arXiv subjects

Christiaan Boersma

Publications and source records attributed to Christiaan Boersma.

At least 19 recordsLinked to original sources

arXiv2026

PDRs4All XXII. Near-Infrared continuum in the Orion Bar

Conspicuous excess emission is present in the near-infrared (NIR) region in various objects, including reflection nebulae, planetary nebulae, and nearby galaxies. However, the spatial distribution and spectral shape of the excess emission remain poorly understood. We studied the NIR continuum emission spectroscopically and obtained its spatial distribution relative to the aromatic infrared band (AIB) at 3.3um in the Orion Bar prototypical photodissociation region (PDR). We aim to characterize its spectral shape and discuss its origin. We employed 3D spectroscopic data of the Orion Bar taken with the integrated field unit of NIRSpec on JWST from the Early Release Science program "PDRs4All." Contribution from the foreground ionized gas was estimated using the Cloudy code and subtracted. The observed regions were divided into nine physically distinct regions and an average spectrum was derived for each region. The nine regions, including the ionized gas, atomic PDR, and molecular PDR, clearly show remaining continuum in the region 1--4.5um. The continuum at wavelengths longer than 2.7um shows good correlations with the 3.3um AIB, while the correlation of the continuum at 1.2um is not significant. We further find that the NIR continuum in the Orion Bar can be approximated by a summation of two blackbodies. The low-temperature component correlates with the AIB well, while the high-temperature component does not. The average spectra also show absorption features at 3.0 and 4.27um, which are attributed to the presence in the spectra of water ice and CO2 ice. We discuss possible origins of the NIR continuum, among which recurrent fluorescence from carbon clusters better explains the observed low-temperature component. The presence of ice species suggests a contribution from a deeper layer of the PDR along the line of sight producing characteristic ice absorption features.

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arXiv2026

Deciphering The Launching of Multi-phase AGN-driven Outflows and Their (Spatially Resolved) Multi-scale Impact

Beyond deepening our understanding of the formation, growth, and evolution of supermassive black holes, it is crucial to uncover the role of feeding and feedback processes from growing black holes (i.e., active galactic nucleus; AGN) in shaping the cosmic ecosystem. Such studies include understanding the dynamics of gas flows in the interstellar (ISM), circumgalactic (CGM), intracluster (ICM), and intergalactic media (IGM). As the output of a sub-group in Habitable Worlds Observatory (HWO) AGN Working Group, this Science Case Development Document (SCDD) proposes to use future HWO observations to solve the following questions. Which mechanism is dominant in triggering inflows/outflows through feedback? How is AGN activity triggered, and is it associated with circumnuclear star formation and what is the overall effect of AGN feedback on star formation (SF)? In AGN feedback, which mode is more influential and does AGN feedback operate similarly or differently in the local universe and at high redshift? To answer these questions, this SCDD proposes to use potential HWO observations as follows. Resolve and characterize the spatial distribution of ionized and cold/warm molecular gas, especially those in inflows/outflows; Explore the spatial coupling and potential stratification of multi-phase inflows/outflows on different physical scales and their resolved and global correlations with AGN and/or SF activities; Investigate whether corresponding outflows/jets induce shocks and/or fluctuations that trigger or suppress the formation of molecular clouds and hence new stars. Specifically, HWO's capabilities will enable us to achieve the above scientific goals while existing facilities lack the required combination of high-throughput ultraviolet (UV) and near-infrared (NIR) integral field unit (IFU) capabilities with simultaneously sufficient spatial resolution and sensitivity.

