When studying the chemistry of PDRs, time dependence becomes important as visual extinction increases, since certain chemical timescales are comparable to the cloud lifetime. Dust temperature is also a key factor, since it significantly influences gas temperature and mobility on dust grains, determining the chemistry occurring on grain surfaces. We present a study of the dust temperature impact and time effects on the chemistry of different PDRs, using an updated version of the Meijerink PDR code and combining it with the time-dependent code Nahoon. We find the largest temperature effects in the inner regions of high $G$$_{\mathrm{0}}$ PDRs, where high dust temperatures favour the formation of simple oxygen-bearing molecules (especially that of O$_2$), while the formation of complex organic molecules is much more efficient at low dust temperatures. We also find that time-dependent effects strongly depend on the PDR type, since long timescales promote the destruction of oxygen-bearing molecules in the inner parts of low $G$$_{\mathrm{0}}$ PDRs, while favouring their formation and that of carbon-bearing molecules in high $G$$_{\mathrm{0}}$ PDRs. From the chemical evolution, we also conclude that, in dense PDRs, CO$_2$ is a late-forming ice compared to water ice, and confirm a layered ice structure on dust grains, with H$_2$O in lower layers than CO$_2$. Regarding steady state, the PDR edge reaches chemical equilibrium at early times ($\lesssim$10$^5$ yr). This time is even shorter ($<$10$^4$ yr) for high $G$$_{\mathrm{0}}$ PDRs. By contrast, inner regions reach equilibrium much later, especially low $G$$_{\mathrm{0}}$ PDRs, where steady state is reached at $\sim$10$^6$-10$^7$ yr.
Aims. Photodissociation by UV light is an important destruction mechanism for CO in many astrophysical environments, ranging from interstellar clouds to protoplanetary disks. The aim of this work is to gain a better understanding of the depth dependence and isotope-selective nature of this process. Methods. We present a photodissociation model based on recent spectroscopic data from the literature, which allows us to compute depth-dependent and isotope-selective photodissociation rates at higher accuracy than in previous work. The model includes self-shielding, mutual shielding and shielding b…
The presence of dust can strongly affect the chemical composition of the interstellar medium. We model the chemistry in photodissociation regions (PDRs) using both gas-phase and dust-phase chemical reactions. Our aim is to determine the chemical compositions of the interstellar medium (gas/dust/ice) in regions with distinct (molecular) gas densities that are exposed to radiation fields with different intensities. We have significantly improved the Meijerink PDR code by including 3050 new gas-phase chemical reactions and also by implementing surface chemistry. In particular, we have included 11…
Non-thermal photons deriving from radiative transitions among the internal ladder of atoms and molecules are an important source of photons in addition to thermal and stellar sources in many astrophysical environments. In the present work the calculation of reaction rates for the direct photodissociation of some molecules relevant in early Universe chemistry is presented; in particular, the calculations include non-thermal photons deriving from the recombination of primordial hydrogen and helium atoms for the cases of H2, HD and HeH+. New effects on the fractional abundances of chemical specie…
We study ten molecular transitions obtained from an unbiased 3 mm molecular line survey using the IRAM 30 m telescope toward 409 compact dust clumps identified by the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) to understand photodissociation regions (PDRs) associated with the clumps. The main goal of this study is to investigate whether the abundances of the selected molecules show any variations resulting from the PDR chemistry in different clump environments. We selected HCO, HOC$^+$, C$_{2}$H, c-C$_{3}$H$_{2}$, CN, H$^{13}$CN, HC$^{15}$N, and HN$^{13}$C as PDR tracers, and H$…
The determination of the physical conditions in molecular clouds is a key step towards our understanding of their formation and evolution of associated star formation. We investigate the density, temperature, and column density of both dust and gas in the photodissociation regions (PDRs) located at the interface between the atomic and cold molecular gas of the NGC 7023 reflection nebula. We study how young stars affect the gas and dust in their environment. Our approach combining both dust and gas delivers strong constraints on the physical conditions of the PDRs. We find dense and warm molecu…
