Photon-dominated Region Models Research Articles

THE DENSE MOLECULAR GAS IN THE CIRCUMNUCLEAR DISK OF NGC 1068

We present a 190-307 GHz broadband spectrum obtained with Z-Spec of NGC 1068 with new measurements of molecular rotational transitions. After combining our measurements with those previously published and considering the specific geometry of this Seyfert 2 galaxy, we conduct a multi-species Bayesian likelihood analysis of the density, temperature, and relative molecular abundances of HCN, HNC, CS, and HCO+. We find that these molecules trace warm (T > 100 K) gas of H2 number densities 10^4.2 - 10^4.9 cm^-3. Our models also place strong constraints on the column densities and relative abundances of these molecules, as well as on the total mass in the circumnuclear disk. Using the uniform calibration afforded by the broad Z-Spec bandpass, we compare our line ratios to X-ray dominated region (XDR) and photon-dominated region models. The majority of our line ratios are consistent with the XDR models at the densities indicated by the likelihood analysis, lending substantial support to the emerging interpretation that the energetics in the circumnuclear disk of NGC 1068 are dominated by accretion onto an active galactic nucleus.

Statistical universal branching ratios for cosmic ray dissociation, photodissociation, and dissociative recombination of the C, CH and C3H2neutral and cationic species

Fragmentation branching ratios of electronically excited molecular species are of first importance for the modeling of gas phase interstellar chemistry. Despite experimental and theoretical efforts that have been done during the last two decades there is still a strong lack of detailed information on those quantities for many molecules such as Cn, CnH or C3H2. Our aim is to provide astrochemical databases with more realistic branching ratios for Cn (n=2 to 10), CnH (n=2 to 4), and C3H2 molecules that are electronically excited either by dissociative recombination, photodissociation, or cosmic ray processes, when no detailed calculations or measurements exist in literature. High velocity collision in an inverse kinematics scheme was used to measure the complete fragmentation pattern of electronically excited Cn (n=2 to 10), CnH (n=2 to 4), and C3H2 molecules. Branching ratios of dissociation where deduced from those experiments. The full set of branching ratios was used as a new input in chemical models and branching ratio modification effects observed in astrochemical networks that describe the dense cold Taurus Molecular Cloud-1 and the photon dominated Horse Head region. The comparison between the branching ratios obtained in this work and other types of experiments showed a good agreement. It was interpreted as the signature of a statistical behavior of the fragmentation. The branching ratios we obtained lead to an increase of the C3 production together with a larger dispersion of the daughter fragments. The introduction of these new values in the photon dominated region model of the Horse Head nebula increases the abundance of C3 and C3H, but reduces the abundances of the larger Cn and hydrocarbons at a visual extinction Av smaller than 4.

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.

AN EXPERIMENTAL AND THEORETICAL STUDY ON THE IONIZATION ENERGIES OF POLYYNES (H-(C≡C)n-H;n= 1-9)

We present a combined experimental and theoretical work on the ionization energies of polyacetylene -- organic molecules considered as important building blocks to form polycyclic aromatic hydrocarbons (PAHs) in the proto planetary nebulae such as of CRL 618. This set of astrophysical data can be utilized with significant confidence in future astrochemical models of photon-dominated regions and also of the proto planetary nebulae CRL 618. We recommend ionization energies of polyacetylenes from diacetylene up to heptaacetylene with an experimental accuracy of +- 0.05 eV: 10.03 eV (diacetylene), 9.45 eV (triacetylene), 9.08 eV (tetraacetylene), 8.75 eV (pentaacetylene), 8.65 eV (hexaacetylene), and 8.50 eV (heptaacetylene); further, ionization energies and with an accuracy of +- 0.1 eV: 8.32 eV (octaacetylene) and 8.24 eV (nonaacetylene) were computed. Implications of these energies to the redox chemistry involved in the multiply charged metal-ion mediated chemistry of hydrocarbon-rich atmospheres of planets and their moons such as Titan are also discussed.

ROTATIONALLY WARM MOLECULAR HYDROGEN IN THE ORION BAR

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 successfully reproduced a wide variety of spectral observations across the Bar. In this model the local cosmic-ray density is enhanced above the galactic background, as is the magnetic field, and which increases the cosmic-ray heating elevating the temperature in the molecular region. The pressure is further enhanced above the gas pressure in the H+ region by the momentum transferred from the absorbed starlight. Here we investigate whether the observed H2 lines can be reproduced with standard assumptions concerning the grain photoelectric emission. We also explore the effects due to the inclusion of recently computed H2 + H2, H2 + H and H2 + He collisional rate coefficients.