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arXiv2026

First detection of HDO ice in a protoplanetary disk

Protoplanetary disks are the birthplace of planets and planetary systems. Investigating the molecular inventory of disks is key to linking the chemical evolution of the interstellar medium and the makeup of planets and their atmospheres. In particular, tracing the history of the deuterium enrichment of water along the journey from interstellar clouds through protoplanetary disks to planetary systems provides critical insights into the chemical inheritance. We aim to investigate the chemical composition of ices in protoplanetary disks; specifically, the presence of HDO ice that ought to be present, but has not been detected in disks thus far. We analyzed JWST/NIRSpec observations of the 132-1832 edge-on disk located in the Orion Nebula Cluster using the ENIIGMA fitting tool and unique laboratory data. We report on the first detections of HDO ice in a protoplanetary disk. The estimated upper limit for the HDO/H$_2$O ratio for 132-1832 is much higher, compared to HDO/H$_2$O ratios obtained for chondrites, comets, and embedded young stellar objects. In the disk ices, beyond HDO, we detected H$_2$O, CO$_2$, $^{13}$CO$_2$, CO, OCN$^-$, and OCS, species, whose presence has also been detected in other disks. The HDO ice detection may point to the efficient ice processing in the disk and confirm the findings of laboratory experiments on deuterated ices.

astro-ph.EP↗

arXiv2026

PDRs4All: XVIII. The evolution of the PAH ionisation and PAH size distribution across the Orion Bar

We investigate the evolution of the PAH population's charge state and size across key physical zones in the Orion Bar, which include the HII region, the atomic PDR (APDR), and three HI/H2 dissociation fronts (DF1, DF2, and DF3). Utilising the NASA Ames PAH Infrared Spectroscopic Database (PAHdb) and the pyPAHdb spectral modelling tool, we analysed the MIRI-MRS observations of the Orion Bar from the "PDRs4All" ERS Program. pyPAHdb modelling reveals the fractional contribution of the different PAH charge states and sizes to the total PAH emission across the Orion Bar. Cationic PAH emission peaks in the APDR region, where neutral PAHs have minimal contribution. Emission from neutral PAHs peaks in the HII region that consists of emission from a face-on PDR associated to the background OMC-1 molecular cloud, and in the molecular cloud regions past DF2. PAH anions are observed deep within the DF2 and DF3 zones. The average PAH size ranges between ~$60-74$ Nc. The modelling reveals regions of top-down PAH formation at the ionisation front, and bottom-up PAH formation within the molecular cloud region. The PAH ionisation parameter $γ$ ranges between ~$2-9 x 10^4$. Intensity ratios tracing PAH ionisation scale well with $γ$ in regions encompassing edge-on or face-on PDR emission, but their correlation weakens within the molecular cloud zone. Modelling of the $5-15$ $μ$m PAH spectrum with pyPAHdb achieves comprehensive characterization of the net contribution of neutral and cationic PAHs across different environments, whereas empirical PAH proxy intensity ratio tracers can be highly variable and unreliable outside regions dominated by PDR emission. The derived average PAH size in the different physical zones is consistent with a view of PAHs being more extensively subjected to ultraviolet processing closer to the ionisation front, and less affected within the molecular cloud.

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arXiv2026

Observation of an Accreting Planetary-Mass Companion with Signs of Disk-Disk Interaction in Orion

Young ($\lesssim 10$ Myr) planetary-mass companions (PMCs) provide valuable insights into the formation and early evolution of planetary systems. To date, only a dozen such objects have been identified through direct imaging. Using JWST/NIRCam observations towards the Orion Nebula, obtained as part of the \textit{PDRs4All} Early Release Science program, we have identified a faint point source near the M-type star V2376 Ori. Follow-up spectroscopic observations with the MUSE instrument on the VLT confirm that the source, V2376 Ori b, is indeed a young planetary-mass companion. It is a member of Orion D, around 80\,pc in the foreground of the Trapezium cluster of Orion and with an age of approximately $7 \pm 3$ Myr. We fit the SED of V2376 Ori b to infer a mass of $ \sim 20~M_{\rm Jup}$. The MUSE spectrum reveals several accretion tracers. Based on the H$α$ line intensity, we estimate an accretion rate of $\sim$10$^{-6.5 \pm 0.7}~\rm M_{Jup}\,yr^{-1}$, which is comparable to that of young PMCs such as PDS~70b. In addition, the MUSE data cube reveals extended emission in the [O\,\textsc{ii}] doublet at 7320 and 7330~Å, which is interpreted as evidence of a dynamical interaction between the two sources that, potentially, involves mass transfer between their individual accretion disks. These results demonstrate that JWST/NIRCam imaging surveys of young stellar associations can uncover new PMCs, which can then be confirmed and characterized through ground-based spectroscopic follow-up.

