRs like the Horsehead Nebula. OPRs in the rotational and rovibrational levels differ across the three PDRs. We also used H2 rovibrational lines and the 0-0 S(3) line to estimate the visual extinction across the field of view, yielding markedly different results that constrain both the geometry and the grain composition. Due to the complex 3D geometry, H2 emission is observed throughout the FOV and is dominated by the thin UV illuminated layer, making H2 an unreliable tracer of gas temperature in deeper and coolers molecular layers of the PDR. Therefore, H2 emission is not sufficient to trace the possible temperature and density gradient across PDRs and must therefore be associated with other tracers.

astro-ph.GA↗
arXiv2026

The JWST Proto-PAH project. Computational modeling of the emission carriers

The infrared spectra of many carbon-rich post-asymptotic giant branch stars are dominated by emission features from aromatic and aliphatic hydrocarbons, but the chemical structure of the carriers remains unidentified. Class D sources show unusual emission profiles with strong aliphatic emission, providing a stringent test of carrier candidates. Here we investigate whether hydrogenated carbonaceous molecular particles can be the carriers of these features. Rather than assuming candidate geometries we generate the initial structures using ab initio molecular dynamics simulations and optimize them using density functional theory. We then compare the computed emission spectra to JWST NIRSpec and MIRI/MRS observations of Class D sources. The structures providing the best agreements contain sizeable domains with defect-bearing aromatics and hydrogenated-aromatics connected by aliphatic bridges, in contrast to the "arophatic" cluster model of isolated two- to three-ring aromatics connected by aliphatic and olefinic bridges. As a representative example, we discuss in detail the molecular particle C130H96 (radius of 5.5 A) and compare its computed spectrum to the JWST spectra of IRAS 05110-6616, a Class D2 source with apparent shifts of the aromatic C-H out-of-plane bands. The calculated spectra are in qualitative agreement with the observed 3 micron profile, the aliphatic C-H bending bands, the broad 8 micron feature, the 11-14 micron C-H out-of-plane region, and the bands at 16 and 21 microns. We hypothesize that UV photo-driven fragmentation of such molecular particles releases aromatics linked by aliphatics, followed by the formation of polycyclic aromatic hydrocarbons and fullerenes as these objects evolve into planetary nebulae.

astro-ph.GA↗
arXiv2026

The JWST Proto-PAH project: Aromatic backbones with extensive aliphatic substitution account for both the aromatic and aliphatic emission

We find that the aromatic and aliphatic emission bands in four carbon-rich post-asymptotic giant branch stars with class D emission arise from a single carrier population: extensive aliphatic decoration of an aromatic skeleton, i.e. molecules with a coherent aromatic backbone and an unusually high aliphatic content. This conclusion is based on JWST NIRSpec and MIRI/MRS spectroscopy, covering 2.88-28.7um. The aliphatic 3.4um feature is stronger than the aromatic 3.3um feature, reversing the ratio seen in sources with typical polycyclic aromatic hydrocarbon (PAH) emission. If the aromatic emission arose from nominal PAHs, the 3.3um feature - unavailable in previous observations - together with the mid-infrared aromatic features, would constrain them to be large (greater than about carbon atoms), compact, and at least partially ionized. These properties predict a strong 8.6um feature. However, the 8.6um feature is weak or absent in all four sources, ruling out PAHs as the source of the aromatic emission. Instead, the emission must arise from the same carriers as the aliphatic emission. The position of the 6.9um feature and the broadened 11-14um out-of-plane emission independently support this. The CH2 and CH3 feature strengths vary across the sample, with the class D2 sources showing higher CH3 content. These carriers are far more aliphatic than the PAHs of planetary nebulae and the interstellar medium. However, they retain the aromatic skeleton that produces their aromatic features.

astro-ph.GA↗
arXiv2026

Searching for the elusive CH2+ with the James Webb Space Telescope. Another carbocation to constrain astrochemical networks

