arXiv subjects

Karl D. Gordon

Publications and source records attributed to Karl D. Gordon.

At least 19 recordsLinked to original sources

arXiv2026

LightCube: A Parsec-Resolution 3D Model of the Local Far-Ultraviolet Interstellar Radiation Field

We present a three-dimensional model of the local interstellar radiation field (ISRF) in the ultraviolet (UV). Using UV flux measurements from the TD1 catalog and stellar distances from Gaia and Hipparcos, we construct a catalog of stars that we expect to dominate the UV flux in the nearby Galaxy. We use the radiative transfer code DIRTY to model the propagation of photons from these stars through a 3D dust map, including the effects of scattering and absorption. The result is LightCube, a model of the ultraviolet ISRF out to 1.25 kpc from the Sun, with a maximum linear resolution of 1 pc. We model the ISRF in the four TD1 bands (1565 A, 1965 A, 2365 A, and 2740 A), as well as a single value for the full FUV range, and calculate the ISRF at the Sun to be 5.65$\times$10$^{-14}$ erg cm$^{-3}$ from 912 A to 2000 A. The modeled ISRF is quite variable, with more than an order of magnitude variation seen in dense regions, and about half that in lower density regions. By comparing LightCube to an estimate of the 3D distribution of total-to-selective extinction ratio, $R_V$, we find a positive correlation between UV flux and $R_V$ in regions of low UV radiation.

astro-ph.GA↗

arXiv2026

JWST MIRI Medium Resolution Spectrometer Point Fixed Pattern Corrections: Cleaner and Higher Signal-to-Noise Spectra of Point Sources

The JWST Mid-Infrared Instrument Medium Resolution Spectrometer provides the capability to obtain spectra from 5-28 micron. The JWST data reduction pipeline removes the majority but not all of the instrument artifacts and the signal-to-noise (S/N) of the resulting spectra are limited by fixed pattern noise. Building on previous work, Point Fixed Pattern Corrections (PFPCs) are constructed using observations of O, A, and G dwarf flux calibration stars and asteroids taken using the default four point dither pattern. The PFPCs can be applied to spectra of point sources taken with target acquisition and the same dither pattern. They can be used alone or with the pipeline residual fringe correction depending on the sources spectral properties. Both narrow and broad artifacts are removed by the PFPCs improving the spectra regardless of their S/N. For higher S/N observations, the PFPCs significantly improve the S/N by up to factors of a few and S/N values of 1000 or more. The MRS-PFPC python package is provided to allow anyone to utilize the PFPCs for their own data.

astro-ph.IM↗

arXiv2026

Scylla: Observational Evidence for an Order of Magnitude in Dust Mass Opacity Evolution with ISM Density in the Large Magellanic Cloud

The emissivity of dust is known to vary greatly with radiative environment, density, grain chemistry, and geometry. Discrepancies between dust mass surface densities derived from far-infrared (FIR) emission and visible extinction persist across and within galaxies in the local Universe. Here, we use new extinction and emission measurements towards the LMC to show that this discrepancy is driven by the dust mass opacity evolving with the intrinsic density of the ISM, and that the ratio between FIR and optical dust mass opacity varies with gas surface density. These new findings imply that the dust mass opacity in the FIR could increase by nearly an order of magnitude (e.g., $\kappa_{160} = 0.3 - 6\ m^2\ kg^{-2}$) across over an order of magnitude of total hydrogen surface density $(\Sigma_H = 4 - 100 M_{\odot}\ pc^{-2})$, corroborating previous theoretical models for dust mass opacity evolution in the FIR, and providing new implications for emission-based dust mass estimates.

astro-ph.GA↗

arXiv2026

Stellar winds of O-type stars traced by high ionization fine-structure emission lines with JWST/MIRI

