We study whether polycyclic aromatic hydrocarbons (PAHs) can be a weighty source of small hydrocarbons in photo-dissociation regions (PDRs). We modeled the evolution of 20 specific PAH molecules in terms of dehydrogenation and destruction of the carbon skeleton under the physical conditions of two well-studied PDRs, the Orion Bar and the Horsehead nebula which represent prototypical examples of PDRs irradiated by "high" and "low" ultraviolet radiation field. PAHs are described as microcanonical systems. The acetylene molecule is considered as the main carbonaceous fragment of the PAH dissociation as it follows from laboratory experiments and theory. We estimated the rates of acetylene production in gas phase chemical reactions and compared them with the rates of the acetylene production through the PAH dissociation. It is found that the latter rates can be higher than the former rates in the Orion Bar at $A_{\rm V}<1$ and also at $A_{\rm V}>3.5$. In the Horsehead nebula, the chemical reactions provide more acetylene than the PAH dissociation. The produced acetylene participate in the reactions of the formation of small hydrocarbons (C$_2$H, C$_3$H, C$_3$H$^{+}$, C$_3$H$_2$, C$_4$H). Acetylene production via the PAH destruction may increase the abundances of small hydrocarbons produced in gas phase chemical reactions in the Orion Bar only at $A_{\rm V}>3.5$. In the Horsehead nebula, the contribution of PAHs to the abundances of the small hydrocarbons is negligible. We conclude that the PAHs are not a major source of small hydrocarbons in both PDRs except some locations in the Orion Bar.
We study evolution of hydrogenated amorphous carbon (HAC) grains under harsh UV radiation in photo-dissociation regions (PDRs) near young massive stars. Our aim is to evaluate the impact of the HAC grains on formation of observed small hydrocarbons: C$_2$H, C$_2$H$_2$, C$_3$H$^+$, C$_3$H, C$_3$H$_2$, C$_4$H, in PDRs. We developed a microscopic model of the HAC grains based on available experimental results. The model includes processes of photo- and thermodesorption, accretion of hydrogen and carbon atoms and subsequent formation of carbonaceous mantle on dust surface. H$_2$, CH$_4$, C$_2$H$_2…
The large disparity in physical conditions from the diffuse interstellar medium (ISM) to denser clouds such as photon-dominated regions (PDRs) triggers an evolution of the dust properties (i.e. composition, size, and shape). The gas physics and chemistry are tightly connected to these dust properties and are therefore affected by dust evolution and especially the nano-grain depletion in the outer irradiated part of PDRs. We highlight the influence of nano-grain depletion on the gas physics and chemistry in the Horsehead nebula, a prototypical PDR. We used a model for atomic and molecular gas i…
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…
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 t…
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…
Water photodissociation in the 114 - 144 nm UV range forms excited OH which emits at mid-infrared wavelengths via highly excited rotational lines. These lines have only been detected with Spitzer in several proto-planetary disks and shocks. Previous studies have shown they are a unique diagnostic for water photodissociation. Thanks to its high sensitivity and angular resolution, the James Webb Space Telescope (JWST) could be able to detect them in other environments such as interstellar Photo-Dissociation Regions (PDRs). In order to predict the emerging spectrum of OH, we use the Meudon PDR Co…
H2 emission in the Orion Bar, NGC7023 and the Horsehead Nebula observed with the James Webb Space Telescope
Instituto de Física Fundamental (CSIC)（基础物理研究所（西班牙科学理事会））；Institut d’Astrophysique Spatiale, Université Paris-Saclay, CNRS（空间天体物理学研究所，巴黎萨克雷大学，法国国家科学研究中心）；Department of Physics and Astronomy, University of Western Ontario（韦仕敦大学物理与天文系）；Institute for Earth and Space Exploration, The University of Western Ontario（韦仕敦大学地球与太空探索研究所）；Department of Astronomy, University of Michigan（密歇根大学天文系）；Astronomy Department, University of Maryland（马里兰大学天文系）；Leiden Observatory, Leiden University（莱顿大学莱顿天文台）