We explore the effect of varying the spectral energy distribution of the incident continuum, by simultaneously and self-consistently computing the structure of an H II region and a photodissociation region that are in pressure equilibrium. The results of the calculations are applied to extragalactic observations. The intensity ratio diagrams of far-infrared (FIR) emission for Herschel bands (70, 110, 160, 250, 350, and 500 μm) and the contribution from H II regions for these specific FIR emissions are presented for the first time. With these diagrams, we compare the predicted FIR continuum int…
A new collection of photodissociation and photoionisation cross sections for 102 atoms and molecules of astrochemical interest has been assembled, along with a brief review of the basic processes involved. These have been used to calculate dissociation and ionisation rates, with uncertainties, in a standard ultraviolet interstellar radiation field (ISRF) and wavelength-dependent radiation fields. The new ISRF rates generally agree within 30% with our previous compilations, with a few notable exceptions. The reduction of rates in shielded regions was calculated as a function of dust, molecular …
Chemical modelling of AGB outflows is typically focused on either non-thermodynamic equilibrium chemistry in the inner region or photon-driven chemistry in the outer region. We include, for the first time, a comprehensive dust-gas chemistry in our AGB outflow chemical kinetics model, including both dust-gas interactions and grain-surface chemistry. The dust is assumed to have formed in the inner region, and follows an interstellar-like dust-size distribution. Using radiative transfer modelling, we obtain dust temperature profiles for different dust types in an O-rich and a C-rich outflow. We c…
In this contribution we discuss about numerical modeling of nebulae. In particular we emphasize on the dynamical evolution of an HII region and the chemical structure of a Photodissociation region. We do this using the Smoothed Particles Hydrodynamics code SEREN and the recently developed astrochemistry code 3D-PDR, respectively. We show an example application by simulating a cometary globule using these two codes.
Small hydrocarbons, such as C2H, C3H and C3H2 are more abundant in photo-dissociation regions (PDRs) than expected based on gas-phase chemical models. To explore the hydrocarbon chemistry further, we observed a key intermediate species, the hydrocarbon ion l-C3H+, in the Horsehead PDR with the Plateau de Bure Interferometer at high-angular resolution (6''). We compare with previous observations of C2H and c-C3H2 at similar angular resolution and new gas-phase chemical model predictions to constrain the dominant formation mechanisms of small hydrocarbons in low-UV flux PDRs. We find that, at th…
Photodissociation regions (PDRs) are parts of the ISM consisting of predominantly neutral gas, located at the interface between H II regions and molecular clouds. The physical conditions within these regions show variations on very short spatial scales, and therefore PDRs constitute ideal laboratories for investigating the properties and evolution of dust grains. We have mapped IC 63 at high resolution from the UV to the NIR (275 nm to 1.6 $μ$m), using the Hubble Space Telescope WFC3. Using a Bayesian SED fitting tool, we simultaneously derive a set of stellar ($T_\text{eff}$, $\log(g)$, dista…
AGB stars are, together with supernovae, the main contributors of stellar dust to the interstellar medium (ISM). Dust grains formed by AGB stars are thought to be large. However, as dust nucleation and growth within their outflows are still not understood, the dust-grain size distribution (GSD) is unknown. This is an important uncertainty regarding our knowledge of the chemical and physical history of interstellar dust, as AGB dust forms $\sim$ 70% of the starting point of its evolution. We expand on our chemical kinetics model, which uniquely includes a comprehensive dust-gas chemistry. The G…
We investigate Galactic bulge planetary nebulae without emission-line central stars for which peculiar infrared spectra have been obtained with the Spitzer Space Telescope, including the simultaneous signs of oxygen and carbon based dust. Three separate sub-groups can be defined characterized by the different chemical composition of the dust and the presence of crystalline and amorphous silicates. We find that the classification based on the dust properties is reflected in the more general properties of these planetary nebulae. However, some observed properties are difficult to relate to the c…
