Dense Photon-dominated Regions Research Articles

Large-scale [C II] 158 μm emission from the Orion-Eridanus superbubble

In this study, we analyzed the [C II] 158 μm emission from the Orion-Eridanus region measured by the Cosmic Background Explorer. Morphologically, the [C II] emission traces prominent star-forming regions this area. The analysis takes into account five different components of the interstellar medium (ISM) that can contribute to the [C II] emission: compact H II regions, dense Photon-Dominated Region, surfaces of molecular clouds, the Warm Ionized Medium, and the Cold Neutral Medium. We estimate the contribution from each object of interest to the observed [C II] emission based upon the physical properties of the object and validate our results by making a comparison with existing “small” scale maps. Inside the ~400 parsec aperture radius that we investigate, surfaces of molecular clouds exposed to radiation from nearby stellar clusters are the dominant contributor to the observed global [C II] flux. These molecular cloud surfaces are exposed to moderate radiation fields (G0 ~ 100 times the average interstellar radiation field) and are moderately dense (nH ~ 103 cm−3). In addition, extended low-density ionized gas, along with large-scale ionized gas structures (Barnard’s Loop; λ Ori) also make a substantial contribution. The implications of this study for the analysis of extragalactic [C II] observations are assessed.

The Star-forming Interstellar Medium of Lyman Break Galaxy Analogs

We present VLT/SINFONI near-infrared (NIR) integral field spectroscopy of six $z \sim 0.2$ Lyman break galaxy "analogs" (LBAs), from which we detect HI, HeI, and [FeII] recombination lines, and multiple H$_2$ ro-vibrational lines in emission. Pa$\alpha$ kinematics reveal high velocity dispersions and low rotational velocities relative to random motions ($\langle v/\sigma \rangle = 1.2 \pm 0.8$). Matched-aperture comparisons of H$\beta$, H$\alpha$, and Pa$\alpha$ reveal that the nebular color excesses are lower relative to the continuum color excesses than is the case for typical local star-forming systems. We compare observed HeI/HI recombination line ratios to photoionization models to gauge the effective temperatures (T$_{\rm eff}$) of massive ionizing stars, finding the properties of at least one LBA are consistent with extra heating from an active galactic nucleus (AGN) and/or an overabundance of massive stars. We use H$_2$ 1-0 S($\cdot$) ro-vibrational spectra to determine rotational excitation temperature $T_{\rm ex} \sim 2000$ K for warm molecular gas, which we attribute to UV heating in dense photon-dominated regions. Spatially resolved NIR line ratios favor excitation by massive, young stars, rather than supernovae or AGN feedback. Our results suggest that the local analogs of Lyman break galaxies are primarily subject to strong feedback from recent star formation, with evidence for AGN and outflows in some cases.

Opening the Treasure Chest in Carina

Pillars and globules are the best examples of the impact of the radiation and wind from massive stars on the surrounding interstellar medium. We mapped the G287.84-0.82 cometary globule (with the Treasure Chest cluster embedded in it) in the South Pillars region of Carina (i) in [C II], 63 μm [O I], and CO(11–10) using the heterodyne receiver array upGREAT on SOFIA and (ii) in J = 2–1 transitions of CO, 13CO, C18O, and J = 3–2 transitions of H2CO using the APEX telescope in Chile. We used these data to probe the morphology, kinematics, and physical conditions of the molecular gas and the photon-dominated regions (PDRs) in G287.84-0.82. The velocity-resolved observations of [C II] and [O I] suggest that the overall structure of the pillar (with red-shifted photoevaporating tails) is consistent with the effect of FUV radiation and winds from η Car and O stars in Trumpler 16. The gas in the head of the pillar is strongly influenced by the embedded cluster, whose brightest member is an O9.5 V star, CPD −59°2661. The emission of the [C II] and [O I] lines peak at a position close to the embedded star, while all the other tracers peak at another position lying to the northeast consistent with gas being compressed by the expanding PDR created by the embedded cluster. The molecular gas inside the globule was probed with the J = 2–1 transitions of CO and isotopologs as well as H2CO, and analyzed using a non-local thermodynamic equilibrium model (escape-probability approach), while we used PDR models to derive the physical conditions of the PDR. We identify at least two PDR gas components; the diffuse part (~ 104 cm−3) is traced by [C II], while the dense (n ~ 2–8 × 105 cm−3) part is traced by [C II], [O I], and CO(11–10). Using the F = 2–1 transition of [13C II] detected at 50 positions in the region, we derived optical depths (0.9–5), excitation temperatures (80–255 K) of [C II], and N(C+) of 0.3–1 × 1019 cm−2. The total mass of the globule is ~1000 M⊙, about half of which is traced by [C II]. The dense PDR gas has a thermal pressure of 107–108 K cm−3, which is similar to the values observed in other regions.