Chemical stratification in the Orion Bar: JCMT Spectral Legacy Survey observations

Photon-dominated regions (PDRs) are expected to show a layered structure in molecular abundances and emerging line emission, which is sensitive to the physical structure of the region as well as the UV radiation illuminating it. We aim to study this layering in the Orion Bar, a prototypical nearby PDR with a favorable edge-on geometry. We present new maps of 2 by 2 arcminute fields at 14-23 arcsecond resolution toward the Orion Bar in the SO 8_8-9_9, H2CO 5_(1,5)-4_(1,4), 13CO 3-2, C2H 4_(9/2)-3_(7/2) and 4_(7/2)-3_(5/2), C18O 2-1 and HCN 3-2 transitions. The data reveal a clear chemical stratification pattern. The C2H emission peaks close to the ionization front, followed by H2CO and SO, while C18O, HCN and 13CO peak deeper into the cloud. A simple PDR model reproduces the observed stratification, although the SO emission is predicted to peak much deeper into the cloud than observed while H2CO is predicted to peak closer to the ionization front than observed. In addition, the predicted SO abundance is higher than observed while the H2CO abundance is lower than observed. The discrepancies between the models and observations indicate that more sophisticated models, including production of H2CO through grain surface chemistry, are needed to quantitatively match the observations of this region.

Sensitivity of PDR Calculations to Microphysical Details

Our understanding of physical processes in photodissociation regions or photon-dominated regions (PDRs) largely depends on the ability of spectral synthesis codes to reproduce the observed infrared emission-line spectrum. In this paper, we explore the sensitivity of a single PDR model to microphysical details. Our calculations use the Cloudy spectral synthesis code, recently modified to include a wealth of PDR physical processes. We show how the chemical/thermal structure of a PDR, along with the calculated spectrum, changes when the treatment of physical processes such as grain physics and atomic/molecular rates are varied. We find a significant variation in the intensities of PDR emission lines, depending on different treatments of the grain physics. We also show how different combinations of the cosmic-ray ionization rate, inclusion of grain-atom/ion charge transfer, and the grain size distribution can lead to very similar results for the chemical structure. In addition, our results show the utility of Cloudy for the spectral modeling of molecular environments.

A clumpy-cloud photon-dominated regions model of the global far-infrared line emission of the Milky Way

Context. The fractal structure of the interstellar medium suggests that the interaction of UV radiation with the ISM as described in the context of photon-dominated regions (PDR) dominates most of the physical and chemical conditions, and hence the far-infrared and submm emission from the ISM in the Milky Way. Aims. We investigate to what extent the Galactic FIR line emission of the important species CO, C, C + , and O, as observed by the Cosmic Background Explorer (COBE) satellite can be modeled in the framework of a clumpy, UV-penetrated cloud scenario. Methods. The far-infrared line emission of the Milky Way is modeled as the emission from an ensemble of clumps with a power law clump mass spectrum and mass-size relation with power-law indices consistent with the observed ISM structure. The individual clump line intensities are calculated using the KOSMA-τ PDR-model for spherical clumps. The model parameters for the cylindrically symmetric Galactic distribution of the mass density and volume filling factor are determined by the observed radial distributions. A constant FUV intensity, in which the clumps are embedded, is assumed. Results. We show that this scenario can explain, without any further assumptions and within a factor of about 2, the absolute FIR-line intensities and their distribution with Galactic longitude as observed by COBE.

Photon-dominated region modeling of the CO and [C i] line emission in Barnard 68

We use the Barnard 68 dark globule as a test case for a spherically symmetric PDR model exposed to low-UV radiation fields. With a roughly spherical morphology and an accurately determined density profile, Barnard 68 is ideal for this purpose. The processes governing the energy balance in the cloud surface are studied in detail. We compare the spherically symmetric PDR model by Stoerzer, Stutzki & Sternberg (1996) to observations of the three lowest rotational transitions of 12CO, 13CO J = 2-1 and J = 3-2 as well as the [CI] 3P_1-3P_0 fine structure transition. We study the role of Polycyclic Aromatic Hydrocarbons (PAHs) in the chemical network of the PDR model and consider the impact of depletion as well as of a variation of the external FUV field. We find it difficult to simultaneously model the observed 12CO and 13CO emission. The 12CO and [CI] emission can be explained by a PDR model with a external FUV field of 1-0.75 chi_0, but this model fails to reproduce the observed 13CO by a factor of ~2. Adding PAHs to the chemical network increases the [CI] emission by 50% in our model but makes [CII] very faint. CO depletion only slightly reduces the 12CO and 13CO line intensity (by <10% and <20%, respectively). Predictions for the [CII] 2P_3/2-2P_1/2, [CI] 3P_2-3P_1 and 12CO J= 5-4 and 4-3 transitions are presented. This allows a test of our model with future observations (APEX, NANTEN2, HERSCHEL, SOFIA).

