Abundant hydrocarbons in a buried galactic nucleus with signs of carbonaceous grain and polycyclic aromatic hydrocarbon processing

While supernova remnants (SNRs) have been observed to produce up to of dust, the amount of dust destroyed by the forward shock is poorly constrained, raising the question of whether they are net dust producers or destroyers. 1 M_⊙ Our aim was to estimate the dust destruction efficiency of SNR forward shocks in a realistically turbulent interstellar medium (ISM) during their most destructive phase, and to assess dust shielding by high-density filaments during this period. We ran 3D high-resolution turbulence simulations for different turbulent Mach numbers (0--3) and average ISM densities ( ) to resemble observations of the turbulent ISM. We then set off a supernova explosion to trace its 3D magnetohydrodynamic evolution for the first . Finally, we ran post-processing simulations to investigate the dust transport and destruction by the SNR forward shock, taking into account gas and plasma drag, kinetic and thermal sputtering, and grain-grain collisions, and considering either silicates or carbonaceous dust. 1--100 cm -3 10 kyr The dust destruction rate of the forward shock strongly depends on the average ISM density and turbulence strength, varying in the range 27--92% ( ) in the studied . Overall, dust is less efficiently destroyed in a low-density medium (unit 0.85--11.0 M_⊙ 10 kyr 1 cm^ -3 , 27--57%) than in intermediate-density (unit 10 cm^ -3 , 46--92%) and high-density (unit 100 cm^ -3 , 73--87%) cases. The forward shock is found to destroy 8--34% less dust in high Mach turbulence compared to a homogeneous medium. Furthermore, carbonaceous grains are up to 21% more robust than silicates. Filaments can partly shield dust from destruction in the first ; however, always more than of dust is destroyed, making most SNRs dust sinks under the conditions explored in this work. The destruction efficiency of the SNRs with less than of destroyed dust has not yet plateaued, so that they are most likely also net dust destroyers by the end of their lifetimes. 10 kyr 0.85 M_⊙ 1 M_⊙

We investigate the impacts of the evolution of dust mass and grain size distribution on the evolution of global attenuation curves, with a focus on the optical-ultraviolet (UV) slope and the $2175$ Å bump, within a Milky Way-like (MW-like) galaxy simulation. In addition, we discuss the contributions of the star-dust geometry, scattering, and dust properties to the attenuation curves. We performed the post-processing dust radiative transfer using the SKIRT code based on a MW-like galaxy simulation. The hydrodynamic simulation was carried out with the GADGET4-OSAKA code, which models the evolution of grain size distributions. For lower inclination angles (i.e., closer to face-on), the attenuation curve flattens over time up to t = 1 Gyr and becomes progressively steeper. The steeper slope of the attenuation curve is caused by the interplay between scattering and the dust disk becoming more extended over time (i.e., changes in the star-dust geometry). At higher inclination angles, the effect of scattering is suppressed and the attenuation curves steepen slightly over time due to the formation of small grains and the bias of observed UV emission toward old stars. The $2175$ Å bump becomes stronger on a timescale of ∼250 Myr due to the formation of small carbonaceous grains. However, the bump strength is affected not only by the abundance of small grains, but also by star-dust geometry. At higher A_V, or at higher inclination angles, the bump strengths become weaker. These results may help interpret flatter attenuation curves and less prominent bumps in high-redshift galaxies. Furthermore, we find that variations in the star-dust geometry alter the amount of scattered photons escaping the galaxy, thereby driving the anti-correlation between the slope and V-band attenuation, A_V. The scatter in this relation arises from differences in dust optical depth along and perpendicular to the line of sight, reflecting differences in the inclination and star-dust geometry. Additional contributions to the scatter come from variations in the grain size distribution and the fraction of obscured young stars.