Models of irradiated molecular shocks

Context. The recent discovery of excited molecules in starburst galaxies observed with ALMA and the Herschel space telescope has highlighted the necessity to understand the relative contributions of radiative and mechanical energies in the formation of molecular lines and explore the conundrum of turbulent gas bred in the wake of galactic outflows. Aims. The goal of the paper is to present a detailed study of the propagation of low velocity (5–25 km s−1) stationary molecular shocks in environments illuminated by an external ultraviolet (UV) radiation field. In particular, we intend to show how the structure, dynamics, energetics, and chemical properties of shocks are modified by UV photons and to estimate how efficiently shocks can produce line emission. Methods. We implemented several key physico-chemical processes in the Paris-Durham shock code to improve the treatment of the radiative transfer and its impact on dust and gas particles. We propose a new integration algorithm to find the steady-state solutions of magnetohydrodynamics equations in a range of parameters in which the fluid evolves from a supersonic to a subsonic regime. We explored the resulting code over a wide range of physical conditions, which encompass diffuse interstellar clouds and hot and dense photon-dominated regions. Results. We find that C-type shock conditions cease to exist as soon as G0 > 0.2 (nH/cm−3)1/2. Such conditions trigger the emergence of another category of stationary solutions, called C*-type and CJ-type shocks, in which the shocked gas is momentarily subsonic along its trajectory. These solutions are shown to be unique for a given set of physical conditions and correspond to dissipative structures in which the gas is heated up to temperatures comprised between those found in C-type and adiabatic J-type shocks. High temperatures combined with the ambient UV field favour the production or excitation of a few molecular species to the detriment of others, hence leading to specific spectroscopic tracers such as rovibrational lines of H2 and rotational lines of CH+. Unexpectedly, the rotational lines of CH+ may carry as much as several percent of the shock kinetic energy. Conclusions. Ultraviolet photons are found to strongly modify the way the mechanical energy of interstellar shocks is processed and radiated away. In spite of what intuition dictates, a strong external UV radiation field boosts the efficiency of low velocity interstellar shocks in the production of several molecular lines which become evident tracers of turbulent dissipation.

Kinematics and properties of the central molecular zone as probed with [C ii

The Galactic Central Molecular Zone (CMZ) is a region containing massive and dense molecular clouds, with dynamics driven by a variety of energy sources including a massive black hole. It is thus the nearest template for understanding physical processes in extragalactic nuclei. The CMZ's neutral interstellar gas has been mapped spectrally in many neutral atomic and molecular gas tracers, but the ionized and CO-dark H2 regions are less well traced spectroscopically. We mapped the fine structure line of C+ at 158 microns, [C II], to identify and characterize features of the ionized gas in the CMZ, including UV irradiated neutral gas, photon dominated regions (PDRs), CO-dark H2 gas, and highly ionized gas. We observed the [C II] 158-micron fine structure line with high spectral resolution using Herschel HIFI with two perpendicular On-the-Fly strip scans, along l = -0.8 to +0.8 and b = -0.8 to +0.8, both centered on (l,b) = (0,0). We analyzed the spatial-velocity distribution of the [C II] data and compared them to those of [C I] and CO, and to dust continuum maps, in order to determine the properties and distribution of the UV irradiated gas and its dynamics within the CMZ. The longitude- and latitude-velocity maps of [C II] trace portions of the orbiting open gas streams of dense molecular clouds, the cloud G0.253+0.016, also known as the Brick, the Arched Filaments, and the ionized gas near Sgr A and Sgr B2. We use the [C II] and auxiliary data to determine the physical and dynamical properties of these CMZ features. The [C II] emission arises primarily from dense PDRs and highly ionized gas, and is an important tracer of the kinematics and physical conditions of this gas.

