arXiv subjects

Xuejuan Yang

Publications and source records attributed to Xuejuan Yang.

At least 19 recordsLinked to original sources

arXiv2026

Detection of Aliphatically Deuterated Aromatic Hydrocarbons in the Large Magellanic Cloud 30 Doradus Star-Forming Complex

The unidentified infrared (IR) emission (UIE) bands at 3.3, 6.2, 7.7, 8.6, 11.3 and 12.7 micron are ubiquitously seen in a wide variety of astrophysical environments. While the exact assignment of these UIE bands remains controversial, they are generally ascribed to C--H and C--C stretching and bending vibrations of aromatic hydrocarbon molecules. Here, based on observations made with the Near Infrared Spectrograph (NIRSpec) and the Mid Infrared Instrument (MIRI) aboard the James Webb Space Telescope (JWST), we report that the UIE emitters in the 30 Doradus star-forming complex in the Large Magellanic Cloud (LMC) are deuterated and have an appreciable amount of aliphatic content. The spatially resolved NIRSpec and MIRI spectra of 30 Doradus reveal a widespread detection of the 3.4 and 6.85 micron emission features attributed to aliphatic C--H stretch and deformation, respectively, as well as the 4.65 micron feature attributed to aliphatic C--D stretch. Notably, the 6.85 micron feature exhibits three complex substructures at ~6.83, 6.86 and 6.88 micron that have never been reported before.

astro-ph.GA↗

arXiv2026

Aromatics and Aliphatics in Local Star-Forming Galaxies as Probed by AKARI

Polycyclic aromatic hydrocarbon (PAH) molecules are abundant and widespread in galaxies and their infrared (IR) emission traces star formation. PAH molecules in astronomical environments often have aliphatic contents as revealed by the detection of the 3.4 micron aliphatic C--H stretch, a weak satellite feature accompanying the 3.3 micron aromatic C--H stretch. Here, we selected 102 local star-forming galaxies from the AKARI archive, including 66 galaxies each of which hosts an active galactic nucleus (AGN). We analyzed their AKARI near-IR spectra, which exhibit pronounced 3.3 micron aromatic and 3.4 micron aliphatic C--H emission. We also compiled their multi-wavelength photometric data and performed a decompositional analysis of their spectral energy distributions (SEDs) from the ultraviolet (UV) to the far-IR to derive the star formation rates (SFRs), stellar masses, metallicities, and luminosity of the galaxies. We explored the 3.3 micron PAH emission luminosity ($L_{3.3}$) as a calibrator of the SFR and found a close agreement with previous studies. We also found that $L_{3.3}/L_{\rm IR}$ and $L_{3.4}/L_{\rm IR}$ exhibit a strong dependence on metallicity, but remain nearly constant above 12+log(O/H)$\sim\,$8.5, where $L_{\rm IR}$ is the total luminosity emitted by dust, and $L_{3.4}$ is the luminosity of the 3.4 micron aliphatic emission. We derived from $L_{3.4}/L_{3.3}$ the PAH aliphatic fractions, defined as the fractions of carbon atoms in aliphatic units, to be in the range of $\sim\,$0.38%--6.8%, with a median fraction of $\sim\,$3.1%. The PAH aliphatic fractions are lower in AGN hosts and show a weak negative correlation with the SFR and $L_{\rm IR}$, suggesting that UV photons in regions with AGN or strong star formation activities may photodissociate the aliphatic structures associated with PAH molecules.

astro-ph.GA↗

arXiv2026

On the Interstellar Extinction Curve toward HD 93222, A Sightline with an Exceedingly Narrow 2175 Angstrom Extinction Bump

The 2175 Angstrom extinction bump, the most prominent spectral feature superimposed on the interstellar extinction curve, is widely seen in the interstellar medium (ISM) of the Milky Way and external galaxies, both near and far. While its central wavelength is remarkably stable and independent with environment, its width shows considerable variation and environmental dependence. Here we examine the extinction curve for the line of sight toward HD 93222, a young star located in the Carina nebula. It is found that the 2175 Angstrom bump is extremely sharp, which is among the narrowest ever found in the Milky Way and external galaxies. We model the derived extinction curve and find that, to explain the extinction characteristics of HD 93222, in addition to the conventional silicate and graphite dust mixture, an additional population of nano-sized graphitic grains is required.