A photon dominated region code comparison study

We present a comparison between independent computer codes, modeling the physics and chemistry of interstellar photon dominated regions (PDRs). Our goal was to understand the mutual differences in the PDR codes and their effects on the physical and chemical structure of the model clouds, and to converge the output of different codes to a common solution. A number of benchmark models have been created, covering low and high gas densities and far ultraviolet intensities. The benchmark models were computed in two ways: one set assuming constant temperatures, thus testing the consistency of the chemical network and photo-processes, and a second set determining the temperature selfconsistently. We investigated the impact of PDR geometry and agreed on the comparison of results from spherical and plane-parallel PDR models. We identified a number of key processes governing the chemical network which have been treated differently in the various codes, and defined a proper common treatment. We established a comprehensive set of reference models for ongoing and future PDR model bench-marking and were able to increase the agreement in model predictions for all benchmark models significantly.

Submillimeter imaging spectroscopy of the Horsehead nebula

We present ~15 arcsecond resolution single-dish imaging of the Horsehead nebula in the CI (1-0) and CO (4-3) lines, carried out using the CHAMP array at the Caltech Submillimeter Observatory (CSO). The data are used together with supporting observations of the (2-1) transitions of the CO isotopologues to determine the physical conditions in the atomic and molecular gas via Photon Dominated Region (PDR) modeling. The CO (4-3)/(2-1) line ratio, which is an excellent tracer of the direction of the incoming UV photons, increases at the western and northern edges of the nebula, confirming that the illumination is provided mostly by the stars σ and گ Orionis. The observed line intensities are consistent with PDR models with an H nuclei volume density of ~3- 7 x 10^4 cm^(-3). The models predict a kinetic temperature of ~12 K and a C^(18)O fractional abundance with respect to H atoms of 2.4 x 10^(-7) in the shielded region, which in turn imply a total molecular mass of ~24 M_☉ in the C^(18)O filament. The outer halo, devoid of C^(18)O, but traced by the CI emission has a comparable density and contributes additional ~13 M_☉ of material, resulting in an upper limit of ~37 M_☉ for the total molecular mass of the nebula.

CII] 158 μm emission and metallicity in photon dominated regions

We study the effects of a metallicity variation on the thermal balance and [CII] fine-structure line strengths in interstellar photon dominated regions (PDRs). We find that a reduction in the dust-to-gas ratio and the abundance of heavy elements in the gas phase changes the heat balance of the gas in PDRs. The surface temperature of PDRs decreases as the metallicity decreases except for high density ( cm-3) clouds exposed to weak () FUV fields where vibrational H2-deexcitation heating dominates over photoelectric heating of the gas. We incorporate the metallicity dependence in our KOSMA-τ PDR model to study the metallicity dependence of [CII]/CO line ratios in low metallicity galaxies. We find that the main trend in the variation of the observed CII/CO ratio with metallicity is well reproduced by a single spherical clump, and does not necessarily require an ensemble of clumps as in the semi-analytical model presented by Bolatto et al. (1999).

Modelling Diffuse Interstellar Environments

Diffuse clouds are defined as low-density clouds permeated by the ultraviolet interstellar radiation field. We discuss the ensemble of atomic, ionic and molecular observations in the context of our Photon Dominated Region model. The comparison with all observational constraints is limited and implies that the diffuse environment is more complex than originally thought.