AHerschel[C ii] Galactic plane survey

[Abridged] The [CII] 158um line is an important tool for understanding the life cycle of interstellar matter. Ionized carbon is present in a variety of phases of the interstellar medium, including the diffuse ionized medium, warm and cold atomic clouds, clouds in transition from atomic to molecular, and dense and warm photon dominated regions (PDRs). The Galactic Observations of Terahertz C+ (GOTC+) project surveys the [CII] line over the entire Galactic disk with velocity-resolved observations using the Herschel/HIFI instrument. We present the first longitude-velocity maps of the [CII] emission for Galactic latitudes b=0deg, +-0.5deg, and +-1.0deg. [CII] emission is mostly associated with spiral arms, mainly emerging from Galactocentric distances between 4 and 10 kpc. We estimate that most of the observed [CII] emission is produced by dense PDRs (47%), with smaller contributions from CO-dark H2 gas (28%), cold atomic gas (21%), and ionized gas (4%). Atomic gas inside the Solar radius is mostly in the form of cold neutral medium (CNM), while the warm neutral medium (WNM) gas dominates the outer galaxy. The average fraction of CNM relative to total atomic gas is 43%. We find that the warm and diffuse CO-dark H2 is distributed over a larger range of Galactocentric distances (4-11 kpc) than the cold and dense H2 gas traced by 12CO and 13CO (4-8kpc). The fraction of CO-dark H2 to total H2 increases with Galactocentric distance, ranging from 20% at 4 kpc to 80% at 10 kpc. On average, CO-dark H2 accounts for 30% of the molecular mass of the Milky Way. When the CO-dark H2 component is included, the radial distribution of the CO-to-H2 conversion factor is steeper than that when only molecular gas traced by CO is considered. Most of the observed [CII] emission emerging from dense PDRs is associated with modest far-ultraviolet fields in the range chi0~1-30.

SPATIALLY RESOLVED CHEMISTRY IN NEARBY GALAXIES. II. THE NUCLEAR BAR IN MAFFEI 2

We present 2" - 10" imaging of eleven transitions from nine molecular species across the nuclear bar in Maffei 2. The data were obtained with the BIMA and OVRO interferometers. The ten detected transitions are compared with existing CO isotopologues, HCN, CS and millimeter continuum data. Dramatic spatial variations among the mapped species are observed across the nuclear bar. A principle component analysis is performed to characterize correlations between the transitions, star formation and molecular column density. The analysis reveals that HCN, HNC, HCO+ and 3 mm continuum are tightly correlated, indicating a direct connection to massive star formation. We find two main morphologically distinct chemical groups, CH3OH, SiO and HNCO comprising the grain chemistry molecules, versus HCN, HNC, HCO+ and C2H, molecules strong in the presence of star formation. The grain chemistry molecules, HNCO, CH3OH and SiO, trace hydrodynamical bar shocks. The near constancy of the HNCO/CH3OH, SiO/CH3OH and SiO/HNCO ratios argue that shock properties are uniform across the nucleus. HCN/HCO+, HCN/HNC, HCN/CS and HCN/CO ratios are explained primarily by variations in density. High HCO+/N2H+ ratios are correlated with the C2H line, suggesting that this ratio may be a powerful new dense photon-dominated region (PDR) probe in external galaxies. C2H reveals a molecular outflow along the minor axis. The morphology and kinematics of the outflow are consistent with an outflow age of 6-7 Myrs.

The APEX-CHAMP+view of the Orion Molecular Cloud 1 core

A high density portion of the Orion Molecular Cloud 1 (OMC-1) contains the prominent, warm Kleinmann-Low (KL) nebula that is internally powered by an energetic event plus a farther region in which intermediate to high mass stars are forming. Its outside is affected by ultraviolet radiation from the neighboring Orion Nebula Cluster and forms the archetypical photon-dominated region (PDR) with the prominent bar feature. Its nearness makes the OMC-1 core region a touchstone for research on the dense molecular interstellar medium and PDRs. Using the Atacama Pathfinder Experiment telescope (APEX), we have imaged the line emission from the multiple transitions of several carbon monoxide (CO) isotopologues over the OMC-1 core region. Our observations employed the 2x7 pixel submillimeter CHAMP+ array to produce maps (~ 300 arcsec x 350 arcsec) of 12CO, 13CO, and C18O from mid-J transitions (J=6-5 to 8-7). We also obtained the 13CO and C18O J=3-2 images toward this region. The 12CO line emission shows a well-defined structure which is shaped and excited by a variety of phenomena, including the energetic photons from hot, massive stars in the nearby Orion Nebula's central Trapezium cluster, active high- and intermediate-mass star formation, and a past energetic event that excites the KL nebula. Radiative transfer modeling of the various isotopologic CO lines implies typical H2 densities in the OMC-1 core region of ~10^4-10^6 cm^-3 and generally elevated temperatures (~ 50-250 K). We estimate a warm gas mass in the OMC-1 core region of 86-285 solar masses.