astro-ph.GA↗

arXiv2025

Unveiling the Aromatic and Aliphatic Universe at Redshifts $z\sim$0.2--0.5 with JWST NIRCam/WFSS

Utilizing deep NIRCam/WFSS data from JWST's FRESCO program, we spectroscopically survey the 3.3 $μm$ aromatic and 3.4 $μm$ aliphatic C--H stretching emission bands of polycyclic aromatic hydrocarbon (PAH) molecules in galaxies at redshifts $z$$\sim$0.2--0.5. Unlike pre-JWST studies, largely limited to infrared (IR)-bright galaxies ($L_{\rm IR}\gtrsim10^{11}~L_\odot$) at $z\lesssim0.1$, we probe 200 galaxies down to $L_{\rm IR}$$\sim$$10^{8.5}$--$10^{10}~L_\odot$ well beyond the local Universe. The 3.3 $μm$ emission is detected at $\geq$3-$σ$ in 88 out of 187 galaxies, correlating tightly with galaxy IR luminosity and star formation rate (SFR) and confirming the 3.3 $μm$ PAH as a viable SFR tracer. Despite a large scatter, the 3.3 $μm$-to-IR luminosity ratio ($L_{3.3}/L_{\rm IR}$) exhibits a strong metallicity dependence with a drop of $L_{3.3}/L_{\rm IR}$ by a factor of $\gtrsim10$ at 12+log(O/H)$\sim$8.4--8.5 towards lower metallicities. The 3.4 $μm$ emission is detected in 37 out of 159 galaxies, with the 3.4 $μm$-to-3.3 $μm$ luminosity ratio ($L_{3.4}/L_{3.3}$) spanning from $\sim$0.05 to $\sim$0.58 (median $\sim$0.19), corresponding to PAH aliphatic fractions of $\sim$0.78%--8.3% (median $\sim$2.9%) in terms of fractional carbon atoms in aliphatic units. While $L_{3.4}/L_{3.3}$ does not depend significantly on redshift, stellar mass, metallicity, or galaxy morphology, it does decrease with various SFR tracers, suggesting that ultraviolet photons in active star-forming regions may strip aliphatic sidegroups from PAH molecules. Our study showcases the unique power of JWST's NIRCam/WFSS to systematically map PAH aromatic and aliphatic content in statistically significant, less-biased galaxy samples, providing critical insights into PAH chemistry and its connection to galaxy properties.

astro-ph.GA↗

arXiv2025

Emission and Absorption Lines in Photospheric Radius Expansion Bursts of 4U 1820$-$30

We analyze the emission and absorption lines during photospheric radius expansion (PRE) X-ray bursts from the ultracompact binary 4U 1820--30, observed with the Neutron Star Interior Composition Explorer (NICER). Using Monte Carlo simulations to estimate the significance, we identified a 1 keV emission line from 14 bursts, a 3 keV absorption line from 12 bursts, and 1.6 keV absorption from one burst. By coadding the burst spectra at the maximum radius phase, we detected a 1.034 keV emission line with significance of $14.2σ$, and absorption lines at 1.64 and 3 keV with significances of $10.8σ$ and $11.7σ$, respectively. The observed energy shifts are consistent with the prediction from the burst-driven wind model, indicating that all three spectral features are produced by the PRE wind. Analysis of the ratios between the emission and absorption line energies suggests that the 1 keV feature is a superposition of several narrower Fe L-shell lines. To evaluate the scientific capabilities of the Hot Universe Baryon Surveyor (HUBS), we simulated mock observations of multiple narrow lines near 1 keV. The results demonstrate that HUBS is well suited for detailed studies of the 1 keV emission line during bursts, offering significant potential to advance our understanding of these phenomena.

astro-ph.HE↗

arXiv2025

Deuterated Polycyclic Aromatic Hydrocarbons in the Interstellar Medium: Constraints from the Orion Bar as Observed by the James Webb Space Telescope