Recent near- and mid-infrared (IR) observations have revealed the existence of appreciable amounts of aromatic and aliphatic hydrocarbon dust in the harsh environments of active galactic nuclei (AGNs), the origins of which are still under discussion. In this paper, we analyze the near-IR spectra of AGNs obtained with AKARI in order to systematically study the properties of the aromatic and aliphatic hydrocarbon dust affected by AGN activity. We performed spectral fitting and spectral energy distribution fitting for our sample of 102 AGNs to obtain the fluxes of the aromatic and aliphatic spectral features, the total IR luminosity ( L IR ), and the fractional luminosity of AGN components ( L AGN / L IR ). As a result, we find that L aromatic / L IR is systematically lower for the AGN sample and especially lower for AGNs with the aliphatic feature seen in the absorption than for star-forming galaxies (SFGs), while L aliphatic / L aromatic is systematically higher for the AGN sample than for the SFG sample, increasing with AGN activity indicated by L AGN / L IR . In addition, the profiles of the aliphatic emission features of the AGN sample are significantly different from those of the SFG sample in that the AGNs have systematically stronger feature intensities at longer wavelengths. We conclude that both aromatic and aliphatic hydrocarbon dust are likely of circumnuclear origin, suggesting that a significant amount of the aliphatic hydrocarbon dust may come from a new population created through processes such as the shattering of large carbonaceous grains by AGN outflows.

Infrared (IR) radiation dominates dense, interstellar clouds, yet its effect on icy grains remains largely unexplored. Its potential role in driving the photodesorption of volatile species from such grains has recently been demonstrated, providing a crucial link between the solid state reservoir of molecules and the gas phase. In this work, we investigate IR-induced photodesorption of CO for astrophysically relevant ice systems containing perhydropyrene (PHP). This fully super-hydrogenated version of pyrene is used as an analogue for large carbonaceous molecules such as polycyclic aromatic hydrocarbons (PAHs) and related species, as well as hydrogenated carbonaceous grains. The abundance and range of strong IR absorption bands of these carbonaceous species make them interesting candidates for studying IR-induced effects. We present IR spectroscopic and mass spectrometric measurements probing the effects of IR radiation on two ice systems: 1) CO:PHP mixed ice and 2) layered ice with CO on top of PHP. The ices were irradiated with the FELIX IR Free Electron Laser (FEL) FEL-2. In accordance with previous studies, we confirm that direct excitation of CO is not an efficient pathway to CO desorption, indicating that another energy dissipation mechanism exists. We demonstrate that vibrational excitation of the PHP CH stretching modes leads to efficient CO photodesorption. The derived photodesorption yields are an order of magnitude higher for the layered than the mixed system and comparable to those previously obtained for CO photodesorption from CO on amorphous solid water upon excitation of H2O vibrational modes. Our results indicate that IR excitation of carbonaceous molecules and grains in dense clouds could potentially play an important role in the desorption of volatile species such as CO from icy grains.

ABSTRACT In this work, we present an off-lattice Monte Carlo model of accretion and migration of hydrogen atoms on a rough surface of carbon dust grain. The migration of physisorbed atoms by means of thermal diffusion and quantum tunnelling through barriers between the surface potential minima is considered. The model is applied to simulations of molecular hydrogen formation in a cold interstellar medium for a temperature range 5–35 K. Eley–Rideal and Langmuir–Hinshelwood mechanisms for the formation of the H$_2$ molecule were taken into account. We found that the surface potential energy minima that hold the accreted hydrogen atoms (binding energy) has wide dispersion of its values. The minimum energy is three times smaller than the maximum energy for the uneven surface of the model grain. The large dispersion of the binding energies results in an extended range of temperatures where H$_2$ formation is efficient: 5–25 K. The dispersion of binding energies also reduces efficiency of diffusion due to tunnelling in comparison to that assumed in kinetic equation codes in which constant values of binding energies are employed. Thus, thermal hopping is the main source for the mobility of the hydrogen atoms in the presented off-lattice model. Finally, the model naturally provides the mean values for the ratio of binding-to-desorption energy. This ratio demonstrates weak dependence on temperature and is in the range of 0.5–0.6.