Gas morphology and energetics at the surface of PDRs: New insights withHerschelobservations of NGC 7023

We investigate the physics and chemistry of the gas and dust in dense photon-dominated regions (PDRs), along with their dependence on the illuminating UV field. Using Herschel-HIFI observations, we study the gas energetics in NGC 7023 in relation to the morphology of this nebula. NGC 7023 is the prototype of a PDR illuminated by a B2V star and is one of the key targets of Herschel. Our approach consists in determining the energetics of the region by combining the information carried by the mid-IR spectrum (extinction by classical grains, emission from very small dust particles) with that of the main gas coolant lines. In this letter, we discuss more specifically the intensity and line profile of the 158 micron (1901 GHz) [CII] line measured by HIFI and provide information on the emitting gas. We show that both the [CII] emission and the mid-IR emission from polycyclic aromatic hydrocarbons (PAHs) arise from the regions located in the transition zone between atomic and molecular gas. Using the Meudon PDR code and a simple transfer model, we find good agreement between the calculated and observed [CII] intensities. HIFI observations of NGC 7023 provide the opportunity to constrain the energetics at the surface of PDRs. Future work will include analysis of the main coolant line [OI] and use of a new PDR model that includes PAH-related species.

Time-dependent models of dense PDRs with complex molecules

We present a study of the chemistry of a dense photon-dominated region (PDR) using a time-dependent chemical model. Our major interest is to study the spatial distribution of complex molecules such as hydrocarbons and cyanopolyynes in the cool dense material bordering regions where star formation has taken place. Our standard model uses a homogeneous cloud of density 2x10e4 cm-3 and temperature T=40 K, which is irradiated by a far-ultraviolet radiation field of intermediate intensity, given by X=100. We find that over a range of times unsaturated hydrocarbons (e.g., C2H, C4H, C3H2) have relatively high fractional abundances in the more external layers of the PDR, whereas their abundances in the innermost layers are several orders of magnitudes lower. On the other hand, molecules that are typical of late-time chemistry are usually more abundant in the inner parts of the PDR. We also present results for models with different density, temperature, intensity of the radiation field and initial fractional abundances. Our results are compared with both high- and moderate-angular resolution observations of the Horsehead nebula. Our standard model is partially successful in reproducing the observations. Additional models run with different physical parameters are able to reproduce the abundance of many of the observed molecules, but we do not find a single model that fits all the observations at the same time. We discuss the suitability of a time-dependent model of a dense PDR such as ours as an estimator of the age of a PDR, provided that enough observational data exist.

First COJ= 6–5 andJ= 4–3 Detections in Local ULIRGs: The Dense Gas in Markarian 231 and Its Cooling Budget

We report on detections of the high-excitation CO J=6-5, J=4-3 lines in Mrk231, a prototypical Ultra Luminous Infrared Galaxy (ULIRG) and Seyfert 1 QSO. These observations are combined with CO J=3-2, HCN J=4-3 (this work), and CO J=2-1, J=1-0, 13CO J=2-1, HCN J=1-0 measurements taken from the literature to provide better constraints on the properties of the molecular gas in an extreme starburst/QSO in the local Universe. We find that the CO J=4-3 and J=6-5 transitions trace a different gas phase from that dominating the lower three CO transitions, with n(H_2) ~ (1-3)x10^4 cm-3 and Tk ~ (40-70) K. This phase is responsible for the luminous HCN emission, and contains most of the H2 gas mass of this galaxy. The total CO line cooling emanating from this dense phase is found similar to that of the [CII] line at 158 micron, suggesting a very different thermal balance to that seen in lower IR-luminosity galaxies, and one likely dominated by dense photon-dominated regions. Our dense "sampling" of the CO rotational ladder and the HCN lines enables us to produce well-constrained Spectral Line Energy Distributions (SLEDs) for the dense molecular gas in Mrk231 and compare them to those of high redshift starbursts, many of which have SLEDs that may be affected by strong lensing. Finally, we use our local molecular line excitation template to assess the capabilities of future cm and mm/sub-mm arrays in detecting CO and HCN transitions in similar systems throughout the local and distant universe.