The gas-phase abundances of deuterium (D) in the local interstellar medium (ISM) exhibit considerable regional variations. Particularly, in some regions the gas-phase D abundances are substantially lower than the primordial D abundance generated in the Big Bang, after subtracting the astration reduction caused by the Galactic chemical evolution. Deuterated polycyclic aromatic hydrocarbon (PAH) molecules have been suggested as a potential reservoir of the D atoms missing from the gas-phase. Recent observations from the James Webb Space Telescope's Near Infrared Spectrograph have revealed the widespread of deuterated PAHs in the Orion Bar through their aliphatic C--D emission at 4.65${\,{\rm μm}}$ and possibly aromatic C--D emission at 4.4${\,{\rm μm}}$ as well. To examine the viability of deuterated PAHs as the D reservoir, we model the infrared (IR) emission spectra of small PAH molecules containing various aromatic and aliphatic D atoms in the Orion Bar. We find that small deuterated PAHs exhibit a noticeable emission band at 4.4 or 4.65${\,{\rm μm}}$ even if they contain only one aromatic or aliphatic D atom. We derive ${N_{\rm D,ali}}/{N_{\rm H}}\approx3.4\%$, the deuteration degree of PAHs measured as the number of aliphatic D atoms (relative to H), from the observed intensity ratios of the 4.65${\,{\rm μm}}$ band to the 3.3${\,{\rm μm}}$ aromatic C--H band. The deuteration degree for aromatically-deuterated PAHs is less certain as C--N stretch also contributes to the observed emission around 4.4${\,{\rm μm}}$. If we attribute it exclusively to aromatic C--D, we derive an upper limit of $\approx14\%$ on the deuteration degree, which is capable of accounting for an appreciable fraction of the missing D budget.

astro-ph.GA↗

arXiv2025

Interstellar dust revealed by light from cosmic dawn

The obscuration of light from a distant galaxy has raised the possibility that a type of carbon dust existed in the earliest epochs of the Universe -- challenging the idea that stars had not yet evolved enough to make such material.

astro-ph.GA↗

arXiv2025

Polycyclic Aromatic Hydrocarbon and the Ultraviolet Extinction Bump at the Cosmic Dawn

First detected in 1965, the mysterious ultraviolet (UV) extinction bump at 2175 Angstrom is the most prominent spectroscopic feature superimposed on the interstellar extinction curve. Its carrier remains unidentified over the past six decades ever since its first detection, although many candidate materials have been proposed. Widely seen in the interstellar medium (ISM) of the Milky Way as well as several nearby galaxies, this bump was recently also detected by the James Webb Space Telescope (JWST) at the cosmic dawn in JADES-GS-z6-0, a distant galaxy at redshift z~6.71, corresponding to a cosmic age of just 800 million years after the Big Bang. Differing from that of the known Galactic and extragalactic interstellar sightlines which always peak at ~2175 Angstrom, the bump seen at z~6.71 in JADES-GS-z6-0 peaks at an appreciably longer wavelength of ~2263 Angstrom and is the narrowest among all known Galactic and extragalactic extinction bumps. Here we show that the combined electronic absorption spectra quantum-chemically computed for a number of polycyclic aromatic hydrocarbon (PAH) molecules closely reproduce the bump detected by JWST in JADES-GS-z6-0. This suggests that PAH molecules have already been pervasive in the Universe at an epoch when asymptotic giant branch stars have not yet evolved to make dust.

astro-ph.GA↗

arXiv2025

What causes the ultraviolet extinction bump at the cosmic dawn?

The enigmatic ultraviolet (UV) extinction bump at 2175 Angstrom, the strongest spectroscopic absorption feature superimposed on the interstellar extinction curve, has recently been detected at the cosmic dawn by the James Webb Space Telescope (JWST) in JADES-GS-z6-0, a distant galaxy at redshift z=6.71, corresponding to a cosmic age of just 800 million years after the Big Bang. Although small graphite grains have historically long been suggested as the carrier of the 2175 Angstrom extinction bump and graphite grains are expected to have already been pervasive in the early Universe, in this work we demonstrate that small graphite grains are not responsible for the UV extinction bump seen at the cosmic dawn in JADES-GS-z6-0, as the extinction bump arising from small graphite grains is too broad and peaks at wavelengths that are too short to be consistent with what is seen in JADES-GS-z6-0.

astro-ph.GA↗

arXiv2025

Spectral properties of the neutron star low-mass X-ray binary 4U 1636-53, XTE J1739-285 and MAXI J1816-195