Abstract We extend previous theoretical works to gain a better understanding of the origin of observed polarisation degree spectra of molecular clouds, which show a so-called V-shape, i.e. a pronounced minimum around 350 μm. For this purpose, we present results of two-phase dust models investigated with POLARIS. We also provide a guideline to calculate individual dust temperatures for different grain types in POLARIS. We show that V-shaped polarisation spectra can only be obtained if two dust phases, one dense and cold as well as one warm and dilute phase, are present along the line of sight. We find that the V-shape is the stronger pronounced the larger the density and temperature contrast between both phases is. In contrast to previous results, no correlation between the alignment efficiency of silicate grains and the dust temperature is required; carbonaceous grains are in general assumed to be not aligned with the magnetic field. By matching our model results with actual observations of V-shaped polarisation spectra, we show that in UV-illuminated regions (here the warm and dilute phase) carbon grain destruction might take place. This leads to a more pronounced V-shape with a minimum around 300 μm. In addition, we show that the dust spectral index and temperature of silicate grains affects the steepness of the polarisation spectrum at long wavelengths. Finally, we present a first polarisation spectrum obtained from a 3D, magneto-hydrodynamical molecular cloud simulation. It shows a flattening or even weakly pronounced minimum around 350 μm demonstrating the potential of such complex 3D simulations to study polarisation spectra.

Context. Protoplanetary discs around very low mass stars (VLMSs) show hydrocarbon-rich MIR spectra indicative of C/O>1 in their inner discs. This is in contrast to such discs around higher-mass hosts, which are typically richer in O-bearing species. Aims. The two scenarios proposed to elevate C/O around the inner discs of VLMSs are the release of C by eroding carbonaceous grains or the advection of O-depleted gas from the outer disc. However, if CO gas remains abundant, sufficiently O-depleted material cannot be produced. We tested whether the chemical transformation of CO into other species allows the transport scenario to produce C/O significantly in excess of 1. Methods. We tracked the inner disc C/H and O/H over time using a 1D disc evolution code that models the transport of gas and ice phase molecules and includes the conversion of some species into others to represent key reaction pathways operating in the midplane. We explored the influence of disc mass, size, ionisation rate, and the presence of a dust trap. Results. The inner disc C/O increases over time due to sequential delivery where O-rich species (e.g. H2O) give way to C-rich species (e.g. CH4). To reach C/O>1, separating C and O is key, and hence the gas phase destruction of CO by He+, which liberates C, is critical. Ionisation drives the midplane chemistry and must have rates ≳10−17 s−1 (at least for VLMSs) for significant chemical evolution within the disc lifetime. However, the rates must be ≲10−17 s−1 for T Tauri stars to ensure their C/O remains less than 1 for the first few megayears. Initially more compact discs lose O-rich ices faster and reach a higher C/O. A warm dust trap between the CH3OH and CH4 snowlines traps CH3 OH ice (formed via hydrogenation of CO ice) for long enough to be photodissociated, providing an alternative way to liberate the C that started in CO in the form of CH4 gas that keeps the inner disc significantly C rich. Conclusions. The destruction of gaseous CO combined with gas advection and radial drift can deplete O enough and produce sufficient hydrocarbons to explain the typical C/O>1 of VLMSs. While their C/O is typically higher than for T Tauri stars due to the faster sequential delivery, achieving values significantly in excess of 1 likely also requires higher ionisation rates and more compact discs than for T Tauri stars. Observations of older discs may distinguish whether a higher ionisation rate is indeed required or if the faster physical evolution timescales alone are sufficient.

We discuss ASASSN-24fw, a 13th-magnitude star that optically faded by Δg=4.12±0.02 mag starting in September 2024 after over a decade of quiescence in ASAS-SN. The dimmimg lasted $$8 months before returning to quiescence in late May 2025. The spectral energy distribution (SED) before the event is that of a pre-main sequence or a modestly evolved F star with some warm dust emission. The shape of the optical SED during the dim phase is unchanged and the optical and near-infrared spectra are those of an F star. The SED and the dilution of some of the F star infrared absorption features near minimum suggest the presence of a $ 0.25M_$ M dwarf binary companion. The 43.8 year period proposed by Nair & Denisenko (2024) appears correct and is probably half the precession period of a circumbinary disk. The optical eclipse is nearly achromatic, although slightly deeper in bluer filters, Δ(g−z)=0.31±0.15 mag, and the V band emission is polarized by up to 4%. The materials most able to produce such small optical color changes and a high polarization are big ($$20 μm) carbonaceous or water ice grains. Particle distributions dominated by big grains are seen in protoplanetary disks, Saturn-like ring systems and evolved debris disks. We also carry out a survey of occultation events, finding 46 additional systems, of which only 7 (4) closely match ε Aurigae (KH 15D), the two archetypes of stars with long and deep eclipses. The full sample is widely distributed in an optical color-magnitude diagram, but roughly half show a mid-IR excess. It is likely many of the others have cooler dust since it seems essential to produce the events.