We investigate the presence of high ionization fine-structure emission lines across a range of 22 OB-type stars observed with JWST-MIRI as part of calibration programmes and the WISCI and MEAD projects. MIR wind emission is detected in 4 late O-dwarfs (O8 V - O9 V), 1 early O-dwarf (O5 V) and there are tentative detections in an additional 3 stars (O8.5 II, O8.5 IV and O9 I). We measure the wind speeds and make estimates of lower limits on the mass-loss rates of 5 O-type stars from broad, flat-topped emission in the fine-structure line of [Ne V] 14.3micron. We find terminal wind speeds that are generally in agreement with empirical trends, but note that in some cases the wind speeds are surprisingly low. We highlight two main takeaways from this sample, which combine to establish an exciting new window into the winds of massive stars. First, a new diagnostic ability is gained from lines formed at much higher ionization, larger spatial extent, and longer wavelengths than typical wind diagnostics. Secondly, there is frequent incidence of MIR emission in O-type stars, even in the 'weak-wind' regime where wind emission is often not detected in the UV and optical.

astro-ph.SR↗

arXiv2026

Survival of very small carbonaceous dust grains in the inner-CGM of NGC 891 from JWST/MIRI MRS

We present new spectroscopic observations of the inner circumgalactic medium (CGM) of NGC 891 taken with the Mid-Infrared Imager/Medium Resolution Spectroscopy instrument onboard JWST, in four positions: two near the bulge and two at galactocentric radii (r) of ~ 1.5, 4.7 kpc. Each pair of pointings has one position along the minor axis (h) at ~ 0.5 kpc and one at ~ 1 kpc away from the mid-plane. We analyse both 1D spectra and 3D cubes using the dust emission model PAHFIT to extract properties of typical mid-IR features. These spectra reveal that the earlier reported mid-IR emission out to 4 kpc is dominated by the emission of polycyclic aromatic hydrocarbons (PAHs), and not hot dust continuum, providing direct evidence of the survival of PAHs in the inner CGM of NGC 891. Comparing PAH band ratios with other environments (Orion, M51), it is obvious that the 11.2 $\mu$m PAH feature -- and not the usual 7.7 ${\mu}$m -- dominates in NGC 891, which seems to imply the presence of more neutral, large PAHs in the CGM. Overall, PAH-to-continuum ratios show little variations with scale-height and radius in NGC 891, which suggests little PAH processing. However, we do see a decrease in the PAH feature strengths with the [Ne III]/[Ne II] ratio, pointing at elevated dust processing with increased radiation field hardness. We also confirm a tight correlation between H2 and PAH features, suggesting that both tracers must be co-spatial and, hence, implying that PAH emission predominantly arises from cool dense parts of cloudlets entrained in galactic outflows. Finally, we report the clear detection of a previously unidentified PAH feature at 16.72 $\mu$m.

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arXiv2026

Summary of Discussion Sessions from "The Dusty Universe 2025: The Fifth Pandust Conference"

"The Dusty Universe: The Fifth Pandust Conference" took place in Tucson (AZ, USA) from November 10 until 14, 2025. The goal of this meeting was to get the dust community together to review where we are, hear exciting new results, and make plans for the future. The meeting encompassed all aspects of dust investigations including observations, theory, modeling, and laboratory studies. The conference consisted of invited review talks, contributed talks and posters. Science topics included interstellar dust (Milky Way & nearby galaxies), circumstellar dust (including Solar System & exoplanets), dust in Galaxies (including high-z), lifecycle of dust, and future needs (laboratory, theory, & observations), with a particular focus on results from JWST and ALMA, and on nanodust (including PAHs). On November 12th, we organized breakout discussion sessions covering a wide range of interesting dust-related topics. The purpose of this document is to capture the main topics/questions that were discussed, the key conclusions of these discussions, the challenges and possible solutions that were brought up, and the open questions that still remain to be answered. We hope that this document records our findings and challenges for the future generation.