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…
JWST observations of photodissociation regions: II. Warm molecular Hydrogen spectroscopy in the Horsehead nebula
Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale（巴黎萨克雷大学）；Sorbonne Université, CNRS, Institut d’Astrophysique de Paris（索邦大学）；Space Telescope Science Institute（太空望远镜科学研究所）；LUX, Observatoire de Paris, Université PSL, Sorbonne Université, CNRS（巴黎天文台）；Université Paris-Cité（巴黎西岱大学）；Steward Observatory, University of Arizona（亚利桑那大学斯图尔德天文台）；Department of Physics & Astronomy, The University of Western Ontario（韦仕敦大学物理与天文学系）；Ritter Astrophysical Research Center, University of Toledo（托莱多大学里特天体物理研究中心）
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…
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…
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…
We present spectroscopic observations obtained with the infrared Spitzer Space Telescope, which provide insight into the H$_2$ physics and gas energetics in photodissociation Regions (PDRs) of low to moderate far-ultraviolet (FUV) fields and densities. We analyze data on six well known Galactic PDRs (L1721, California, N7023E, Horsehead, rho Oph, N2023N), sampling a poorly explored range of excitation conditions ($χ\sim 5-10^3$), relevant to the bulk of molecular clouds in galaxies. Spitzer observations of H$_2$ rotational lines are complemented with H$_2$ data, including ro-vibrational line m…
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$…
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…
Sgr B1 is a luminous H II region in the Galactic Center immediately next to the massive star-forming giant molecular cloud Sgr B2 and apparently connected to it from their similar radial velocities. In 2018 we showed from SOFIA FIFI-LS observations of the [O III] 52 and 88 micron lines that there is no central exciting star cluster and that the ionizing stars must be widely spread throughout the region. Here we present SOFIA FIFI-LS observations of the [O I] 146 and [C II] 158 micron lines formed in the surrounding photodissociation regions (PDRs). We find that these lines correlate neither wi…
Photodissociation regions (PDRs) contain a large fraction of all of the interstellar matter in galaxies. Classical examples include the boundaries between ionized regions and molecular clouds in regions of massive star formation, marking the point where all of the photons energetic enough to ionize hydrogen have been absorbed. In this paper we determine the physical properties of the PDRs associated with the star forming regions IRAS 23133+6050 and S 106 and present them in the context of other Galactic PDRs associated with massive star forming regions. We employ Herschel PACS and SPIRE spectr…
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…
PDRs4All XVII: Formation and excitation of HD in photodissociation regions. Application to the Orion Bar
Institut d’Astrophysique Spatiale, Université Paris-Saclay, CNRS（空间天体物理研究所，巴黎萨克雷大学，法国国家科学研究中心）；LUX, Observatoire de Paris, Université PSL, Sorbonne Université, CNRS（LUX，巴黎天文台，巴黎文理研究大学，索邦大学，法国国家科学研究中心）；Université Paris-Cité（巴黎西岱大学）；Department of Physics, College of Science, United Arab Emirates University (UAEU)（阿拉伯联合酋长国大学理学院物理学系）；Instituto de Física Fundamental (CSIC)（基础物理研究所（西班牙国家研究委员会））；Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory（查尔姆斯理工大学空间、地球与环境系，翁萨拉空间观测站）
We consider the evolution of an isentropic thermal instability in the atomic zone of a photodissociation region (PDR). In this zone, gas heating and cooling are associated mainly with photoelectric emission from dust grains and fine-structure lines ([\ion{C}{ii}] 158, [\ion{O}{i}] 63, and [\ion{O}{i}] 146 {\micron}), respectively. The instability criterion has a multi-parametric dependence on the conditions of the interstellar medium. We found that instability occurs when the intensity of the incident far-ultraviolet field $G_0$ and gas density $n$ are high. For example, we have $3\times10^3<G…