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arXiv2025

SPHEREx: Aromatics, Aliphatics and PAH Size across the Iris Nebula

Observations by the SpectroPhotometer for the History of the Universe, Epoch of Reionization, and Ices Explorer (SPHEREx) are combined with Spitzer spectral map data to study the aromatic, aliphatic, and PAH size evolution across the northwest photo-dissociation region (PDR) of the Iris Nebula (NGC7023). The 3.3-3.4 $μ$m complex (I$_{3.3}$) and 11.2 $μ$m (I$_{11.2}$) PAH band strength are determined through direct integration. In addition, the former is decomposed into a 3.3 (I'$_{3.3}$) and 3.4 $μ$m (I'$_{3.4}$) sub-feature by fitting SPHEREx bandpass-integrated photometry using a modeled, highly sampled, multi-component spectrum. I$_{3.3}$, I$_{11.2}$, I'$_{3.3}$, and I'$_{3.4}$ all peak at the PDR. The NASA Ames PAH IR Spectroscopic Database is used to obtain the average number of carbon atoms ($\overline{\rm N_{C}}$) and small PAH fraction ($f_{\rm small}$) by fitting the isolated PAH component of the Spitzer segment; $70\lesssim\overline{N_{C}}\lesssim76$ and $0.24\lesssim\text{f}_{\rm small}\lesssim0.36$. I'$_{3.4}$/I'$_{3.4}$, I$_{11.2}$/I$_{3.3}$, $\overline{\rm N_{C}}$, and $f_{\rm small}$ all show a demarcation that matches the large-scale morphology of the region. For I'$_{3.3}$ and I'$_{3.4}$ this is reflected by two distinct trends when plotted against each other, one associated with the dense, the other with the diffuse medium; $[N_{\rm H,ali}/N_{\rm H,aro}]_{\rm dense}$ = 0.42$\pm$0.01 and $[N_{\rm H,ali}/N_{\rm H,aro}]_{\rm diffuse}$ = 0.10$\pm$0.01. $\overline{\rm N_{C}}$ and $f_{\rm small}$ are tentatively correlated with I$_{11.2}$/I$_{3.3}$ (R=0.54$\pm$0.05 and -0.45$\pm$0.05, respectively). A wider variety of large(r) extended interstellar medium objects is required to tighten the correlations, turn them into quantitative calibrators for PAH size, and pin down the discrepancy of correlations with I'$_{3.3}$ involved.

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arXiv2025

PDRs4All XIX. The 6 to 9 $μ$m region as a probe of PAH charge and size in the Orion Bar

Infrared emission from polycyclic aromatic hydrocarbons (PAHs) play a major role in determining the charge balance of their host environments that include photo-dissociation regions (PDRs) in galaxies, planetary nebulae, and rims of molecular clouds. We aim to investigate the distribution and sizes of charged PAHs across the key zones of the Orion Bar PDR. We employ JWST MIRI-MRS observations of the Orion Bar from the Early Release Science program ''PDRs4All'' and synthetic images in the JWST MIRI filters. We investigate the spatial morphology of the AIBs at 6.2, 7.7, 8.6, and 11.0 $μ$m that commonly trace PAH cations, and the neutral PAH-tracing 11.2 $μ$m AIB, their (relative) correlations, and the relationship with existing empirical prescriptions for AIBs. The 6.2. 7.7, 8.6, 11.0, and 11.2 $μ$m AIBs are similar in spatial morphology, on larger scales. Analyzing three-feature intensity correlations, two distinct groups emerge: the 8.6 and 11.0 $μ$m vs. the 6.2 and 7.7 $μ$m AIBs. We attribute these correlations to PAH size. The 6.2 and 7.7 $μ$m AIBs trace cationic, medium-sized PAHs. Quantum chemical calculations reveal that the 8.6 $μ$m AIB is carried by large, compact, cationic PAHs, and the 11.0 $μ$m AIB's correlation to it implies, so is this band. The 6.2/8.6 and 7.7/8.6 PAH band ratios thus probe PAH size. We conclude that the 6.2/11.2 AIB ratio is the most reliable proxy for charged PAHs, within the cohort. We outline JWST MIRI imaging prescriptions that serve as effective tracers of the PAH ionization fraction as traced. This study showcases the efficacy of the 6-9 $μ$m AIBs to probe the charge state and size distribution of the emitting PAHs, offering insights into the physical conditions of their host environments. JWST MIRI photometry offers a viable alternative to IFU spectroscopy for characterizing this emission in extended objects.