We study a group of evolved M-stars in the Large Magellanic Cloud, characterized by a peculiar spectral energy distribution. While the $9.7~μ$m feature arises from silicate particles, the whole infrared data seem to suggest the presence of an additional featureless dust species. We propose that the circumstellar envelopes of these sources are characterized by a dual dust chemistry, with an internal region, harbouring carbonaceous particles, and an external zone, populated by silicate, iron and alumina dust grains. Based on the comparison with results from stellar modelling that describe the du…
The interaction of ionizing and far-ultraviolet radiation with the interstellar medium is of great importance. It results in the formation of regions in which the gas is ionized, beyond which are photodissociation regions (PDRs) in which the gas transitions to its atomic and molecular form. Several numerical codes have been implemented to study these two main phases of the interstellar medium either dynamically or chemically. In this paper we present TORUS-3DPDR, a new self-consistent code for treating the chemistry of three-dimensional photoionization and photodissociation regions. It is an i…
The PhotoDissociation Region Toolbox provides comprehensive, easy-to-use, public software tools and models that enable an understanding of the interaction of the light of young, luminous, massive stars with the gas and dust in the Milky Way and in other galaxies. It consists of an open-source Python toolkit and photodissociation region models for analysis of infrared and millimeter/submillimeter line and continuum observations obtained by ground-based and sub-orbital telescopes, and astrophysics space missions. Photodissociation regions (PDRs) include all of the neutral gas in the ISM where fa…
Molecular hydrogen is the most abundant molecule in the Universe. It is thought that a large portion of H2 forms by association of hydrogen atoms to polycyclic aromatic hydrocarbons (PAHs). We model the influence of PAHs on total H2 formation rates in photodissociation regions (PDRs) and assess the effect of these formation rates on the total cloud structure. We set up a chemical kinetic model at steady state in a PDR environment and included adiative transfer to calculate the chemistry at different depths in the PDR. This model includes known dust grain chemistry for the formation of H2 and a…
We present a simple analytic procedure for generating atomic-to-molecular (HI-to-H$_2$) density profiles for optically thick clouds illuminated by far-ultraviolet radiation. Our procedure is based on the analytic theory for the structure of one-dimensional HI/H$_2$ photon-dominated regions, presented by Sternberg et al. (2014). Depth-dependent HI and H$_2$ density fractions may be computed for arbitrary gas density, far-ultraviolet field intensity, and the metallicity dependent H$_2$ formation rate coefficient, and dust absorption cross section. We use our procedure to generate a set of HI-to-…
NeuralPDR: Neural Differential Equations as surrogate models for Photodissociation Regions
Leiden Observatory, Leiden University（莱顿天文台，莱顿大学）；SURF；Research Center for Astronomical Computing, Zhejiang Lab（天文计算研究中心，浙江实验室）；Transdisciplinary Research Area (TRA) ‘Matter’/Argelander-Institut für Astronomie, University of Bonn（跨学科研究领域（TRA）‘物质’/阿格拉德勒天文台研究所，波恩大学）；Department of Physics and Astronomy, University College London（物理与天文学系，伦敦大学学院）
At the first Galactic quadrant, at l=33.134, b=-0.091, an extended photodissociation region generated by an HII region complex lies. This region is related to abundant molecular gas, and particularly, a hot molecular core, known as G33.133-mm3, appears embedded in a molecular clump. Using data from the James Clerk Maxwell Telescope with an angular resolution of about 15", we studied the 13CO/C18O abundance ratio towards the mentioned molecular clump and its relation with the ultraviolet radiation. At smaller spatial scales, using data from the Atacama Large Millimeter Array (angular resolution…
To explain the properties of dust in the interstellar medium (ISM), the presence of a refractory organic mantle is necessary. The outflows of AGB stars are among the main contributors of stellar dust to the ISM. We present the first study of the refractory organic contribution of AGB stars to the ISM. Based on laboratory experiments, we included a new reaction in our extended chemical kinetics model: the photoprocessing of volatile complex ices into inert refractory organic material. The refractory organic feedback of AGB outflows to the ISM is estimated using observationally motivated parent …