The physical and chemical conditions in photodissociation regions (PDRs) are largely determined by the influence of far ultraviolet radiation. Far-UV photons can efficiently dissociate molecular hydrogen, a process that must be balanced at the HI/H2 interface of the PDR. Given that reactions involving hydrogen atoms in the gas phase are highly inefficient under interstellar conditions, H2 formation models mostly rely on catalytic reactions on the surface of dust grains. Additionally, molecular hydrogen formation in polycyclic aromatic hydrocarbons (PAHs) through the Eley-Rideal mechanism has b…
Various studies have emphasized variations of the charge state and composition of the interstellar polycyclic aromatic hydrocarbon (PAH) population in photodissociation regions (PDRs). We aim to model the spatial evolution of the charge and hydrogenation states of PAHs in PDRs. We focus on the specific case of the north-west (NW) PDR of NGC 7023 and also discuss the case of the diffuse interstellar medium (ISM). The physical conditions in NGC 7023 NW are modelled using a state-of-the-art PDR code. We then use a new PAH chemical evolution model that includes recent experimental data on PAHs and…
In bright photodissociation regions (PDRs) associated to massive star formation, the presence of dense "clumps" that are immersed in a less dense interclump medium is often proposed to explain the difficulty of models to account for the observed gas emission in high-excitation lines. We aim at presenting a comprehensive view of the modeling of the CO rotational ladder in PDRs, including the high-J lines that trace warm molecular gas at PDR interfaces. We observed the 12CO and 13CO ladders in two prototypical PDRs, the Orion Bar and NGC 7023 NW using the instruments onboard Herschel. We also co…
The ionization fraction plays a key role in the chemistry and dynamics of molecular clouds. We study the H13CO+, DCO+ and HOC+ line emission towards the Horsehead, from the shielded core to the UV irradiated cloud edge, i.e., the Photodissociation Region (PDR), as a template to investigate the ionization fraction gradient in molecular clouds. We analyze a PdBI map of the H13CO+ J=1-0 line, complemented with IRAM-30m H13CO+ and DCO+ higher-J line maps and new HOC+ and CO+ observations. We compare self-consistently the observed spatial distribution and line intensities with detailed depth-depend…
The [CII] 158$μ$m fine-structure line is the dominant cooling line of moderate-density photodissociation regions (PDRs) illuminated by moderately bright far-ultraviolet (FUV) radiation fields. We aim to understand the origin of [CII] emission and its relation to other tracers of gas and dust in PDRs. One focus is a study of the heating efficiency of interstellar gas as traced by the [CII] line to test models of the photoelectric heating of neutral gas by polycyclic aromatic hydrocarbon (PAH) molecules and very small grains. We make use of a one-square-degree map of velocity-resolved [CII] line…
We examine new and pre-existing wide-field, continuum-corrected, narrowband images in H$_2$ 1-0 S(1) and Br$γ$ of three regions of massive star formation: IC 1396, Cygnus OB2, and Carina. These regions contain a variety of globules, pillars, and sheets, so we can quantify how the spatial profiles of emission lines behave in photodissociation regions (PDRs) that differ in their radiation fields and geometries. We have measured 450 spatial profiles of H$_2$ and Br$γ$ along interfaces between HII regions and PDRs. Br$γ$ traces photoevaporative flows from the PDRs, and this emission declines more …
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…
We present a charge distribution based model that computes the infrared spectrum of polycyclic aromatic hydrocarbon (PAH) molecules using recent measurements or quantum chemical calculations of specific PAHs. The model is applied to a sample of well-studied photodissociation regions (PDRs) with well-determined physical conditions (the radiation field strength, $G_{0}$, electron density $n_{e}$, and the gas temperature, $T_{\rm gas}$). Specifically, we modelled the emission of five PAHs ranging in size from 18 to 96 carbon atoms, over a range of physical conditions characterized by the ionizati…