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arXiv2026

Scylla VI: Parsec-Scale Dust Extinction Maps in the SMC and LMC

We present a novel methodology for mapping dust extinction in nearby galaxies at parsec-scale resolution. We apply it to HST 68 fields within the Small and Large Magellanic Clouds (23 fields in the SMC and 45 fields in the LMC) using multi-band HST photometry from the Scylla and METAL surveys. Our technique leverages \textit{kriging}, a geostatistical interpolation method built on the principles of Gaussian Process regression, combined with Gaussian mixture modeling to statistically isolate background stellar sources and account for line-of-sight depth effects. 3D dust simulations demonstrate the method's capability to recover column densities to an accuracy of $A_V \approx 0.1$ mag in fields with at least 1000 sources. The resulting $4^{\prime\prime}$ resolution ($\sim1$-pc) dust maps reveal detailed structure and strong spatial correlation with ancillary ISM tracers, especially in star-forming regions like 30 Doradus. Global extinction of total column densities follows log-normal profiles in both galaxies, with the SMC exhibiting slightly higher mean extinction ($e^{\mu}=0.47$ mag) than the broader LMC ($e^{\mu}=0.43$ mag), likely due to significant line-of-sight depths. We find systematic offsets between dust mass surface densities ($\Sigma_{D}$) derived from extinction versus FIR emission in both galaxies, with $\Sigma_{D, FIR}/\Sigma_{D, A_V}$ ratios ranging from $0.6-1.8$. This work provides the highest-resolution dust extinction maps in SMC and LMC to date, which offer a vital independent benchmark for constraining dust emissivity, $\text{CO}$-dark gas fractions, and the multi-scale structure of the ISM in low-metallicity environments.

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arXiv2026

JWST Observations of Starbursts: Dust Processing in the M82 Superwind

We present JWST MIRI and NIRCam imaging of the inner ~5 kpc of the M82 superwind at 0.05-0.375'' (~0.9-6.5 pc) resolution. Targeted filters probe emission from polycyclic aromatic hydrocarbons (PAHs; F335M, F360M, F770W, F1130W) and continuum (F250M, F360M). The images reveal a network of cool wind filaments traced by PAHs. PAH surface brightness declines with the inverse square of distance to the midplane, suggesting that the incident radiation field from the starburst drives the observed PAH intensity out to 2.5 kpc. The 3.3/11.3 and 3.3/7.7 band ratios show uniformity with distance from the starburst, though comparisons with mid-IR dust emission models indicate a modest shift toward larger PAHs. Outside the disk, 11.3/7.7 increases moderately, reflecting that PAHs become more neutral with distance from the starburst as they are exposed to a declining radiation field and ionization parameter. Overall, PAHs in the wind are consistent with standard-to-large sizes and standard-to-high ionization states. Including Spitzer and Herschel data, PAH abundance (qPAH) is set at ~1% in the starburst and remains unchanging out to 5 kpc off the disk. This flat qPAH profile suggests that PAHs are shielded from the hot wind, perhaps residing in the surface layers of cool clouds, with possible replenishment from cloud interiors and enrichment of the halo from previous bursts. In this picture, clouds are not dense enough to promote PAH growth, and they likely undergo radiative cooling and mixing with the hot phase to survive the gauntlet for at least ~20 Myr.

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arXiv2026

Large Variations Seen in First Ultraviolet Spectroscopic M33 Dust Extinction Curves

Dust extinction curves provide one of the main avenues to understanding the detailed nature of dust grains and accounting for the effects of dust on observations of many astrophysical objects. For the first time, spectroscopic ultraviolet (UV) extinction curves are measured in M33 expanding the sample of Local Group galaxies with such measurements to five. These curves are based on Hubble Space Telescope/Space Telescope Imaging Spectrograph spectra and literature photometry from the UV to the near-infrared. The four measured curves show large variations in their UV shapes including their 2175 A bump and UV slope strengths. The average extinction of these four sightlines is lower than the averages for other Local Group Galaxies and does not follow the Milky Way R(V) dependent relationship. The variations between UV extinction shape parameters and gas-to-dust ratios for the M33 sightlines fall within the variations seen in the combined sample of UV extinction curves in the Milky Way, Large and Small Magellanic Clouds, and M31. The correlation with gas-to-dust ratio is much stronger than the correlation with global metallicity. This strengthens the picture that local conditions like radiation field density and shocks dominate over global galaxy properties like metallicity in determining the wavelength dependence of dust extinction.