We discuss ASASSN-24fw, a 13th-magnitude star that optically faded by Δg=4.12±0.02 mag starting in September 2024 after over a decade of quiescence in ASAS-SN. The dimmimg lasted $$8 months before returning to quiescence in late May 2025. The spectral energy distribution (SED) before the event is that of a pre-main sequence or a modestly evolved F star with some warm dust emission. The shape of the optical SED during the dim phase is unchanged and the optical and near-infrared spectra are those of an F star. The SED and the dilution of some of the F star infrared absorption features near minimum suggest the presence of a $ 0.25M_$ M dwarf binary companion. The 43.8 year period proposed by Nair & Denisenko (2024) appears correct and is probably half the precession period of a circumbinary disk. The optical eclipse is nearly achromatic, although slightly deeper in bluer filters, Δ(g−z)=0.31±0.15 mag, and the V band emission is polarized by up to 4%. The materials most able to produce such small optical color changes and a high polarization are big ($$20 μm) carbonaceous or water ice grains. Particle distributions dominated by big grains are seen in protoplanetary disks, Saturn-like ring systems and evolved debris disks. We also carry out a survey of occultation events, finding 46 additional systems, of which only 7 (4) closely match ε Aurigae (KH 15D), the two archetypes of stars with long and deep eclipses. The full sample is widely distributed in an optical color-magnitude diagram, but roughly half show a mid-IR excess. It is likely many of the others have cooler dust since it seems essential to produce the events.

ABSTRACT Dust is a fundamental component of the interstellar medium within galaxies, as dust grains are highly efficient absorbers of ultraviolet (UV) and optical photons. Accurately quantifying this obscuration is crucial for interpreting galaxy spectral energy distributions (SEDs). The extinction curves in the Milky Way (MW) and Large Magellanic Cloud exhibit a strong feature known as the 2175 Å UV bump, most often attributed to small carbonaceous dust grains. This feature was recently detected in faint galaxies out to $z=7.55$, suggesting rapid formation channels. Here, we report the detection of a strong UV bump in a luminous Lyman-break galaxy at $z_\mathrm{prism}=7.11235$, GNWY-7379420231, through observations taken as part of the NIRSpec Wide GTO survey. We fit a dust attenuation curve that is consistent with the MW extinction curve within $1\sigma$, in a galaxy just $\sim 700$ Myr after the big bang. From the integrated spectrum, we infer a young mass-weighted age ($t_\star \sim 22\!-\!59$ Myr) for this galaxy, however spatially resolved SED fitting unveils the presence of an older stellar population ($t_\star \sim 252$ Myr). Furthermore, morphological analysis provides evidence for a potential merger. The underlying older stellar population suggests the merging system could be pre-enriched, with the dust illuminated by a merger-induced starburst. Moreover, turbulence driven by stellar feedback in this bursty region may be driving polycyclic aromatic hydrocarbon formation through top-down shattering. The presence of a UV bump in GNWY-7379420231 solidifies growing evidence for the rapid evolution of dust properties within the first billion years of cosmic time.