We investigated simultaneous NICER plus NuSTAR observations of three neutron star low-mass X-ray binary 4U 1636-53, XTE J1739-285 and MAXI J1816-195 using the latest reflection models, with the seed photons feeding into the corona originating from either the neutron star (NS) or the accretion disk. We found that, for the sources in the hard spectral state, more than $\sim$ 50% of the NS photons enter into the corona if NS provides seed photons, while only $\sim$ 3%-5% disk photons go to the corona if seed photons come from the disk. This finding, together with the derived small height of the corona, favors the lamp-post geometry or boundary layer scenario where the corona is close to the central neutron star. Additionally, we found that the source of the seed photons has big influence in the significance of the NS radiation, especially for the soft spectral state. This result may help explain why the NS radiation in MAXI J1816-195 is weak in the previous work. More importantly, for the first time, we explored the properties of the corona in the NS systems with the compactness ($l-θ$) diagram. We found that the corona in the NS systems all lie in the left side of the pair-production forbidden region, away from the predicted pair-production lines. This finding indicates that either the corona in these NS systems is not pair-dominated, possibly due to the additional cooling from NS photons, or the corona is composed of both thermal and non-thermal electrons.

astro-ph.HE↗

arXiv2024

General Stacking Theory for Altermagnetism in Bilayer Systems

Two-dimensional (2D) altermagnetism was recently proposed to be attainable in twisted antiferromagnetic bilayers providing an experimentally feasible approach to realize it in 2D materials. Nevertheless, a comprehensive understanding of the mechanism governing the appearance of altermagnetism in bilayer systems is still absent. In present letter, we address this gap by introducing a general stacking theory (GST) as a key condition for the emergence of altermagnetism in bilayer systems. The GST provides straightforward criteria to predict whether a bilayer demonstrates altermagnetic spin splitting, solely based on the layer groups of the composing monolayers. According to the GST, only seven point groups of bilayers facilitate the emergence of altermagnetism. It is revealed that, beyond the previously proposed antiferromagnetic twisted vdW stacking, altermagnetism can even emerge in bilayers formed through the symmetrically restricted direct stacking of two monolayers. By combining the GST and first-principles calculations, we present illustrative examples of bilayers demonstrating altermagnetism. Our work establishes a robust framework for designing diverse bilayer systems with altermagnetism, thereby opening up new avenues for both fundamental research and practical applications in this field.

cond-mat.mtrl-sci↗

arXiv2024

NICER views moderate, strong, and extreme photospheric expansion bursts from the ultracompact X-ray binary 4U 1820$-$30

Type I X-ray bursts in the ultracompact X-ray binary 4U 1820$-$30 are powered by the unstable thermonuclear burning of hydrogen-deficient material. We report the detection of 15 type I X-ray bursts from 4U 1820$-$30 observed by NICER in between 2017 and 2023. All these bursts occurred in the low state for the persistent flux in the range of $2.5-8\times10^{-9}~{\rm erg~s^{-1}~cm^{-2}}$ in 0.1$-$250 keV. The burst spectra during the tail can be well explained by blackbody model. However, for the first $\sim$5 s after the burst onset, the time-resolved spectra showed strong deviations from the blackbody model. The significant improvement of the fit can be obtained by taking into account of the enhanced persistent emission due to the Poynting-Robterson drag, the extra emission modelled by another blackbody component or by the reflection from the surrounding accretion disk. The reflection model provides a self-consistent and physically motivated explanation. We find that the accretion disk density changed with 0.5 s delay as response to the burst radiation, which indicates the distortion of the accretion disk during X-ray bursts. From the time-resolved spectroscopy, all bursts showed the characteristic of photospheric radius expansion (PRE). We find one superexpansion burst with the extreme photospheric radius $r_{\rm ph}>10^3$ km and blackbody temperature of $\sim 0.2$ keV, thirteen strong PRE bursts for $r_{\rm ph}>10^2$ km, and one moderate PRE burst for $r_{\rm ph}\sim55$ km.