astro-ph.GA↗

arXiv2025

JWST Captures Growth of Aromatic Hydrocarbon Dust Particles in the Extremely Metal-poor Galaxy Sextans A

The mid-infrared spectrum of star-forming, high metallicity galaxies is dominated by emission features from aromatic and aliphatic bonds in small carbonaceous dust grains, often referred to as polycyclic aromatic hydrocarbons (PAHs). In metal-poor galaxies, the abundance of PAHs relative to the total dust sharply declines, but the origin of this deficit is unknown. We present JWST observations that detect and resolve emission from PAHs in the 7% Solar metallicity galaxy Sextans A, representing the lowest metallicity detection of PAH emission to date. In contrast to higher metallicity galaxies, the clumps of PAH emission are compact (0.5-1.5'' or 3-10 pc), which explains why PAH emission evaded detection by lower resolution instruments like Spitzer. Ratios between the 3.3, 7.7, and 11.3 $\mu$m PAH features indicate that the PAH grains in Sextans A are small and neutral, with no evidence of significant processing from the hard radiation fields within the galaxy. These results favor inhibited grain growth over enhanced destruction as the origin of the low PAH abundance in Sextans A. The compact clumps of PAH emission are likely active sites of in-situ PAH growth within a dense, well-shielded phase of the interstellar medium. Our results show that PAHs can form and survive in extremely metal-poor environments common early in the evolution of the Universe.

astro-ph.GA↗

arXiv2025

Scylla V: Constraints on the spatial and temporal distribution of bursts and the interaction history of the Magellanic Clouds from their resolved stellar populations

We measure the star formation histories (SFHs) from the Scylla survey in approximately 98,000 pc^2 and 75,000 pc^2 of the SMC and LMC, respectively, using deep Hubble Space Telescope imaging (80% complete to more than 1 mag below the ancient main-sequence turnoff, 25.1 and 26.0 mag in F475W and F814W) from 74 pointings. We group the fields into eight sub-regions in the SMC and seven in the LMC. We use the birth rate parameter to identify bursts of star formation and measure their properties in each sub-region. Our methodology provides a standardized framework for burst identification and reveals both broad and fine burst characteristics. We identify global and local bursts, defined as those occurring in at least half or less than half of a galaxy's sub-regions, respectively. In the SMC we find two global (about 5 and 1.5 Gyr ago) and one local burst (about 3 Gyr ago). In the LMC we find one global burst (about 3 Gyr ago). Comparing these findings with dynamical models of the LMC and SMC orbital histories, we find that when models predict a shared dynamical trigger for bursts across both galaxies, the burst begins earlier in the SMC with a greater enhancement in star formation rate than in the LMC. Finally, using age-metallicity relations (AMRs) and cumulative SFHs, we report that the Wing/Bridge region in the SMC resembles the southwestern LMC both chemically and in stellar mass assembly over the last about 7 Gyr, possibly due to stellar material stripped from the LMC during their last interaction.

astro-ph.GA↗

arXiv2025

A Tale of Two Sightlines: Comparison of Hydrocarbon Dust Absorption Bands toward Cygnus OB2-12 and the Galactic Center

Infrared spectra of hydrocarbon dust absorption bands toward the bright hypergiant Cygnus OB2-12 are compared to published spectra of the Quintuplet Cluster, a sightline to the Galactic center. The Cyg OB2-12 data include a new ground-based 2.86-3.70 microns spectrum and a previously published, but here further analyzed, spectrum of the 5.50-7.34 microns region. Higher spectral resolution data for the Cyg OB2-12 sightline in the 3 micron region allows a detailed comparison of the 3.4 micron aliphatic bands to those observed toward the Quintuplet. Despite differences in interstellar environments along each sightline, strong similarities are observed in the central wavelengths and relative strengths for bands at 3.3, 3.4, 5.85, 6.2, and 6.85 microns. Analysis of these bands, produced by aromatic, aliphatic, olefinic, hydrogenated, and oxygenated components, shows that carbonaceous dust is a significant component of the diffuse interstellar medium, second in abundance only to silicates, and is primarily aromatic in nature. The grains producing these bands likely consist of large aromatic carbon cores with thin aliphatic mantles composed of hydrogenated amorphous carbon (HAC). Laboratory analog spectra reproduce the observed aliphatic absorption bands well, supporting the presence of such mantles. We present evidence that the carriers of both the 3.4 micron aliphatic and the 3.3 micron aromatic bands reside exclusively in the diffuse ISM, and that the 3.3 micron bands observed in the diffuse ISM differ from the 3.25 micron band seen in dense clouds, implying chemically distinct carriers.