Carbocations are key species in interstellar chemistry, providing entry points for building larger hydrocarbons. CH+, and more recently, CH3+, have been detected. Other carbocations await detection to provide a comprehensive view of the astrochemical network that is at work in the interstellar medium. We search for CH2+ in objects in which CH3+ was detected and evaluate the most favorable conditions for detecting the elusive CH2+ reactive cation. We calculated the CH2+ rotational and rovibrational transitions expected to contribute in the mid- to far-infrared, focusing on the lower-energy rovibrational levels. We then calculated CH2+ infrared emission spectra at different excitation temperatures and compared them to JWST spectra of the externally irradiated disk d203-506 in Orion, where CH+ and CH3+ have already been detected. We used thermochemical models to predict the abundance and spatial morphology of CH2+ to better understand its nondetection. The comparison to JWST spectra allowed us to provide excitation-temperature-dependent upper limits to the excited column density. These are several times lower than those detected for CH+ and CH3+ in their excited states. Based on model calculations for photodissociation regions and assuming similar excitation temperatures, the upper limit derived from observations and CH2+ model spectrum is either slightly above or below the column density expected from models of photodissociation regions. We provide a list of tabulated transitions to allow the community to search for this carbocation in future observations as CH2+ is key in providing observational constraints on astrochemical models.

astro-ph.GA↗
arXiv2026

MUSE Imaging Spectroscopy of the Fullerene Planetary Nebula Tc 1

The planetary nebula Tc 1 (PN G345.2 -08.8), one of the rare group of Galactic PNe showing fullerene emission in the infrared, was observed with MUSE wide field mode with adaptive optics, wavelength range 4750-9300A. Extinction, electron temperature (T_e) and density (N_e) images are presented from collisionally excited and recombination line ratios. The nebula has a high surface brightness 12 arcsec core, an elliptical ring of major axis 2.8 arcsec around the central star and some low ionization knots, and an extended halo 55 arcsec in size; between the core and halo is an annulus with intermediate properties, including higher T_e and lower N_e than in the core. The image of optical extinction from H Balmer line ratios is highly structured, and shows an annulus, adjacent to the core, of low extinction, lower than the line-of-sight interstellar extinction. Instrumental effects to account for this anomalously low extinction area are investigated and intrinsic effects from the scattering properties of nebular dust; neither can entirely explain the low-extinction region and the most likely cause is a local non-standard dust reddening law. This low extinction region also shows an anomalously high He I 7281/6678A line ratio, possibly caused by a contaminating line, but none were conclusively identified. The spectrum of the central star was extracted and fitted by a 31000K model atmosphere and is of type O7.5I(f). Over extended regions an enhancement of the detected continuum above the nebular continuum was found, as also seen in some other PNe observed with MUSE. The annulus of low extinction occurs outside the region of strongest fullerene emission, in the zone where N_e declines and T_e rises. A change in dust properties linked to conditions in this transition region between the higher density core nebula and lower density halo is deduced. (Abridged)

astro-ph.SR↗
arXiv2026

PDRs4All XVIII. JWST-NIRCam Photometric properties of protoplanetary disks in the Orion Nebula Cluster

We use the high angular resolution NIRCam images from the PDRs4All program, combined with those of GTO program 1256, to extract key properties of disks in the Orion Nebula Cluster. We measure disk radii in silhouette against the bright background, identify dissociation fronts (DFs) and ionization fronts (IFs), determine Paschen $α$ intensities, and derive near-infrared spectral energy distributions (SEDs). From these diagnostics we define a typology of ONC disks. Type I sources show merged IFs and DFs close to the disk surface. Type II sources have DFs at the disk surface and IFs located tens of astronomical units away. Type III sources show a DF at the disk surface but no IF. For all types, PAH emission traces the PDR. We find that the disk radius $r_{\rm disk}$ increases with projected distance to the ionizing source $d_{\rm proj}$, following $r_{\rm disk} \propto d_{\rm proj}^{0.30}$, consistent with disk truncation by photoevaporation. Disk radii measured in the infrared are larger than those measured at millimeter wavelengths, suggesting radial dust segregation within the disks. In agreement with PDR models, the thermal pressure in the disk PDR increases with the FUV radiation field $G_0$, but with a flatter slope. Finally, the SEDs of candidate Jupiter Mass Binary Objects (JuMBOs) are similar to those of Type III disks, except for JuMBO24, which resembles a Type I or Type II source. Its SED is consistent with a young low-mass binary hosting an unresolved ionized disk.