The infrared emission from polycyclic aromatic hydrocarbons (PAHs), along with emission from atomic carbon and simple hydrocarbons, is a robust tracer of the interaction between stellar far-UV (FUV) radiation and molecular clouds. We present subarcsecond-resolution ALMA mosaics of the Orion Bar photodissociation region (PDR) in [C I] 609 μm (3P1−3P0), C2H (N = 4−3), and C18O (J = 3−2) emission lines complemented by JWST images of H2 and aromatic infrared band (AIB) emission. We interpreted the data using up-to-date PDR and radiative transfer models, including high-temperature C2H (X2 Σ+)-o/p-H2 and C (3P)-o/p-H2 inelastic collision rate coefficients (we computed the latter up to 3000 K). The rim of the Bar shows very corrugated and filamentary structures made of small-scale H2 dissociation fronts (DFs). The [C I] 609 μm 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 ×10−7. 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 μm AIB intensity ratio, which is 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.

The reason for the abundance of molecular hydrogen (H2) in space remains unresolved. Here we study collision dynamics under spacelike conditions to test H2 formation mechanisms where carbonaceous dust grains may have a catalytic role. Density functional theory molecular dynamics simulates atomic hydrogen capture and H2 formation on the surface of buckminsterfullerene as a carbonaceous cosmic dust model. Maximally localized Wannier functions are applied to examine the electronic bonding during transition states. The fullerene surface is shown to be effective at warm (50K) and low (10K) temperatures in achieving atomic H chemisorption, potentially explaining the observed broad temperature range for efficient H2 formation. We revise the Eley-Rideal mechanism and propose that both it and the Langmuir-Hinshelwood mechanism, induced by thermal hopping, contribute to bursts of H2 formation during energetic events. Additionally, we show how fullerene maintains the abundance of H2 in space by selectively preventing H2 molecules from capture.

Abstract The gas present in planet-forming disks typically exhibits strong emission features of abundant carbon and oxygen molecular carriers. In some instances, protoplanetary disks show an elevated C/O ratio above interstellar values, which leads to a rich hydrocarbon chemistry evidenced in the mid-infrared spectra. The origin of this strengthened C/O ratio may stem from the release of less complex hydrocarbons from the chemical processing of carbonaceous grains. We have explored a set of 42 single-cell models in which we match the physical conditions to the inner regions of planet-forming disks, while varying the C/O ratio by exploring different levels of CH4, C, H2O, and CO to the gas-phase chemistry, which we evaluate in both the cosmic/X-ray- and UV-driven limit. We find that the carbon-bearing species in our models exhibit high dependencies on the driver of the chemistry, where both CO and long chain hydrocarbons act as carbon sinks in the cosmic/X-ray-driven chemistry limit, while the vast majority ends up in atomic carbon and CO in the UV-driven limit. We also find moderate dependencies upon the C/O ratio, where this and the ionization rate/UV field determines the point of peak production of a species, as well as its equilibrium abundance. We also find that the production of several hydrocarbons, specifically C2H2, is strongly dependent up to an order of magnitude on the initial water abundance. We finally find that in the X-ray-driven limit, both CH4 and C serve as highly transient donor species to the carbon chemistry.

Abstract The James Webb Space Telescope (JWST) has opened up a new window to study highly reddened explosive transients. We present results from late-time JWST follow-up spectroscopic observations with NIRSpec and MIRI-LRS of the intermediate-luminosity red transient (ILRT) AT 2019abn. ILRTs represent a mysterious class of transients that exhibit peak luminosities between those of classical novae and supernovae and that are known to be highly dust obscured. Similar to the prototypical examples of this class of objects, NGC 300 2008-OT and SN 2008S, AT 2019abn has an extremely red and dusty progenitor detected only in pre-explosion Spitzer/IRAC imaging at 3.6 and 4.5 μm and not in deep optical or near-infrared Hubble Space Telescope images. We find that late-time observations of AT 2019abn from NEOWISE and JWST are consistent with the late-time evolution of SN 2008S. In part because they are so obscured by dust, it is unknown what produces an ILRT, with hypotheses including high-mass stellar merger events, nonterminal stellar outbursts, and terminal supernova explosions through electron capture in super-AGB (SAGB) stars. Our JWST observations show strong mid-IR class C polycyclic aromatic hydrocarbon features at 6.3 and 8.25 μm typical of carbon-rich post-AGB sources. These features suggest that the dust around AT 2019abn is composed of carbonaceous grains, which are not typically observed around red supergiants. However, depending on the strength and temperature of hot bottom burning, SAGB stars may be expected to exhibit a carbon-rich chemistry. Thus, our JWST observations are consistent with AT 2019abn having an SAGB progenitor and exploding as an electron-capture supernova.