astro-ph.GA↗

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.

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arXiv2025

Photometric Mapping of Carbonaceous/Siliceous Dust and Water Ice in the ISM with JWST: Applications to the Dense Sightlines

We introduce a new photometric mapping method for the James Webb Space Telescope (JWST) to measure the spatial distribution of carbonaceous dust, siliceous dust and water ice by using absorption features arising from the grains in the dense interstellar medium (ISM). Employing NIRCam and MIRI imaging filters, low-resolution spectroscopic data can be obtained to measure the optical depths of the 3.0-$μ$m water ice -OH feature, the 3.4-$μ$m aliphatic hydrocarbon -CH feature, and the 10-$μ$m silicate -SiO feature for large fields of view. This method provides extensive statistical data of the grains across wide fields in the ISM at minimal observing cost. In this study, we present its application on observational data from the literature to validate the measured optical depths and simulations to assess the accuracy of the method under various conditions. We showed that the photometric method can be employed to obtain reasonably accurate measurements of optical depth. We demonstrate that JWST optical depth maps enable the independent exploration of abundance distributions of major grain components across a wide spatial coverage in the ISM.

astro-ph.IM↗

arXiv2025

Nature or Nurture: LMC-like Dust in the Solar Metallicity Galaxy M31

Using the {\it Hubble Space Telescope}/Space Telescope Imaging Spectrograph, ultraviolet (UV) extinction curves have been measured in M31 along thirteen new sightlines, increasing the M31 sample to seventeen. This sample covers a wide area of M31 having galactocentric distances of 5 to 16 kpc, enabling the analysis of UV extinction curve variations over a large region of an external galaxy similar to the Milky Way with global galactic characteristics such as metallicity for the first time. No correlation is found between the extinction parameters and galactocentric distance which might be expected if there is a radial metallicity gradient in M31. Most of the new UV extinction curves presented here are significantly different from the average extinction curves of the Milky Way, LMC, and SMC, but the average M31 extinction curve is similar to the average extinction curve in the 30-Dor region of the LMC. The wide range of extinction curves seen in each individual Local Group galaxy suggests that global galactic properties such as metallicity may be less important than the local environmental conditions such as density, UV radiation field, and shocks along each sightline. The combined behavior of the Milky Way, LMC, SMC, and now M31 UV extinction curves supports the idea that there is a family of curves in the Local Group with overlapping dust grain properties between different galaxies.