Photoprocesses in protoplanetary disks

Circumstellar disks are exposed to intense ultraviolet (UV) radiation from the young star. In the inner disks, the UV radiation can be enhanced by more than seven orders of magnitude compared with the average interstellar radiation field, resulting in a physical and chemical structure that resembles that of a dense photon-dominated region (PDR). This intense UV field affects the chemistry, the vertical structure of the disk, and the gas temperature, especially in the surface layers. The parameters which make disks different from more traditional PDRs are discussed, including the shape of the UV radiation field, grain growth, the absence of PAHs, the gas/dust ratio and the presence of inner holes. Illustrative infrared spectra from the Spitzer Space Telescope are shown. New photodissociation cross sections for selected species, including simple ions, are presented. Also, a summary of cross sections at the Lyman alpha 1216 A line, known to be strong for some T Tauri stars, is made. Photodissociation and ionization rates are computed for different radiation fields with color temperatures ranging from 30000 to 4000 K and grain sizes up to a few microm. The importance of a proper treatment of the photoprocesses is illustrated for the transitional disk toward HD 141569A which includes grain growth.

The abundance of + in diffuse clouds

We used the Plateau de Bure Interferometer to search for λ3mm absorption lines of HOC+ from local diffuse and translucent clouds occulting compact extragalactic mm-wave continuum sources. We detected HOC+ in three directions with column densities only 70-120 times below that of the HCO+ isomer, a factor 5-50 higher than typically found in dense dark gas but comparable to recent observations of dense photon-dominated regions. The observed amounts of HOC+ N(HOC+)/N(H2) can be made in quiescent diffuse gas at thermal gas-kinetic rates if the H2O/OH ratio is of order unity, in mild violation of extant observational limits.

Molecular Hydrogen Excitation in Ultraluminous Infrared Galaxies

We report medium-resolution Very Large Telescope ISAAC K-band spectroscopy of the nuclei of seven ultraluminous infrared galaxies. After accounting for stellar absorption features, we have detected several molecular hydrogen (H2) v = 1-0, 2-1, and 3-2 vibrational emission lines, as well as the H I, Brγ, and He I 21P-21S recombination lines. The relative H2 line intensities show little variation between the objects, suggesting that the H2 excitation mechanisms in the nuclei are similar in all the objects. The 1-0 emissions appear thermalized at temperatures T ~ 1000 K. However, the 2-1 and 3-2 emissions show evidence of being radiatively excited by far-ultraviolet (FUV) photons, suggesting that the H2 excitation in the ultraluminous infrared galaxies (ULIRGs) might arise in dense photon-dominated regions (PDRs). We show that the line ratios in the nuclei are consistent with PDRs with cloud densities between 104 and 105 cm-3, exposed to FUV radiation fields at least 103 times more intense than the ambient FUV intensity in the local interstellar medium. We have constructed starburst models for the ULIRGs based on their H2 properties as well as on the intensities of the recombination lines. Our models provide a consistent picture of young 1-5 Myr star clusters surrounded by relatively dense PDRs that are irradiated by intense FUV fluxes. Comparison to the inner few hundred parsecs of the Milky Way indicates that the star formation efficiency in ULIRGs is 10-100 times higher than in the Galactic center.

Near‐Infrared H2and Associated O0and C+Emission from Dense Photon‐dominated Regions

We have observed near-infrared H2 vibrational line emission from dense (n ≳ 104 cm-3) photon-dominated regions (PDRs) along the Orion A molecular cloud "ridge" and toward the Orion B molecular cloud near NGC 2024. Specifically, we have observed the 1.601 μm v = 6-4 Q(1), 2.121 μm v = 1-0 S(1), and 2.247 μm v = 2-1 S(1) lines of H2. To assess the physical state of the PDRs, we combine our H2 data with observations of the [O I] 63 μm, [O I] 146 μm, and [C II] 158 μm atomic fine-structure lines along the same lines of sight. We use our data to study how the character of the H2 emission depends on the PDR conditions. For the range of densities (104-105 cm-3) and incident far-ultraviolet fluxes (103-105 times the mean interstellar field) in the PDRs, we find that the relative H2 line strengths are characteristic of both low gas density and high gas density UV-excited emission. In Orion, we observe direct evidence of the transition from the (low-density) regime in which the H2 vibrational cascade is dominated by radiative transitions to the (high-density) regime in which collisions influence the vibrational cascade. Our observations show that, for Orion-type abundances and physical conditions, collisions begin to influence the H2 ro-vibrational line ratios near a gas density of 5 × 104 cm-3.