Context. The total deuterium abundance [D/H] in the universe is set by just two processes: the creation of deuterium in Big Bang nucleosynthesis at an abundance of [D/H] = 2.58 ± 0.13 × 10−5, and its destruction within stellar interiors (astration). Measurements of variations in the total [D/H] abundance can thus potentially provide a probe of Galactic chemical evolution. However, most observational measurements of [D/H] are only sensitive to the gas-phase deuterium, and the amount of deuterium sequestered in dust grains is debated. With the launch of the James Webb Space Telescope (JWST), it is now possible to measure the gas-phase [D/H] at unprecedented sensitivity and distances through observation of mid-IR lines of H2 and HD. Comparisons of gas-phase [D/H] with the constraints on the total [D/H] from the primordial abundance and Galactic chemical evolution models can provide insight into the degree of Deuterium lock-up in grains and the star formation history of our Galaxy. Aims. We use data from the JWST Observations of Young protoStars (JOYS) program of 5 nearby and resolved low-mass protostellar outflows and 5 distant high-mass protostellar outflows taken with the JWST Mid Infrafred Instrument (MIRI) Medium Resolution Spectrometer (MRS) to measure gas-phase [D/H] via H2 and HD lines, assuming the gas is fully molecular. Methods. We extract spectra from various locations in the outflows. Using a rotational diagram analysis covering lines of H2 and HD with similar excitation energies, we derive the column density of HD and H2 or their upper limits. We then calculate the gas-phase [D/H] from the column density results, and additionally apply a correction factor for the effect of chemical conversion of HD to atomic D and non-LTE excitation on the HD abundance in the shocks. To investigate the spatial distribution of the bulk gas and species refractory species associated with the dust grains, we also construct integrated line intensity maps of H2, HD, [Fe II], [Fe I], and [S I] lines. Results. A comparison of gas-phase [D/H] between our low-mass sources shows variations of up to a factor of ~4, despite these sources likely having formed in nearly the same region of the Galactic disk that would be expected to have nearly constant total [D/H]. Most measurements of gas-phase [D/H] from our work or previous studies produce [D/H] ≲ 1.0 × 10−5, a factor of 2-4 lower than found from local UV absorption lines and as expected from Galactic chemical evolution models. In the integrated line intensity maps, the morphology of the HD R(6) line emission is strongly correlated with the H2 S(7), [S I], and [Fe I] lines which mostly trace high velocity jet knots and bright bow-shocks. In our extracted spectra along the outflows, there is similarly a strong correlation between the H2 and HD column density and the [S I] and [Fe I] line flux, however, no correlation is seen between [D/H] and the [S I] or [Fe I] line flux. Conclusions. The variations in [D/H] between our low-mass sources and the low [D/H] with respect to Galactic chemical evolution models suggest that our observations are not sensitive to the total [D/H]. Significant depletion of deuterium onto carbonaceous dust grains is a possible explanation, and tentative evidence of enhanced [D/H] toward positions with higher gas-phase Fe abundance is seen in the HH 211 outflow. Deeper observations of HD and H2 across a wider range of shock conditions and modeling of the carbonaceous dust-grain destruction and shock conditions are warranted to test for the effects of depletion.

Molecular hydrogen (H2) stands as the most abundant molecule within the interstellar medium (ISM), primarily originating from the coupling of two H atoms on the surfaces of dust grains. The role of dust grains during the H2 formation is of third bodies, dissipating the nascent reaction energy and thereby stabilizing the newly formed molecule and preventing it from dissociating back. Whether the formed H2 remains adsorbed or not on the surface (in this latter case undergoing chemical desorption, CD) largely depends on the type of grain and its capability to absorb the reaction energy excess. In diffuse interstellar clouds, dust grains are typically bare and are composed primarily of silicates or carbonaceous materials, while in denser regions they are covered in ices mostly of water. While water-ice-covered grains have been elucidated to be efficient third bodies, the behavior of carbonaceous grains is still unknown. In this study, ab initio molecular dynamics (AIMD) simulations are employed to analyze how the reaction energy is distributed between the newly formed H2 and a large graphene slab, as a model of carbonaceous grains in diffuse clouds, and assess the feasibility of CD. The results indicate that only a fraction of the reaction energy is absorbed by the surface, leaving the newly formed H2 with sufficient internal energy for CD to occur.