The interaction of polycyclic aromatic hydrocarbons (PAHs) with vacuum ultraviolet (VUV) photons triggers the emission of the well-known aromatic infrared bands (AIBs) but other mechanisms such as fragmentation can be involved in this interaction. Fragmentation leads to selection effects that favor specific sizes and structures. We investigate the impact of aliphatic bonds on the VUV photo-stability of PAH cations under conditions applicable for photodissociation regions (PDRs). Cations of pyrene (C16H10) and coronene (C24H12) derivatives containing aliphatic bonds (methyl or ethyl sidegroups,…
We observationally investigate the relation between the photoelectric heating efficiency in PDRs and the charge of PAHs, which are considered to play a key role in photoelectric heating. Using PACS onboard Herschel, we observed six PDRs spanning a wide range of FUV radiation fields (G_0=100-10^5). To measure the photoelectric heating efficiency, we obtained the intensities of the main cooling lines, i.e., the [OI]63um, 145um, and [CII]158um, as well as the FIR continuum intensity. We used Spitzer/IRS spectroscopic mapping observations to investigate the MIR PAH features in the same regions. We…
Hydrocarbons are ubiquitous in the interstellar medium, but their formation is still not well understood, depending on the physical environment they are found in. M8 is host to one of the brightest HII regions and PDRs in our galaxy. Using the APEX, and the IRAM 30 m telescopes, we performed a line survey toward Herschel 36 (Her 36), which is the main ionizing stellar system in M8, and an imaging survey within 1.3 $\times$ 1.3 pc around Her 36 of various transitions of C$_{2}$H and c-C$_{3}$H$_{2}$. We used both LTE and non-LTE methods to determine the physical conditions of the emitting gas a…
The results of a Caltech Submillimeter Observatory (CSO) search for $l$-C$_3$H$^+$, first detected by Pety et al. (2012) in observations toward the Horsehead photodissociation region (PDR), are presented. A total of 39 sources were observed in the 1 mm window. Evidence of emission from $l$-C$_3$H$^+$ is found in only a single source - the Orion Bar PDR region, which shows a rotational temperature of 178(13) K and a column density of 7(2) x $10^{11}$ cm$^{-2}$. In the remaining sources, upper limits of ~10$^{11} - 10^{13}$ cm$^{-2}$ are found. These results are discussed in the context of guidi…
We present 5--14~$μ$m spectra at two different positions across the Orion Bar photodissociation region (PDR) obtained with the Infrared Spectrograph onboard the Spitzer Space Telescope and 3.3~$μ$m PAH observations obtained with the Stratospheric Observatory for Infrared Astronomy (SOFIA). We aim to characterize emission from Polycyclic Aromatic Hydrocarbon (PAH), dust, atomic and molecular hydrogen, argon, sulfur, and neon as a function of distance from the primary illuminating source. We find that all the major PAH bands peak between the ionization front and the PDR front, as traced by H$_{2…
The Orion Bar is one of the nearest and best-studied photodissociation or photon-dominated regions (PDRs). Observations reveal the presence of H2 lines from vibrationally or rotationally excited upper levels that suggest warm gas temperatures (400 to 700 K). However, standard models of PDRs are unable to reproduce such warm rotational temperatures. In this paper we attempt to explain these observations with new comprehensive models which extend from the H+ region through the Bar and include the magnetic field in the equation of state. We adopt the model parameters from our previous paper which…