We study the effects of grain surface reactions on the chemistry of protoplanetary disks where gas, ice surface layers and icy mantles of dust grains are considered as three distinct phases. Gas phase and grain surface chemistry is found to be mainly driven by photo-reactions and dust temperature gradients. The icy disk interior has three distinct chemical regions: (i) the inner midplane with low FUV fluxes and warm dust ($\gtrsim 15$K) that lead to the formation of complex organic molecules, (ii) the outer midplane with higher FUV from the ISM and cold dust where hydrogenation reactions domin…
Interstellar carbonaceous dust is mainly formed in the innermost regions of circumstellar envelopes around carbon-rich asymptotic giant branch (AGB) stars. In these highly chemically stratified regions, atomic and diatomic carbon, along with acetylene are the most abundant species after H$_2$ and CO. In a previous study, we addressed the chemistry of carbon (C and C$_2$) with H$_2$ showing that acetylene and aliphatic species form efficiently in the dust formation region of carbon-rich AGBs whereas aromatics do not. Still, acetylene is known to be a key ingredient in the formation of linear po…
Asymptotic giant branch (AGB) stars are significant contributors to the chemical enrichment of the interstellar medium (ISM) of galaxies. It is therefore essential to constrain the AGB contribution to the dust budget in galaxies. Recent estimates of the total dust injection rate to the Large and Small Magellanic Clouds (LMC and SMC; Riebel et al. 2012, Boyer et al. 2012, Srinivasan et al. in prep) have used data from the Spitzer Space Telescope SAGE (Surveying the Agents of Galaxy Evolution; Meixner et al. 2006) and SAGE-SMC (Gordon et al. 2011) surveys. When sorted by dust chemistry, the data…
The [CII] 158$μ$m fine-structure line is the dominant cooling line of moderate-density photodissociation regions (PDRs) illuminated by moderately bright far-ultraviolet (FUV) radiation fields. We aim to understand the origin of [CII] emission and its relation to other tracers of gas and dust in PDRs. One focus is a study of the heating efficiency of interstellar gas as traced by the [CII] line to test models of the photoelectric heating of neutral gas by polycyclic aromatic hydrocarbon (PAH) molecules and very small grains. We make use of a one-square-degree map of velocity-resolved [CII] line…
Resolving dense photodissociation regions: the structure of photochemical fronts in three-dimensional gas distributions
Faculty of Physics, University of Duisburg-Essen（杜伊斯堡-埃森大学物理学院）；Research Center for Computational Earth and Space Science, Zhejiang Lab（浙江实验室计算地球与空间科学研究中心）
Six low-mass embedded sources (L1489, L1551-IRS5, TMR1, TMC1-A, L1527, and TMC1) in Taurus have been observed with Herschel-PACS to cover the full spectrum from 50 to 210 $μ$m as part of the Herschel key program, "Dust, Ice, and Gas In Time (DIGIT)". The relatively low intensity of the interstellar radiation field surrounding Taurus minimizes contamination of the [C II] emission associated with the sources by diffuse emission from the cloud surface, allowing study of the [C II] emission from the source. In several sources, the [C II] emission is distributed along the outflow, as is the [O I] e…
Within the framework of The Heterogeneous dust Evolution Model at the IaS (THEMIS) this work investigates in detail the surface processes and chemistry relating to core/mantle interstellar and cometary grain structures and its influence on the nature of these fascinating particles. It appears that a realistic consideration of the nature and chemical reactivity of interstellar grain surfaces could self-consistently and within a coherent framework explain: the anomalous oxygen depletion, the nature of the CO dark gas, the formation of 'polar ice' mantles, the red wing on the 3 um water ice band,…
The degree of porosity in interstellar dust-grain material is poorly defined, although recent work has suggested that the grains could be highly porous. Aside from influencing the optical properties of the dust, porosity has the potential to affect the chemistry occurring on dust-grain surfaces, via increased surface area, enhanced local binding energies, and the possibility of trapping of molecules within the pores as ice mantles build up on the grains. Through computational kinetics simulations, we investigate how interstellar grain-surface chemistry and ice composition are affected by the p…
We present an extensive review of gas phase reactions producing methylacetylene and propene showing that these relatively abundant unsaturated hydrocarbons cannot be synthesized through gas-phase reactions. We explain the formation of propene and methylacetylene through surface hydrogenation of C3 depleted onto interstellar ices, C3 being a very abundant species in the gas phase.