Chemistry in Dense Photon-dominated Regions

view Abstract Citations (433) References (102) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Chemistry in Dense Photon-dominated Regions Sternberg, A. ; Dalgarno, A. Abstract We present a detailed chemical model of a photon-dominated region produced in a dense molecular cloud exposed to an intense far-ultraviolet radiation field. We use it to illustrate the gas-phase processes that lead to the production of atomic and molecular species in the several distinct zones that occur in the cloud. We identify molecular diagnostics of photon-dominated chemistry in dense molecular clouds. Publication: The Astrophysical Journal Supplement Series Pub Date: August 1995 DOI: 10.1086/192198 Bibcode: 1995ApJS...99..565S Keywords: ISM: MOLECULES; MOLECULAR PROCESSES; GALAXIES: ISM full text sources ADS |

CH+ in shocks, cloud-intercloud interfaces, and dense photon-dominated regions

A resolution of the long-standing CH+ problem almost certainly will yield considerable insight into processes (e.g. mass and energy transfer at interfaces between plasma in different phases) affecting a wide range of astrophysical environments. CH+ column densities are large |$(\geqslant\,{10}^{12}\,\text{cm}^{-2})$| along at least three types of lines of sight: (i) those with little molecular hydrogen; (ii) those passing through diffuse molecular clouds such as ζ Oph, and (iii) those that provide high reddening of the background stars (visual extinctions of about 2–4 mag). This indicates either that one process of ubiquitous importance is responsible for the CH+ production or that a number of different scenarios must be considered. In this paper we explore the latter view. For type (i) we have investigated the production of interstellar CH+ by the destruction, in shocks, of hydrogenated amorphous carbon in grains and the subsequent photoabsorption sequence. We argue that much of the eroded carbon passes through CH+. Maximum column densities |$(\simeq {10}^{12}\,\text{cm}^{-2})\,\text{of CH}^{+}$| are obtained in the absence of H2 For type (ii), we have calculated the relative abundances of CH+, CH, OH, and various v = 0 rotationally excited levels of H2 in hot diffuse molecular gas in which the velocity distributions are all Maxwellian. We argue that such gas exists in turbulent boundary layers at molecular-cloud–intercloud interfaces and has speeds lower than those associated with shocks, which have been advocated as the sources of hot gas containing CH+. We demonstrate that plausible conditions can be adopted which lead to relative abundances of several important species close to those observed. The presence of components of CH associated with CH+ in warm interfaces places fairly stringent constraints on the pressures in the interfaces. For type (iii) we show that CH+ column densities of |${10}^{14}\,\text{cm}^{-2}$| can be produced in photon-dominated regions only if the conditions in them are similar to those under which hydroxyl maser regions, surrounding young stars, form. A more likely alternative is that a CH+ column density of |${10}^{14}\,\text{cm}^{-2}$| towards a highly reddened star arises in one thick boundary layer or several boundary layers like those considered for type (ii).

A 158μm [CII] Map of NGC 6946: Detection in Extragalactic Atomic and Ionized Gas

The first observations of the [CII] line toward the nuclei of gas-rich external galaxies, showed that the far-infrared line emission contributes up to 1% of the total luminosity and most likely originates from dense photon-dominated regions (PDRs) associated with the surfaces of molecular clouds exposed to FUV from external or embedded OB stars (Crawford et al. 1985, Lugten et al. 1986, Stacey et al. 1991). We have mapped the [CII] emission toward NGC 6946 over an 8' × 6' (23 × 17 kpc) (Madden et al. 1991) using the Max-Planck Instutute/U.C.Berkeley Far-Infrared Imaging Fabry-Perot Interferometer (FIFI) on the Kuiper Airborne Observatory (KAO).

The Chemistry of Photon-Dominated Regions

Theoretical models of photon-dominated regions (PDRs) of molecular cloud surfaces are described. Key aspects of the chemistry of dense PDRs are presented.