We investigate the emission properties of polycyclic aromatic hydrocarbons (PAHs) in various metallicity environments with the Infrared Spectrograph on board Spitzer. Local giant HII regions are used as references as they enable access to the distinct interstellar medium components that contribute to the mid-infrared spectrum of star-forming galaxies: photodissociation regions (PDRs), photoionized gas, stellar clusters, and embedded regions. Three objects are considered, NGC3603 in the Milky Way, 30Doradus in the Large Magellanic Cloud, and N66 in the Small Magellanic Cloud. From the variation…
Context. The combined use of carbon radio recombination lines (CRRLs) and the 158 $μ$m-[CII] line is a powerful tool for the study of the energetics and physical conditions (e.g., temperature and density) of photodissociation regions (PDRs). However, there are few observational studies that exploit this synergy. Aims. Here we explore the relation between CRRLs and the 158 $μ$m-[CII] line in light of new observations and models. Methods. We present new and existing observations of CRRLs in the frequency range 0.15--230 GHz with ALMA, VLA, the GBT, Effelsberg 100m, and LOFAR towards Orion~A (M42…
PDRs4All XXII. Near-Infrared continuum in the Orion Bar
The University of Tokyo（东京大学）；Université Paris-Saclay（巴黎萨克雷大学）；CNRS（法国国家科学研究中心）；The University of Western Ontario（韦仕敦大学）；Institute for Earth and Space Exploration（地球与太空探索研究所）；SETI Institute（搜寻地外文明研究所）；Institut de Recherche en Astrophysique et Planétologie（天体物理学与行星学研究所在）；Université Toulouse III - Paul Sabatier（图卢兹第三大学-保罗·萨巴蒂耶大学）；CNES（法国国家空间研究中心）；Institut d’Astrophysique Spatiale（空间天体物理研究所）；NASA Ames Research Center（美国宇航局艾姆斯研究中心）；Centro de Astrobiología (CSIC-INTA)（天体生物学中心（西班牙高等科研委员会-国家航空航天技术研究所））；CSIC（西班牙高等科研理事会）；INTA（国家航空航天技术研究所）；Instituto de Física Fundamental（基础物理研究所）
本研究以猎户座 Bar 为对象，利用 JWST 的 NIRSpec 数据研究近红外连续谱，发现其可近似为两个黑体叠加，碳簇 recurrent fluorescence 或为低温成分的起源，还探测到水冰与CO₂冰的吸收特征。
Polarized emission from polycyclic aromatic hydrocarbons (PAHs) potentially provides a new way to test basic physics of the alignment of ultrasmall grains. In this paper, we present a new model of polarized PAH emission that takes into account the effect of PAH alignment with the magnetic field. We first generate a large sample of the grain angular momentum $\bf J$ by simulating the alignment of PAHs due to resonance paramagnetic relaxation that accounts for various interaction processes. We then calculate the polarization level of PAH emission features, for the different phases of the ISM, in…
The formation of hydrides by gas-phase reactions between H2 and a heavy element atom is a very selective process. Reactions with ground-state neutral carbon, oxygen, nitrogen, and sulfur atoms are very endoergic and have high energy barriers because the H2 molecule has to be fragmented before a hydride bond is formed. In cold interstellar clouds, these barriers exclude the formation of CH, OH, NH, and SH radicals through hydrogen abstraction reactions. Here we study a very energetically unfavorable process, the reaction of N(4S) atoms with H2 molecules. We calculated the reaction rate coeffici…
PAHs constitute a major component of the interstellar medium carbon budget, locking up to 10--20% of the elemental carbon. Sequential fragmentation induced by energetic photons leads to the formation of new species, including fullerenes. However, the exact chemical routes involved in this process remain largely unexplored. In this work, we focus on the first photofragmentation steps, which involve the dehydrogenation of these molecules. For this, we consider a multidisciplinary approach, taking into account the results from experiments, DFT calculations, and modeling using dedicated Monte-Carl…
NGC 1097 is a nearby Seyfert 1 galaxy with a bright circumnuclear starburst ring, a strong large-scale bar and an active nucleus. We present a detailed study of the spatial variation of the far infrared (FIR) [CII]158um and [OI]63um lines and mid-infrared H2 emission lines as tracers of gas cooling, and of the polycyclic aromatic hydrocarbon (PAH) bands as tracers of the photoelectric heating, using Herschel-PACS, and Spitzer-IRS infrared spectral maps. We focus on the nucleus and the ring, and two star forming regions (Enuc N and Enuc S). We estimated a photoelectric gas heating efficiency ([…