We present deep high-resolution (R~15,000) and high-quality UVES optical spectrophotometry of nine planetary nebulae with dual-dust chemistry. We compute physical conditions from several diagnostics. Ionic abundances for a large number of ions of N, O, Ne, S, Cl, Ar, K, Fe and Kr are derived from collisionally excited lines. Elemental abundances are computed using state-of-the-art ionization correction factors. We derive accurate C/O ratios from optical recombination lines. We have re-analyzed additional high-quality spectra of 14 PNe from the literature following the same methodology. Compari…
Pre-stellar cores within molecular clouds provide the very initial conditions in which stars are formed. We use the IRAM 30m telescope and the PdBI to study the chemical and physical properties of the starless core FeSt 1-457 (Core 109), in the Pipe nebula. We fit the hyperfine structure of the N2H+(1-0) IRAM 30m data. This allow us to measure with high precision the velocity field, line widths and opacity and derive the excitation temperature and column density in the core. We use a modified Bonnor-Ebert sphere model adding a temperature gradient towards the center to fit the 1.2 mm continuum…
Aims. Our goal is to complete the inventory of S-bearing molecules and their abundances in the prototypical photodissociation region (PDR) the Horsehead nebula to gain insight into sulphur chemistry in UV irradiated regions. Based on the WHISPER millimeter (mm) line survey, our goal is to provide an improved and more accurate description of sulphur species and their abundances towards the core and PDR positions in the Horsehead. Methods. The Monte Carlo Markov chain (MCMC) methodology and the molecular excitation and radiative transfer code RADEX were used to explore the parameter space and de…
Interstellar dust is a key element in our understanding of the interstellar medium and star formation. The manner in which dust populations evolve with the excitation and the physical conditions is a first step in the comprehension of the evolution of inter- stellar dust. Within the framework of the Evolution of interstellar dust Herschel key program, we have acquired PACS and SPIRE spec- trophotometric observations of various photodissociation regions, to characterise this evolution. The aim of this paper is to trace the evolution of dust grains in the Orion Bar photodissociation region. We u…
Gas chemistry in the dust depleted inner regions of protoplanetary disks. I. Near-IR spectra and overtones
Max-Planck Institute for extraterrestrial physics（马克斯·普朗克地外物理研究所）；Kapteyn Astonomical Institute, University of Groningen（格罗宁根大学卡普坦天文研究所）；University Observatory, Faculty of Physics, Ludwig-Maximilians-University at Munchen（慕尼黑路德维希-马克西米利安大学物理学院天文台）；Max-Planck-Institut fur Astronomie（马克斯·普朗克天文学研究所）
The role and importance of nano-particles for interstellar chemistry and beyond is explored within the framework of The Heterogeneous dust Evolution Model at the IaS (THEMIS), focussing on their active surface chemistry (nascence), the effects of nitrogen doping and the natural selection of interesting nano-particle sub-structures. Nano-particle driven chemistry, and in particular the role of intrinsic epoxide-type structures could provide a viable route to the observed gas phase OH in tenuous interstellar clouds en route to becoming molecular clouds. The aromatic-rich moieties present in asph…
We investigate the chemistry in a radiation-hydrodynamics model of star-forming core which evolves from a cold ($\sim 10$ K) prestellar core to the main accretion phase in $\sim 10^5$ yr. A rotationally-supported gravitationally unstable disk is formed around a protostar. We extract the temporal variation of physical parameters in $\sim 1.5 \times 10^3$ SPH particles which end up in the disk, and perform post-processing calculations of the gas-grain chemistry adopting a three-phase model. Inside the disk, the SPH particles migrate both inward and outward. Since a significant fraction of volati…