astro-ph.GA↗
arXiv2026

JWST MIRI-MRS observations of the Red Rectangle: AIB class transformation in the outer nebula

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 SW whisker, extracted from three distinct environments: the biconical outflow, the whisker itself, and the shadow region outside the outflow. We compare these with an archival ISO-SWS observation of the inner nebula. Results: The JWST spectra display only classical 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 - in marked contrast to the exclusively Class B profiles previously reported for the inner regions - with systematic differences between the outflow and shadow regions pointing to environmentally driven PAH processing.

astro-ph.GA↗
arXiv2025

PDRs4All XV: CH radical and H$_3^+$ molecular ion in the irradiated protoplanetary disk d203-506

Most protoplanetary disks experience a phase in which they are subjected to strong ultraviolet radiation from nearby massive stars. This UV radiation can substantially alter their chemistry by producing numerous radicals and molecular ions. In this Letter we present detailed analysis of the JWST-NIRSpec spectrum of the d203-506 obtained as part of the PDRs4All Early Release Science program. Using state-of-the-art spectroscopic data, we searched for species using a multi-molecule fitting tool, PAHTATmol, that we developed for this purpose. Based on this analysis, we report the clear detection of ro-vibrational emission of the CH radical and likely detection of the H$_3^+$ molecular ion, with estimated abundances of a few times 10$^{-7}$ and approximately 10$^{-8}$, respectively. The presence of CH is predicted by gas-phase models and well explained by hydrocarbon photochemistry. H$_3^+$ is usually formed through reactions of H$_2$ with H$_2^+$ originating from cosmic ray ionization of H$_2$. However, recent theoretical studies suggest that H$_3^+$ also forms through UV-driven chemistry in strongly irradiated ($G_0>$10$^3$), dense ($n_{\rm H} >10^{6}$ cm$^{-3}$) gas. The latter is favored as an explanation for the presence of ``hot'' H$_3^+$ ($T_{\rm ex}\gtrsim$1000 K) in the outer disk layers of d203-506, coinciding with the emission of FUV-pumped H$_2$ and other ``PDR species'', such as CH$^+$, CH$_3^+$, and OH. Our detection of infrared emission from vibrationally excited H$_3^+$ and CH raises questions about their excitation mechanisms and, underscore that UV radiation can have a profound impact on the chemistry of planet forming disks. They also demonstrate the power of JWST pushing the limit for the detection of elusive species in protoplanetary disks.

astro-ph.GA↗
arXiv2025

PDRs4All. XII. FUV-driven formation of hydrocarbon radicals and their relation with PAHs

We present subarcsecond-resolution ALMA mosaics of the Orion Bar PDR in [CI] 609um, C2H (4-3), and C18O (3-2) emission lines complemented by JWST images of H2 and aromatic infrared band (AIB) emission. The rim of the Bar shows very corrugated structures made of small-scale H2 dissociation fronts (DFs). The [CI] 609 um emission peaks very close (~0.002 pc) to the main H2-emitting DFs, suggesting the presence of gas density gradients. These DFs are also bright and remarkably similar in C2H emission, which traces "hydrocarbon radical peaks" characterized by very high C2H abundances, reaching up to several x10^-7. The high abundance of C2H and of related hydrocarbon radicals, such as CH3, CH2, and CH, can be attributed to gas-phase reactions driven by elevated temperatures, the presence of C+ and C, and the reactivity of FUV-pumped H2. The hydrocarbon radical peaks roughly coincide with maxima of the 3.4/3.3 um AIB intensity ratio, a proxy for the aliphatic-to-aromatic content of PAHs. This implies that the conditions triggering the formation of simple hydrocarbons also favor the formation (and survival) of PAHs with aliphatic side groups, potentially via the contribution of bottom-up processes in which abundant hydrocarbon radicals react in situ with PAHs. Ahead of the DFs, in the atomic PDR zone (where [H]>>[H2]), the AIB emission is the brightest, but small PAHs and carbonaceous grains undergo photo-processing due to the stronger FUV field. Our detection of trace amounts of C2H in this zone may result from the photoerosion of these species. This study provides a spatially resolved view of the chemical stratification of key carbon carriers in a PDR. Overall, both bottom-up and top-down processes appear to link simple hydrocarbon molecules with PAHs in molecular clouds; however, the exact chemical pathways and their relative contributions remain to be quantified.