astro-ph.HE↗

arXiv2024

Gas-phase formation of fullerene/9-hydroxyfluorene cluster cations

In interstellar environment, fullerene species readily react with large molecules (e.g., PAHs and their derivatives) in the gas phase, which may be the formation route of carbon dust grains in space. In this work, the gas-phase ion-molecule collision reaction between fullerene cations (Cn+, n=32, 34, ..., 60) and functionalized PAH molecules (9-hydroxyfluorene, C13H10O) are investigated both experimentally and theoretically. The experimental results show that fullerene/9-hydroxyfluorene cluster cations are efficiently formed, leading to a series of large fullerene/9-hydroxyfluorene cluster cations (e.g., [(C13H10O)C60]+, [(C13H10O)3C58+, and [(C26H18O)(C13H10O)2C48]+). The binding energies and optimized structures of typical fullerene/9-hydroxyfluorene cluster cations were calculated. The bonding ability plays a decisive role in the cluster formation processes. The reaction surfaces, modes and combination reaction sites can result in different binding energies, which represent the relative chemical reactivity. Therefore, the geometry and composition of fullerene/9-hydroxyfluorene cluster cations are complicated. In addition, there is an enhanced chemical reactivity for smaller fullerene cations, which is mainly attributed to the newly formed deformed carbon rings (e.g., 7 C-ring). As part of the coevolution network of interstellar fullerene chemistry, our results suggest that ion-molecule collision reactions contribute to the formation of various fullerene/9-hydroxyfluorene cluster cations in the ISM, providing insights into different chemical reactivity caused by oxygenated functional groups (e.g., hydroxyl, OH, or ether, C-O-C) on the cluster formations.

astro-ph.GA↗

arXiv2024

Gas-phase hydrogenation of large, astronomically relevant PAH cations

To investigate the gas-phase hydrogenation processes of large, astronomically relevant cationic polycyclic aromatic hydrocarbon (PAH) molecules under the interstellar environments, the ion-molecule collision reaction between six PAH cations and H-atoms is studied. The experimental results show that the hydrogenated PAH cations are efficiently formed, and no even-odd hydrogenated mass patterns are observed in the hydrogenation processes. The structure of newly formed hydrogenated PAH cations and the bonding energy for the hydrogenation reaction pathways are investigated with quantum theoretical calculations. The exothermic energy for each reaction pathway is relatively high, and the competition between hydrogenation and dehydrogenation is confirmed. From the theoretical calculation, the bonding ability plays an important role in the gas-phase hydrogenation processes. The factors that affect the hydrogenation chemical reactivity are discussed, including the effect of carbon skeleton structure, the side-edged structure, the molecular size, the five- and six-membered C-ring structure, the bay region structure, and the neighboring hydrogenation. The IR spectra of hydrogenated PAH cations are also calculated. These results we obtain once again validate the complexity of hydrogenated PAH molecules, and provide the direction for the simulations and observations under the coevolution interstellar chemistry network. We infer that if we do not consider other chemical evolution processes (e.g., photo-evolution), then the hydrogenation states and forms of PAH compounds are intricate and complex in the interstellar medium (ISM).

astro-ph.GA↗

arXiv2024

Reverse Shock Revisited in Cassiopeia A with Chandra

Using data from the Chandra X-Ray Observatory, we revisited the reverse shock in the supernova remnant (SNR) Cassiopeia A.Based on the spectroscopic of a series of annuli in the northwest (NW) and southeast (SE), we get the radial profiles of the S/Si K-alpha line flux ratio and Fe K-alpha line centroid energy. They both show monotonic increase, confirming that the Si- and Fe-rich ejecta are heated by the reverse shock.The abrupt change of the S and Si line flux ratio is clearly observed in Cassiopeia A, leading to the determination of the reverse shock location (~1.71+-0.16 arcmin and ~1.35+-0.18 arcmin in the NW and SE, with respect to the central source). By comparing the radial profiles of S and Si line flux, we find that the reverse shock is moving outward in the frame of the observer, and the velocities are ~3950+-210 km/s and ~2900+-260 km/s in the NW and SE, respectively. In contrast, the velocities become ~1150 km/s (NW) and ~1300 km/s (SE) in the ejecta frame. Our measured reverse shock velocities are quite consistent with those obtained from the X-ray and/or optical images. It therefore supplies a crosscheck of the accuracy for the two available methods to measure the reverse shock velocity in SNRs. Both the location and the velocity of the reverse shock show apparent asymmetry, suggesting that the asymmetric explosion of the progenitor plays a key role in the interaction between the reverse shock and the ejecta, ultimately shaping complex features observed in SNRs.

astro-ph.HE↗

arXiv2024

Infrared Emission of Specific Polycyclic Aromatic Hydrocarbon Molecules: Cyanonaphthalenes