The radiation from stars and active galactic nuclei (AGN) creates photodissociation regions (PDRs) and X-ray dominated regions (XDRs), where the chemistry or heating are dominated by far-ultraviolet (FUV) radiation or X-ray radiation, respectively. PDRs include a wide range of environments from the diffuse interstellar medium to dense star-forming regions. XDRs are found in the center of galaxies hosting AGN, in protostellar disks, and in the vicinity of X-ray binaries. In this review, we describe the dominant thermal, chemical, and radiation transfer processes in PDRs and XDRs, as well as a b…
Recent ALMA observations suggest that the highest velocity part of molecular protostellar jets are launched from the dust-sublimation regions of the accretion disks (<0.3 au). However, formation and survival of molecules in inner protostellar disk winds, in the presence of a harsh FUV radiation field and the absence of dust, remain unexplored. We aim at determining if simple molecules can be synthesized and spared in fast and collimated dust-free disk winds or if a fraction of dust is necessary to explain the observed molecular abundances. This work is based on the Paris-Durham shock code desi…
The evaporation of molecules from dust grains is crucial to understand some key aspects of the star- and the planet-formation processes. During the warm-up phase the presence of young protostellar objects induces molecules to evaporate from the dust surface into the gas phase, enhancing its chemical complexity. Similarly, in circumstellar disks, the position of the so-called snow-lines is determined by evaporation, with important consequences for the formation of planets. The amount of molecules that are desorbed depends on the interaction between the species and the grain surface, which is co…
The origin of the reservoirs of water on Earth is debated. The Earth's crust may contain at least three times more water than the oceans. This crust water is found in the form of phyllosilicates, whose origin probably differs from that of the oceans. We test the possibility to form phyllosilicates in protoplanetary disks, which can be the building blocks of terrestrial planets. We developed an exploratory rate-based warm surface chemistry model where water from the gas-phase can chemisorb on dust grain surfaces and subsequently diffuse into the silicate cores. We apply the phyllosilicate forma…
We present an unbiased large-scale (9 deg$^2$) CN ($N$=1-0) and C$^{18}$O ($J$=1-0) survey of Cygnus-X conducted with the Nobeyama 45m Cygnus-X CO survey. CN and C$^{18}$O are detected in various objects towards the Cygnus-X North and South (e.g., DR17, DR18, DR21, DR22, DR23, and W75N). We find that CN/C$^{18}$O integrated intensity ratios are systematically different from region to region, and are especially enhanced in DR17 and DR18 which are irradiated by the nearby OB stars. This result suggests that CN/C$^{18}$O ratios are enhanced via photodissociation reactions. We investigate the rela…
Revisiting the formation of molecules and dust in core collapse supernovae
Université de Montpellier, CNRS（蒙彼利埃大学，法国国家科学研究中心）；Consejo Superior de Investigaciones Cientificas, Instituto de Fisica Fundamental（西班牙高等科学研究理事会，基础物理研究所）
We present a study of emission lines of small hydrocarbons C$_2$H and $c$-C$_3$H$_2$, and COMs precursors H$_2$CO and CH$_3$OH in order to better understand the possible chemical link between the molecular abundances and UV radiation field in photodissociation regions (PDRs). We study two PDRs around extended and compact HII regions with $G \leq 50$~Habings in the S235 star-forming complex. We find the highest abundances of both hydrocarbons on the edges of molecular clumps, while $c$-C$_3$H$_2$ is also abundant in the low-density expanding PDR around compact HII region S235\,A. We see the hig…
The ULIRG Mrk 231 exhibits very strong water rotational lines between λ= 200-670μm, comparable to the strength of the CO rotational lines. High redshift quasars also show similar CO and H2O line properties, while starburst galaxies, such as M82, lack these very strong H2O lines in the same wavelength range, but do show strong CO lines. We explore the possibility of enhancing the gas phase H2O abundance in X-ray exposed environments, using bare interstellar carbonaceous dust grains as a catalyst. Cloud-cloud collisions cause C and J shocks, and strip the grains of their ice layers. The internal…