astro-ph.GA↗

arXiv2025

The Resolved Structure of a Low Metallicity Photodissociation Region

Photodissociation Regions (PDRs) are key to understanding the feedback processes that shape interstellar matter in galaxies. One important type of PDR is the interface between HII regions and molecular clouds, where far-ultraviolet (FUV) radiation from massive stars heats gas and dissociates molecules. Photochemical models predict that the C/CO transition occurs deeper in the PDR compared to the H/H$_2$ transition in low-metallicity environments, increasing the extent of CO-dark H$_2$ gas. This prediction has been difficult to test outside the Milky Way due to the lack of high spatial resolution observations tracing H$_2$ and CO. This study examines a low-metallicity PDR in the N13 region of the Small Magellanic Cloud (SMC) where we spatially resolve the ionization front, the H$_2$ dissociation front, and the C/CO transition using 12CO J=2-1, 3-2 and [CI] (1-0) observations from the Atacama Large Millimeter/sub-mm Array (ALMA) and near-infrared spectroscopy of the H$_2$ 2.12 1-0S(1) vibrational line, and H recombination lines from the James Webb Space Telescope (JWST). Our analysis shows that the separation between the H/H$_2$ and C/CO boundaries is approximately 0.043 $\pm$ 0.013(stat.) $\pm$ 0.0036(syst.) pc (equivalent to 0".146 $\pm$ 0".042(stat.) $\pm$ 0".012(syst.) at the SMC's distance of 62 kpc), defining the spatial extent of the CO-dark H$_2$ region. Compared to our plane-parallel PDR models, we find that a constant pressure model matches the observed structure better than a constant density one. Overall, we find that the PDR model does well at predicting the extent of the CO-dark H$_2$ layer in N13. This study represents the first resolved benchmark for low metallicity PDRs.

astro-ph.GA↗

arXiv2025

Investigating silicate, carbon, and water in the diffuse interstellar medium: the first shots from WISCI

The dusty interstellar medium (ISM) of the Milky Way is distributed in a complex, cloudy structure. It is fundamental to the radiation balance within the Milky Way, provides a reaction surface to form complex molecules, and is the feedstock for future generations of stars and planets. The life cycle of interstellar dust is not completely understood, and neither are its structure nor composition. The abundance, composition, and structure of dust in the diffuse ISM can be determined by combining infrared, optical and ultraviolet spectroscopy. JWST enables measurement of the faint absorption of ISM dust grains against bright stars at kiloparsec distances across the infrared spectrum. Here we present an overview of the project `Webb Investigation of Silicates, Carbons, and Ices' (WISCI) along with interpretation of two targets, GSC 08152-02121 and CPD-59 5831. Observations of 12 WISCI target stars were taken by JWST, the Hubble Space Telescope, Himalayan Chandra Telescope, and the Very Large Telescope. We use these to characterize the targets' spectral types and calculate their line-of-sight extinction parameters, $A_{\rm V}$ and $R_{\rm V}$. We find absorption in the JWST spectra of GSC 08152-02121, and CPD-59 5831 associated with carbonaceous dust around 3.4 and 6.2 micron and amorphous silicates at 9.7 micron. In GSC 08152-02121 we also find indications of absorption by trapped water around 3 micron. This first look from WISCI demonstrates the line-of-sight variability within the sample, and the program's potential to identify and correlate features across ultraviolet to mid-infrared wavelengths.

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arXiv2025

Detection of Deuterated Hydrocarbon Nanoparticles in the Whirlpool Galaxy, M51

Deuteration of hydrocarbon material, including polycyclic aromatic hydrocarbons (PAHs), has been proposed to account for the low gas-phase abundances of D in the interstellar medium. JWST spectra of four star-forming regions in M51 show an emission feature, with central wavelength $\sim$4.647$μ$m and FWHM 0.0265$μ$m, corresponding to the C-D stretching mode in aliphatic hydrocarbons. The emitting aliphatic material is estimated to have (D/H)$_{\rm aliph}\approx 0.17\pm0.02$ -- a factor $\sim$$10^4$ enrichment relative to the overall interstellar medium (ISM). On $\sim$$50\,$pc scales, deuteration levels toward four H$\,$II regions in M51 are 2-3 times higher than in the Orion Bar photodissociation region (PDR), with implications for the processes responsible for the formation and evolution of hydrocarbon nanoparticles, including PAHs. The deuteration of the aliphatic material is found to anticorrelate with helium ionization in the associated H$\,$II, suggesting that harsh FUV radiation may act to lower the deuteration of aliphatics in PDRs near massive stars. No evidence is found for deuteration of aromatic material, with (D/H)$_{\rm arom} \lesssim 0.016$: deuteration of the aliphatic material exceeds that of the aromatic material by at least a factor 10. The observed levels of deuteration may account for the depletion of D observed in the Galactic interstellar medium. If so, the $4.65μ$m feature may be detectable in absorption.

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