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arXiv2025

PDRs4All XVI. Tracing aromatic infrared band characteristics in photodissociation region spectra with PAHFIT in the JWST era

Photodissociation regions (PDRs) exhibit emission between 3-20 um known as the Aromatic Infrared Bands (AIBs), originating from small carbonaceous species such as polycyclic aromatic hydrocarbons (PAHs). The AIB spectra observed in Galactic PDRs, such as the Orion Bar observations by the PDRs4All JWST program, are considered a local analog for those seen in extragalactic star-forming regions. We present the Python version of PAHFIT, a spectral decomposition tool that separates the contributions by AIB subcomponents, thermal dust emission, gas lines, stellar light, and dust extinction. By fitting segments of the Orion Bar spectra, we provide a configuration to decompose JWST spectra of PDRs in detail. The resulting central wavelengths and FWHM of the AIB subcomponents are compiled into a "PDR pack" for PAHFIT. We applied PAHFIT with this PDR pack and the default continuum model to spectra of the central star forming ring of the galaxy NGC7469. We introduce an alternate dust continuum model to fit the Orion Bar spectra, as the default PAHFIT continuum model mismatches the intensity at 15-26 um. Using the PDR pack and the alternate continuum model, PAHFIT reproduces the Orion Bar spectra with residuals of a few percent, and similar performance is achieved for the NGC7469 spectra. We provide PAHFIT-based diagnostics that trace the profile variations of the 3.3, 3.4, 5.7, 6.2, and 7.7 um AIBs, and thus the photochemical evolution of the AIB carriers. The 5.7 um AIB emission originates from at least two subpopulations, one more prominent in highly irradiated environments and one preferring more shielded environments. Smaller PAHs as well as very small grains or PAH clusters both thrive in the more shielded environments of the molecular zone in the Orion Bar. Based on these new diagnostics, we quantify the similarities between the AIB profiles observed in the Orion Bar and NGC7469.

astro-ph.GA↗

arXiv2025

Solar C/O ratio in planet-forming gas at 1 au in a highly irradiated disk

The chemical composition of exoplanets is thought to be influenced by the composition of the disks in which they form. JWST observations have unveiled a variety of species in numerous nearby disks, showing significant variations in the C/O abundance ratio. However, little is known about the composition and C/O ratio of disks around young stars in clusters exposed to strong ultraviolet (UV) radiation from nearby massive stars, which are representative of the environments where most planetary systems form, including ours. We present JWST spectroscopy of d203-504, a young 0.7 $\rm M_{\odot}$ star in the Orion Nebula with a 30 au disk irradiated by nearby massive stars. These observations reveal spectroscopic signatures of CO, H$_2$O, CH$_3^+$, and PAHs. Water and CO are detected in absorption in the inner disk ($r\lesssim 1$ au), where the estimated gas-phase C/O ratio is 0.48, consistent with the Solar value and that of the Orion Nebula. In contrast, \ch{CH3+} and PAHs are found in the extended surface layers of the disk. These results suggest that gas in the inner disk is chemically shielded from UV radiation while the surface layers of the disk experience UV-induced chemistry, potentially depleting their carbon content.