astro-ph.GA↗
arXiv2024

PDRs4All. X. ALMA and JWST detection of neutral carbon in the externally irradiated disk d203-506: Undepleted gas-phase carbon

The gas-phase abundance of carbon, x_C = C/H, and its depletion factors are essential parameters for understanding the gas and solid compositions that are ultimately incorporated into planets. The majority of protoplanetary disks are born in clusters and, as a result, are exposed to external FUV radiation. These FUV photons potentially affect the disk's evolution, chemical composition, and line excitation. We present the first detection of the [CI]609um fine-structure line of neutral carbon (CI), achieved with ALMA, toward one of these disks, d203-506, in the Orion Nebula Cluster. We also report the detection of CI forbidden and permitted lines (from electronically excited states up to 10 eV) observed with JWST in the IR. These lines trace the irradiated outer disk and photo-evaporative wind. Contrary to the common belief that these IR lines are C+ recombination lines, we find that they are dominated by FUV-pumping of CI followed by fluorescence cascades. They trace the transition from atomic to molecular gas, and their intensities scale with G0. The lack of outstanding IR OI fluorescent emission, however, implies a sharper attenuation of external FUV radiation with E > 12 eV (~Lyman-beta). This is related to a lower effective FUV dust absorption cross section compared to that of interstellar grains, implying a more prominent role for FUV shielding by the CI photoionization continuum. The [CI]609um intensity is proportional to N(CI) and can be used to infer x_C. We derive x_C ~ 1.4E-4. This implies that there is no major depletion of volatile carbon compared to x_C measured in the natal cloud, hinting at a young disk. We also show that external FUV radiation impacts the outer disk and wind by vertically shifting the water freeze-out depth, which results in less efficient grain growth and settling. This shift leads to nearly solar gas-phase C/O abundance ratios in these irradiated layers.

astro-ph.GA↗
arXiv2024

PDRs4All. V. Modelling the dust evolution across the illuminated edge of the Orion Bar

We study the emission of dust grains within the Orion Bar - a well-known, highly far-UV (FUV)-irradiated PDR. The Orion Bar because of its edge-on geometry provides an exceptional benchmark for characterizing dust evolution and the associated driving processes under varying physical conditions. Our goal is to constrain the local properties of dust by comparing its emission to models. Taking advantage of the recent JWST PDRs4All data, we follow the dust emission as traced by JWST NIRCam (at 3.35 and 4.8 micron) and MIRI (at 7.7, 11.3, 15.0, and 25.5 micron), along with NIRSpec and MRS spectroscopic observations. First, we constrain the minimum size and hydrogen content of carbon nano-grains from a comparison between the observed dust emission spectra and the predictions of the THEMIS dust model coupled to the numerical code DustEM. Using this dust model, we then perform 3D radiative transfer simulations of dust emission with the SOC code and compare to data obtained along well chosen profiles across the Orion Bar. The JWST data allows us, for the first time, to spatially resolve the steep variation of dust emission at the illuminated edge of the Orion Bar PDR. By considering a dust model with carbonaceous nano-grains and submicronic coated silicate grains, we derive unprecedented constraints on the properties of across the Orion Bar. To explain the observed emission profiles with our simulations, we find that the nano-grains must be strongly depleted with an abundance (relative to the gas) 15 times less than in the diffuse ISM. The NIRSpec and MRS spectroscopic observations reveal variations in the hydrogenation of the carbon nano-grains. The lowest hydrogenation levels are found in the vicinity of the illuminating stars suggesting photo-processing while more hydrogenated nano-grains are found in the cold and dense molecular region, potentially indicative of larger grains.

astro-ph.GA↗
arXiv2023

PDRs4All VI: Probing the Photochemical Evolution of PAHs in the Orion Bar Using Machine Learning Techniques