Dust grains play a central role in the physics and chemistry of cosmic environments. They influence the optical and thermal properties of the medium due to their interaction with stellar radiation; provide surfaces for the chemical reactions that are responsible for the synthesis of a significant fraction of key astronomical molecules; and they are building blocks of pebbles, comets, asteroids, planetesimals, and planets. In this paper, we review experimental studies of physical and chemical processes, such as adsorption, desorption, diffusion, and reactions forming molecules, on the surface o…
The mid-infrared (MIR) spectra of dense photodissociation regions (PDRs) are typically dominated by emission from polycyclic aromatic hydrocarbons (PAHs) and the lowest pure rotational states of molecular hydrogen (H2); two species which are probes of the physical properties of gas and dust in intense UV radiation fields. We utilize the high angular resolution of the Infrared Spectrograph on the Spitzer Space Telescope to construct spectral maps of the PAH and H2 features for three of the best studied PDRs in the galaxy, NGC 7023, NGC 2023 and IC 63. We present spatially resolved maps of the p…
Recent studies have confirmed the presence of buckminsterfullerene (C$_{60}$) in different interstellar and circumstellar environments. However, several aspects regarding C$_{60}$ in space are not well understood yet, such as the formation and excitation processes, and the connection between C$_{60}$ and other carbonaceous compounds in the interstellar medium, in particular polycyclic aromatic hydrocarbons (PAHs). In this paper we study several photodissociation regions (PDRs) where C$_{60}$ and PAHs are detected and the local physical conditions are reasonably well constrained, to provide obs…
Unlike gas-phase reactions, chemical reactions taking place on interstellar dust grain surfaces cannot always be modeled by rate equations. Due to the small grain sizes and low flux,these reactions may exhibit large fluctuations and thus require stochastic methods such as the moment equations. We evaluate the formation rates of H2, HD and D2 molecules on dust grain surfaces and their abundances in the gas phase under interstellar conditions. We incorporate the moment equations into the Meudon PDR code and compare the results with those obtained from the rate equations. We find that within the …
A brief introduction and overview of the astrochemistry of dust, ice and gas and their interplay is presented, aimed at non-specialists. The importance of basic chemical physics studies of critical reactions is illustrated through a number of recent examples. Such studies have also triggered new insight into chemistry, illustrating how astronomy and chemistry can enhance each other. Much of the chemistry in star- and planet-forming regions is now thought to be driven by gas-grain chemistry rather than pure gas-phase chemistry, and a critical discussion of the state of such models is given. Rec…
Spatial distribution of organics in the Horsehead nebula: Signposts of chemistry driven by atomic carbon
Pontificia Universidad Católica de Chile（智利天主教 Pontificia 大学）；Millennium Nucleus on Young Exoplanets and their Moons (YEMS)（年轻系外行星及其卫星千年核）；European Southern Observatory（欧洲南方天文台）；IRAM（红外毫米波研究所）；Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités（巴黎天文台，PSL研究大学，CNRS，索邦大学）；Uppsala University（乌普萨拉大学）；Instituto de Física Fundamental (CSIC)（基础物理研究所（CSIC））
Interstellar chemistry is important for galaxy formation, as it determines the rate at which gas can cool, and enables us to make predictions for observable spectroscopic lines from ions and molecules. We explore two central aspects of modelling the chemistry of the interstellar medium (ISM): (1) the effects of local stellar radiation, which ionises and heats the gas, and (2) the depletion of metals onto dust grains, which reduces the abundance of metals in the gas phase. We run high-resolution (400 M$_\odot$ per baryonic particle) simulations of isolated disc galaxies, from dwarfs to Milky Wa…