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arXiv2025

PDRs4All XIII. Empirical prescriptions for the interpretation of JWST imaging observations of star-forming regions

(Abridged) JWST continues to deliver incredibly detailed infrared (IR) images of star forming regions in the Milky Way and beyond. IR emission from star-forming regions is very spectrally rich due to emission from gas-phase atoms, ions, and polycyclic aromatic hydrocarbons (PAHs). Physically interpreting IR images of these regions relies on assumptions about the underlying spectral energy distribution in the imaging bandpasses. We aim to provide empirical prescriptions linking line, PAH, and continuum intensities from JWST images, to facilitate the interpretation of JWST images in a wide variety of contexts. We use JWST PDRs4All Near-Infrared Camera (NIRCam) and Mid-Infrared Instrument (MIRI) imaging and Near-Infrared Spectrograph (NIRSpec) integral field unit (IFU) and MIRI Medium Resolution Spectrograph (MRS) spectroscopic observations of the Orion Bar photodissociation region (PDR), to directly compare and cross-calibrate imaging and IFU data at ~100 AU resolution over a region where the radiation field and ISM environment evolves from the hot ionized gas to the cold molecular gas. We measure the relative contributions of line, PAH, and continuum emission to the NIRCam and MIRI filters as functions of local physical conditions. We provide empirical prescriptions based on NIRCam and MIRI images to derive intensities of emission lines and PAH features. Within the range of the environments probed in this study, these prescriptions accurately predict Pa-$α$, Br-$α$, PAH 3.3 $μ$m and 11.2 $μ$m intensities, while those for FeII 1.644 $μ$m, H$_2$ 1--0 S(1) 2.12 $μ$m and 1--0 S(9) 4.96 $μ$m, and PAH 7.7 $μ$m show more complicated environmental dependencies. Linear combinations of JWST NIRCam and MIRI images provide effective tracers of ionized gas, H$_2$, and PAH emission in PDRs. We expect these recipes to be useful for both the Galactic and extragalactic communities.

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arXiv2024

A sensitivity analysis of the modeling of Polycyclic Aromatic Hydrocarbon emission in galaxies

We have conducted a sensitivity analysis on the mid-infrared spectral decomposition of galaxies and the modeling of the PAH emission spectrum with the NASA Ames PAH Infrared Spectroscopic Database (PAHdb) to assess the variance on the average galaxy PAH population properties under a grid of different modeling parameters. We find that the SL and SL+LL Spitzer-IRS decomposition with PAHFIT provides consistent modeling and recovery of the 5-15 $μ$m PAH emission spectrum. For PAHdb modeling, application of a redshift to the calculated spectra to account for anharmonic effects introduces a $15\%$-$20\%$ variance on the derived parameters, while its absence improves the fits by $\sim13\%$. The 4.00-$α$ release of PAHdb achieves the complete modeling of the 6-15 $μ$m PAH spectrum, including the full 6.2 $μ$m band, improving the average fitting uncertainty by a factor of 2. The optimal PAHdb modeling configuration requires selection of pure PAHs without applying a redshift to the bands. Although quantitatively the PAHdb-derived parameters change under different modeling configurations or database versions, their variation follows a linear scaling, with previously reported trends remaining qualitatively valid. PAHdb modeling of JWST observations, and JWST observations smoothed and resampled to the Spitzer-IRS resolution and dispersion have consistent PAHdb derived parameters. Decomposition with different codes, such as PAHFIT and CAFE, produce PAH emission spectra with noticeable variation in the 11-15~$μ$m region, driving a $\sim7\%$ difference in the neutral PAH fraction under PAHdb modeling. A new library of galaxy PAH emission templates is delivered to be utilized in galaxy SED modeling.

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arXiv2024

The Background Interstellar Medium as Observed from Off-Order Low-Resolution Spitzer-IRS Spectra