[Abridged] JWST observations of the Orion Bar have shown the incredible richness of PAH bands and their variation on small scales. We aim to probe the photochemical evolution of PAHs across the key zones of the photodissociation region (PDR) that is the Orion Bar using unsupervised machine learning. We use NIRSpec and MIRI IFU data from the JWST ERS Program PDRs4All. We lever bisecting k-means clustering to generate detailed spatial maps of the spectral variability in several wavelength regions. We discuss the variations in the cluster profiles and connect them to the local physical conditions. We interpret these variations with respect to the key zones: the HII region, the atomic PDR zone, and the three dissociation fronts. The PAH emission exhibits spectral variation that depends strongly on spatial position in the PDR. We find the 8.6um band to behave differently than all other bands which vary systematically with one another. We find uniform variation in the 3.4-3.6um bands and 3.4/3.3 intensity ratio. We attribute the carrier of the 3.4-3.6um bands to a single side group attached to very similarly sized PAHs. Cluster profiles reveal a transition between characteristic profiles classes of the 11.2um feature from the atomic to the molecular PDR zone. We find the carriers of each of the profile classes to be independent, and reason the latter to be PAH clusters existing solely deep in the molecular PDR. Clustering also reveals a connection between the 11.2 and 6.2um bands; and that clusters generated from variation in the 10.9-11.63um region can be used to recover those in the 5.95-6.6um region. Clustering is a powerful tool for characterizing PAH variability on both spatial and spectral scales. For individual bands as well as global spectral behaviours, we find UV-processing to be the most important driver of the evolution of PAHs and their spectral signatures in the Orion Bar.

astro-ph.GA↗
arXiv2023

JWST: Deuterated PAHs, PAH-nitriles, and PAH Overtone and Combination Bands I: Program Description and First Look

A first look is taken at the NIRSpec 1-5 $μ$m observations from JWST program 1591 that targets 7 objects along the low-mass stellar life cycle with PAH emission. Spectra extracted from a 1.5$^{\prime\prime}$ radius sized circular aperture are explored, showing a wealth of features, including the 3 $μ$m PAH complex, the PAH-continuum, and atomic and molecular emission lines from HI, He, H$_{\rm 2}$, and other species. CO$_{\rm 2}$- and H$_{\rm 2}$O-ice absorption and CO emission is also seen. Focusing on the bright-PDR position in M17, the PAH CH stretch falls at 3.29 $μ$m (FWHM=0.04 $μ$m). Signs of its 1.68 $μ$m overtone are confused by line emission in all targets. Multi-component decomposition reveals a possible aliphatic deuterated PAH feature centered at 4.65 $μ$m (FWHM=0.02 $μ$m), giving [D/H]$_{\rm alip.}$=31$\pm$12.7%. However, there is little sign of its aromatic counterpart between 4.36-4.43 $μ$m. There is also little sign of PAH-nitrile emission between 4.34-4.39 $μ$m. A PAH continuum rises from $\sim$1 to 3.2 $μ$m, after which it jumps by about a factor of 2.5 at 3.6 $μ$m, with bumps at 3.8, 4.04, and 4.34 $μ$m adding structure. The CO$_{\rm 2}$ absorption band in M17 is matched with 10:1 H$_{\rm 2}$O:CO$_{\rm 2}$ ice at 10 K. The $v$=0 pure rotational molecular hydrogen population diagram reveals $>$2200 K UV-pumped gas. The hydrogen Pfund series runs from levels 10 to $>$30. Considering Br$α$/Br$β$=0.381$\pm$0.01966 and Case B recombination results in A$_{\rm V}{\simeq}$8. CO emission in IRAS21282+5050 originates from 258 K gas. In-depth spectral-spatial analysis of all features and targets are planned for a series of forthcoming papers.