The unidentified infrared emission (UIE) features at 3.3, 6.2, 7.7, 8.6, 11.3 and 12.7 micron are ubiquitously seen in a wide variety of astrophysical regions and commonly attributed to polycyclic aromatic hydrocarbon (PAH) molecules. However, the unambiguous identification of any individual, specific PAH molecules has proven elusive until very recently two isomers of cyanonapthalene, which consists of two fused benzene rings and substitutes a nitrile (-CN) group for a hydrogen atom, were discovered in the Taurus Molecular Cloud based on their rotational transitions at radio frequencies. To facilitate the James Webb Space Telescope (JWST) to search for cyanonapthalenes in astrophysical regions, we model the vibrational excitation of cyanonapthalenes and calculate their infrared emission spectra in a number of representative astrophysical regions. The model emission spectra and intensities will allow JWST to quantitatively determine or place an upper limit on the abundance of cyanonapthalenes.

astro-ph.GA↗

arXiv2024

Infrared Emission of Specific Polycyclic Aromatic Hydrocarbon Molecules: Indene

Polycyclic aromatic hydrocarbon (PAH) molecules have long been suggested to be present in the interstellar medium (ISM). Nevertheless, despite their expected ubiquity and sustained searching efforts, identifying specific interstellar PAH molecules from their infrared (IR) spectroscopy has so far been unsuccessful. However, due to its unprecedented sensitivity, the advent of the James Webb Space Telescope (JWST) may change this. Meanwhile, recent years have witnessed breakthroughs in detecting specific PAH molecules (e.g., indene, cyanoindene, and cyanonaphthalene) through their rotational lines in the radio frequencies. As JWST holds great promise for identifying specific PAH molecules in the ISM based on their vibrational spectra in the IR, in this work we model the vibrational excitation of indene, a molecule composed of a six-membered benzene ring fused with a five-membered cyclopentene ring, and calculate its IR emission spectra for a number of representative astrophysical regions. This will facilitate JWST to search for and identify indene in space through its vibrational bands and to quantitatively determine or place an upper limit on its abundance.

astro-ph.GA↗

arXiv2017

The Carriers of the "Unidentified" Infrared Emission Features: Clues from Polycyclic Aromatic Hydrocarbons with Aliphatic Sidegroups

The "unidentified" infrared emission (UIE) features at 3.3, 6.2, 7.7, 8.6, and 11.3 $μ$m are ubiquitously seen in various astrophysical regions. The UIE features are characteristic of the stretching and bending vibrations of aromatic hydrocarbons. The 3.3 $μ$m feature resulting from aromatic C--H stretches is often accompanied by a weaker feature at 3.4 $μ$m often attributed to aliphatic C--H stretches. The ratio of the observed intensity of the 3.3 $μ$m aromatic C--H feature ($I_{3.3}$) to that of the 3.4 $μ$m aliphatic C--H feature ($I_{3.4}$) allows one to estimate the aliphatic fraction (i.e. $N_{\rm C,aliph}/N_{\rm C,arom}$, the number of C atoms in aliphatic units to that in aromatic rings) of the UIE carriers, provided the intrinsic oscillator strengths of the 3.3 $μ$m aromatic C--H stretch ($A_{3.3}$) and the 3.4 $μ$m aliphatic C--H stretch ($A_{3.4}$) are known. In this article we summarize the computational results on $A_{3.3}$ and $A_{3.4}$ and their implications for the aromaticity and aliphaticity of the UIE carriers. We use density functional theory and second-order perturbation theory to derive $A_{3.3}$ and $A_{3.4}$ from the infrared vibrational spectra of seven PAHs with various aliphatic substituents (e.g., methyl-, dimethyl-, ethyl-, propyl-, butyl-PAHs, and PAHs with unsaturated alkyl-chains). The mean band strengths of the aromatic ($A_{3.3}$) and aliphatic ($A_{3.4}$) C--H stretches are derived and then employed to estimate the aliphatic fraction of the UIE carriers by comparing $A_{3.4}$/$A_{3.3}$ with $I_{3.4}$/$I_{3.3}$. We conclude that the UIE emitters are predominantly aromatic, as revealed by the observationally-derived ratio <$I_{3.4}$/$I_{3.3}$> ~ 0.12 and the computationally-derived ratio <$A_{3.4}$/$A_{3.3}$> ~ 1.76 which suggest an upper limit of $N_{\rm C,aliph}/N_{\rm C,arom}$ ~ 0.02 for the aliphatic fraction of the UIE carriers.

astro-ph.GA↗