Spitzer 'hidden' observations of the background are used to construct a catalog of 4,090 spectra and examine the signature of polycyclic aromatic hydrocarbon (PAH) molecules and their connection to extinction by dust. A strong positive correlation is recovered between WISE12, E(B-V), and the 11.2 $μ$m PAH band. For 0.06 $\leq$ E(B-V) $\leq$ 5.0, correlations of the 6.2, 11.2, and 12.7 $μ$m PAH band are positive with E(B-V). Three dust temperature regimes are revealed. Correlations with WISE12 are well-constrained and that with 12.7/11.2 is flat. Decomposition with the NASA Ames PAH IR Spectroscopic Database reveals a tentative positive correlation between the 6.2/11.2 and the PAH ionization fraction, while that with 12.7/11.2 is slightly negative, suggesting PAH structural changes. The relation with PAH size and 6.2/11.2 is negative, while that with 12.7/11.2 is positive. Averaging spectra into five E(B-V) and three T$_{\rm dust}$ bins shows an evolution in PAH emission and variations in 12.7/11.2. Database-fits show an increase in $f_{\rm i}$ and the PAH ionization parameter $γ$, but a more stable large PAH fraction. While the largest $γ$s are associated with the highest T$_{\rm dust}$, there is no one-to-one correlation. The analysis is hampered by low-quality data at short wavelengths. There are indications that PAHs in the more-diffuse backgrounds behave differently from those in the general interstellar medium. However, they are often still associated with larger scale filamentary cloud-like structures. The spectra and auxiliary data have been made available through the Ames Background Interstellar Medium Spectral Catalog and may guide JWST programs.

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arXiv2024

The High-Resolution Far- to Near-Infrared Anharmonic Absorption Spectra of Cyano-Substituted Polycyclic Aromatic Hydrocarbons from 300-6200 cm$^{-1}$

Cyano-substituted polycyclic aromatic hydrocarbons (CN-PAHs) may contribute to the emission detected in the 7 - 9 $μ$m (1430 - 1100 cm$^{-1}$) and 11 - 15 $μ$m (900 - 670 cm$^{-1}$) regions of astronomical IR spectra. Anharmonic quantum chemical computations of 17 CN-PAH isomers for 4 small PAHs and Benzene reveal strong, broad absorption features across the entire 300 - 6200 cm$^{-1}$ (33 - 1.6 $μ$m) frequency range. In particular, when a FWHM of 15 cm$^{-1}$ is applied, the composite CN-PAH spectrum is almost indistinguishable from the unsubstituted-PAH spectrum. At high resolution, however, the infrared absorption spectra reveal unique, identifiable features of CN-PAHs in the 700 - 950, 1100 - 1300, 2000 - 2500, and 3400 - 3600 cm$^{-1}$ ranges. The in-plane and out-of-plane CH bending vibrational frequencies of CN-PAHs are shifted when comparing isomers and to their unsubstituted counterparts, making their differentiation in mixed laboratory experiments possible. The overall aromatic CH stretch fundamental (2950 - 3200 cm$^{-1}$) and first overtone (5950 - 6200 cm$^{-1}$) regions are relatively unaffected by the cyano-substitution, with changes only to the breadth and intensity of the bands. Detailed spectroscopic data on the normal mode components of each state reported herein provide the means to directly assign future laboratory spectra and to guide direct IR observations of astronomical regions with, e.g., JWST.

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arXiv2024

PDRs4All VIII: Mid-IR emission line inventory of the Orion Bar

Mid-infrared emission features probe the properties of ionized gas, and hot or warm molecular gas. The Orion Bar is a frequently studied photodissociation region (PDR) containing large amounts of gas under these conditions, and was observed with the MIRI IFU aboard JWST as part of the "PDRs4All" program. The resulting IR spectroscopic images of high angular resolution (0.2") reveal a rich observational inventory of mid-IR emission lines, and spatially resolve the substructure of the PDR, with a mosaic cutting perpendicularly across the ionization front and three dissociation fronts. We extracted five spectra that represent the ionized, atomic, and molecular gas layers, and measured the most prominent gas emission lines. An initial analysis summarizes the physical conditions of the gas and the potential of these data. We identified around 100 lines, report an additional 18 lines that remain unidentified, and measured the line intensities and central wavelengths. The H I recombination lines originating from the ionized gas layer bordering the PDR, have intensity ratios that are well matched by emissivity coefficients from H recombination theory, but deviate up to 10% due contamination by He I lines. We report the observed emission lines of various ionization stages of Ne, P, S, Cl, Ar, Fe, and Ni, and show how certain line ratios vary between the five regions. We observe the pure-rotational H$_2$ lines in the vibrational ground state from 0-0 S(1) to 0-0 S(8), and in the first vibrationally excited state from 1-1 S(5) to 1-1 S(9). We derive H$_2$ excitation diagrams, and approximate the excitation with one thermal (~700 K) component representative of an average gas temperature, and one non-thermal component (~2700 K) probing the effect of UV pumping. We compare these results to an existing model for the Orion Bar PDR and highlight the differences with the observations.