astro-ph.GA↗
arXiv2023

Polycyclic Aromatic Hydrocarbon Size Tracers

We examine the dependence of polycyclic aromatic hydrocarbon (PAH) band intensity ratios as a function of the average number of carbon atoms and assess their effectiveness as tracers for PAH size, utilising the data, models, and tools provided by the NASA Ames PAH Infrared Spectroscopic Database. To achieve this, we used spectra from mixtures of PAHs of different ionisation fractions, following a size distribution. Our work, congruent with earlier findings, shows that band ratios that include the 3.3 $μ$m PAH band provide the best PAH size tracers for small-to-intermediate sized PAHs. In addition, we find that band ratios that include the sum of the 15-20 $μ$m PAH features (I$_{Σ_{15-20}}$) and the 6.2 or 7.7 $μ$m bands also serve as good tracers for PAH size in the case of small-to-intermediate sized PAHs, for objects under a similar PAH size distribution as with the presented models. For different PAH size distributions, the application of a scaling factor to the I$_{6.2}$/I$_{Σ_{15-20}}$ ratio can provide estimates for the size of the small-to-intermediate PAH population within sources. Employment of the I$_{6.2}$/I$_{Σ_{15-20}}$ and I$_{7.7}$/I$_{Σ_{15-20}}$ ratios can be of particular interest for JWST observations limited only to $\sim$ 5-28 $μ$m MIRI(-MRS) coverage.

astro-ph.GA↗
arXiv2023

Spectral variations among different scenarios of PAH processing or formation

We examine the variations in the spectral characteristics and intensities of PAHs in two different scenarios of PAH processing (or formation): (1) small PAHs are being destroyed (or equivalently large PAHs are being formed, referred to as SPR i.e. small PAHs removed), and (2) large PAHs are being destroyed (or equivalently small PAHs are being formed referred to as LPR i.e. large PAHs removed). PAH emission was measured considering both the presence or absence of plateau components. The variation in the PAH band intensities as a function of the average number of carbon atoms has the highest dynamic range in the SPR case suggesting that smaller PAHs have higher impact on the PAH band strengths. The plateaus show overall declining emission with , and their higher dynamic range in the SPR case also suggests that smaller PAHs are mainly contributing to the plateau emission. The 7.7/(11.0+11.2) $μ$m PAH band ratio presents the least amount of variance with the lowest dynamic range, rendering this ratio as the better choice for tracing PAH charge. The 3.3/(11.2+11.0) $μ$m PAH band ratio is the only ratio that has both a monotonic variance and fully separated values among the SPR and LPR scenarios, highlighting its efficiency as PAH size tracer but also allowing the characterization of the dominant scenario of processing or formation in a given region or source. We present new PAH charge $-$ size diagnostic diagrams, which can provide insights on the average, maximum, or minimum N$_{C}$ within astrophysical sources.

astro-ph.GA↗
arXiv2021

Characterizing Spatial Variations of PAH Emission in the Reflection Nebula NGC 1333

Infrared emission features at 3.3, 6.2, 7.7, 8.6, and 11.2 $μ$m, attributed to polycyclic aromatic hydrocarbons, show variations in relative intensity, shape, and peak position. These variations depend on the physical conditions of the photodissociation region (PDR) in which strong PAH emission arises, but their relationship has yet to be fully quantified. We aim to better calibrate the response of PAH species to their environment using observations with matching apertures and spatial resolution. We present observations from the Field-Imaging Far-Infrared Line Spectrometer (FIFI-LS) on board the Stratospheric Observatory for Infrared Astronomy (SOFIA) of the gas cooling lines [OI] 63, 146 $μ$m and [CII] 158 $μ$m in the reflection nebula NGC 1333 and use archival dust continuum observations from the Photodetector Array Camera and Spectrometer (PACS) on board Herschel. We employ PDR modelling to derive the physical conditions and compare these with the characteristics of the PAH emission as observed with the Infrared Spectrometer (IRS) on board Spitzer. We find distinct spatial characteristics for the various PAH spectral components. We conclude that the ionic bands (6.2, 7.7, 8.6, and 11.0) and the 7-9 $μ$m emission are due to multiple PAH sub-populations and that the plateaus are distinct from the features perched on top. The 6-9 $μ$m PAH emission exhibit a significant change in behaviour between the irradiated PDR and diffuse outskirts, confirming these bands arise from multiple PAH sub-populations with different underlying molecular properties. We find multiple promising relationships between PAH ratios and the FUV radiation field strength but no clear correlations with the PAH ionization parameter.

astro-ph.GA↗