Context. Cosmic dust is ubiquitous in astrophysical environments, where it significantly influences the chemistry and the spectra. Dust grains are likely to grow through the accretion of atoms and molecules from the gas-phase onto them. Despite their importance, only a few studies have computed the sticking coefficients for relevant temperatures and species, along with their direct impact on grain growth. Overall, the formation of dust and its growth are not well understood. Aims. This study is aimed at calculating the sticking coefficients, binding energies, and grain growth rates over a broad range of temperatures, for various gas species interacting with carbonaceous dust grains. Methods. We performed molecular dynamics simulations with a reactive force field algorithm to compute accurate sticking coefficients and obtain the binding energies. These results were used to build an astrophysical model of nucleation regions to study dust growth. Results. We present, for the first time, the sticking coefficients of H, H2 , C, O, and CO on amorphous carbon structures for temperatures ranging from 50 K to 2250 K. In addition, we estimated the binding energies of H, C, and O in carbonaceous dust to calculate the thermal desorption rates. Combining accretion and desorption allows us to determine an effective accretion rate and sublimation temperature for carbonaceous dust. Conclusions. We find that sticking coefficients can differ substantially from what is commonly used in astrophysical models. This offers us new insights into carbonaceous dust grain growth via accretion in dust-forming regions.

We report on the discovery of a very prominent mid-infrared (18–25 μm) excess associated with the trans-Neptunian dwarf planet (136472) Makemake. The excess, detected by the Mid-Infrared Instrument of the James Webb Space Telescope, along with previous measurements from the Spitzer and Herschel space telescopes, indicates the occurrence of temperatures of ∼150 K, much higher than what solid surfaces at Makemake’s heliocentric distance could reach by solar irradiation. We identify two potential explanations: a continuously visible, currently active region powered by subsurface upwelling and possibly cryovolcanic activity covering ≤1% of Makemake’s surface or an as-yet-undetected ring containing very small carbonaceous dust grains, which have not been seen before in trans-Neptunian or Centaur rings. Both scenarios point to unprecedented phenomena among trans-Neptunian objects and could greatly impact our understanding of these distant worlds.

Dust grains play a key role in various astrophysical processes and serve as indicators of interstellar medium structures, density, and mass. Understanding their physical properties and chemical composition is a crucial goal in astrophysics. Dust polarisation is a valuable tool for studying these properties. The radiative torque (RAT) paradigm, which includes radiative torque alignment (RAT-A) and radiative torque disruption (RAT-D), is essential to interpreting the dust polarisation data and constraining the fundamental properties of dust grains. However, it has been used primarily to interpret observations at a single wavelength. In this study, we analyse the thermal dust polarisation spectrum obtained from observations with SOFIA/HAWC+ and JCMT/POL-2 in the Orion molecular cloud 1 (OMC-1) region and compare the observational data with our numerical results using the RAT paradigm. In general, we show that the dense gas exhibits a positive spectral slope, whereas the warm regions show a negative one. We demonstrate that a one-layer dust (one-phase) model can only reproduce the observed spectra at certain locations and cannot match those with prominent V-shaped spectra (for which the degree of polarisation initially decreases with wavelength from 54 to ~300µm and then increases at longer wavelengths). To address this, we improved our model by incorporating two dust components (warm and cold) along the line of sight, resulting in a two-phase model. This improved model successfully reproduces the V-shaped spectra. The best model corresponds to a mixture composition of silicate and carbonaceous grains in the cold medium. Finally, by assuming the plausible model of grain alignment, we were able to infer the inclination angle of the magnetic fields in OMC-1. This approach is an important step towards a better understanding the physics of grain alignment and constraining 3D magnetic fields using dust polarisation spectra.