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arXiv2024

A far-ultraviolet-driven photoevaporation flow observed in a protoplanetary disk

Most low-mass stars form in stellar clusters that also contain massive stars, which are sources of far-ultraviolet (FUV) radiation. Theoretical models predict that this FUV radiation produces photo-dissociation regions (PDRs) on the surfaces of protoplanetary disks around low-mass stars, impacting planet formation within the disks. We report JWST and Atacama Large Millimetere Array observations of a FUV-irradiated protoplanetary disk in the Orion Nebula. Emission lines are detected from the PDR; modelling their kinematics and excitation allows us to constrain the physical conditions within the gas. We quantify the mass-loss rate induced by the FUV irradiation, finding it is sufficient to remove gas from the disk in less than a million years. This is rapid enough to affect giant planet formation in the disk.

astro-ph.GA↗

arXiv2024

The Infrared Absorption Spectrum of Phenylacetylene and its Deuterated Isotopologue in the Mid- to Far-IR

Anharmonicity strongly influences the absorption and emission spectra of polycyclic aromatic hydrocarbon (PAH) molecules. Here, IR-UV ion-dip spectroscopy experiments together with detailed anharmonic computations reveal the presence of fundamental, overtone, as well as 2- and 3-quanta combination band transitions in the far- and mid-infrared absorption spectrum of phenylacetylene and its singly deuterated isotopologue. Strong absorption features in the 400-900 cm$^{\rm -1}$ range originate from CH(D) in-plane and out-of-plane wags and bends, as well as bending motions including the C$\equiv$C and CH bonds of the acetylene substituent and the aromatic ring. For phenylacetylene, every absorption feature is assigned either directly or indirectly to a single or multiple vibrational mode(s). The measured spectrum is dense, broad, and structureless in many regions but well characterized by computations. Upon deuteration, large isotopic shifts are observed. At frequencies above 1500 cm$^{\rm -1}$ for d$_1$-phenylacetylene, a one-to-one match is seen when comparing computations and experiment with all features assigned to combination bands and overtones. The C$\equiv$C stretch observed in phenylacetylene is not observed in d$_1$-phenylacetylene due to a computed 40-fold drop in intensity. Overall, a careful treatment of anharmonicity that includes 2- and 3-quanta modes is found to be crucial to understand the rich details of the infrared spectrum of phenylacetylene. Based on these results, it can be expected that such an all-inclusive anharmonic treatment will also be key for unraveling the infrared spectra of PAHs in general.

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arXiv2024

Formation of the Methyl Cation by Photochemistry in a Protoplanetary Disk

Forty years ago it was proposed that gas phase organic chemistry in the interstellar medium was initiated by the methyl cation CH3+, but hitherto it has not been observed outside the Solar System. Alternative routes involving processes on grain surfaces have been invoked. Here we report JWST observations of CH3+ in a protoplanetary disk in the Orion star forming region. We find that gas-phase organic chemistry is activated by UV irradiation.

astro-ph.GA↗

arXiv2023

OH as a probe of the warm water cycle in planet-forming disks

Water is a key ingredient for the emergence of life as we know it. Yet, its destruction and reformation in space remains unprobed in warm gas. Here, we detect the hydroxyl radical (OH) emission from a planet-forming disk exposed to external far-ultraviolet (FUV) radiation with the James Webb Space Telescope. The observations are confronted with the results of quantum dynamical calculations. The highly excited OH infrared rotational lines are the tell-tale signs of H2O destruction by FUV. The OH infrared ro-vibrational lines are attributed to chemical excitation via the key reaction O+H=OH+H which seeds the formation of water in the gas-phase. We infer that the equivalent of the Earth ocean's worth of water is destroyed per month and replenished. These results show that under warm and irradiated conditions water is destroyed and efficiently reformed via gas-phase reactions. This process, assisted by diffusive transport, could reduce the HDO/H2O ratio in the warm regions of planet-forming disks.

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