Photon dominated regions in the spiral arms of M 83 and M 51

We present [C ] 3 P1– 3 P0 spectra at four spiral arm positions and the nuclei of the nearby galaxies M 83 and M 51 obtained at the JCMT. The spiral arm positions lie at galacto-centric distances of between 2 kpc and 6 kpc. This data is complemented with maps of CO 1–0, 2–1, and 3–2, and ISO/LWS far-infrared data of [C ] (158 µm), [O  ]( 63µm), and [N ] (122 µm) allowing for the investigation of a complete set of all major gas cooling lines. From the intensity of the [N ] line, we estimate that between 15% and 30% of the observed [C ] emission originates from the dense ionized phase of the ISM. The analysis indicates that emission from the diffuse ionized medium is negligible. In combination with the FIR dust continuum, we find gas heating efficiencies below ∼0.21% in the nuclei, and between 0.25 and 0.36% at the outer positions. Comparison with models of photon-dominated regions (PDRs) with the standard ratios [O ](63)/[C ]PDR and ([O ](63)+[C ]PDR )v s. TIR, the total infrared intensity, yields two solutions. The physically most plausible solution exhibits slightly lower densities and higher FUV fields than found when using a full set of line ratios, [C ]PDR/[C ](1–0), [C ](1–0)/CO(3–2), CO(3–2)/CO(1–0), [C ]/CO(3–2), and, [O ](63)/[C ]PDR. The best fits to the latter ratios yield densities of 10 4 cm −3 and FUV fields of ∼G0 = 20–30 times the average interstellar field without much variation. At the outer positions, the observed total infrared intensities are in agreement with the derived best fitting FUV intensities. The ratio of the two intensities lies at 4–5 at the nuclei, indicating the presence of other mechanisms heating the dust. The [C ] area filling factors lie below 2% at all positions, consistent with low volume filling factors of the emitting gas. The fit of the model to the line ratios improves significantly if we assume that [C ] stems from a larger region than CO 2–1. Improved modelling would need to address the filling factors of the various submm and FIR tracers, taking into consideration the presence of density gradients of the emitting gas by including cloud mass and size distributions within the beam.

Carbon budget and carbon chemistry in Photon Dominated Regions

We present a study of small carbon chains and rings in Photon Dominated Regions (PDRs) performed at millimetre wavelengths. Our sample consists of the Horsehead nebula (B33), the rho,Oph L1688 cloud interface, and the cometary-shaped cloud IC63. Using the IRAM 30-m telescope, the SEST and the Effelsberg 100-m teles cope at Effelsberg., we mapped the emission of \cch, c-C3H2 and C4H, and searched for heavy hydrocarbons such as c-C3H, l-C3H, l-C3H2, l-C4H2 and C6H. The large scale maps show that small hydrocarbons are present until the edge of all PDRs, which is surprising as they are expected to be easily destroyed by UV radiation. Their spatial distribution reasonably agrees with the aromatic emission mapped in mid-IR wavelength bands. Their abundances relative to H2 are relatively high and comparable to the ones derived in dark clouds such as L134N or TMC-1, known as efficient carbon factories. In particular, we report the first detection of C6H in a PDR. We have run steady-state PDR models using several gas-phase chemical networks (UMIST95 and the New Standard Model) and conclude that both networks fail in reproducing the high abundances of some of these hydrocarbons by an order of magnitude. The high abundance of hydrocarbons in the PDR may suggest that the photo-erosion of UV-irradiated large carbonaceous compounds could efficiently feed the ISM with small carbon clusters or molecules. This new production mechanism of carbon chains and rings could overcome their destruction by the UV radiation field. Dedicated theoretical and laboratory measurements are required in order to understand and implement these additional chemical routes.

Some empirical estimates of the H2formation rate in photon-dominated regions

We combine recent ISO observations of the vibrational ground state lines of H 2 towards Photon-Dominated Regions (PDRs) with observations of vibrationally excited states made with ground-based telescopes in order to constrain the formation rate of H 2 on grain surfaces under the physical conditions in the layers responsible for H 2 emission. We briefly review the data available for five nearby PDRs. We use steady state PDR models in order to examine the sensitivity of different H 2 line ratios to the H 2 formation rate R f . We show that the ratio of the 0-0 S(3) to the I-0 S(1) line increases with R f but that one requires independent estimates of the radiation field incident upon the PDR and the density in order to infer R f from the H 2 line data. We confirm earlier work by Habart et al. (2003) on the Oph W PDR which showed that an H 2 formation rate higher than the standard value of 3 x 10 -17 cm 3 s -1 inferred from UV observations of diffuse clouds is needed to explain the observed H 2 excitation. From comparison of the ISO and ground-based data, we find that moderately excited PDRs such as Oph W, S140 and IC 63 require an H 2 formation rate of about five times the standard value whereas the data for PDRs with a higher incident radiation field such as NGC 2023 and the Orion Bar can be explained with the standard value of R f . We compare also the H 2 1-0 S(1) line intensities with the emission in PAH features and find a rough scaling of the ratio of these quantities with the ratio of local density to radiation field. This suggests but does not prove that formation of H 2 on PAHs is important in PDRs. We also consider some empirical models of the H 2 formation process with the aim of explaining these results. Here we consider both formation on classical grains of size roughly 0.1 μm and on very small (∼ 10 A) grains by either direct recombination from the gas phase (Eley-Rideal mechanism) or recombination of physisorbed H atoms with atoms in a chemisorbed site. We conclude that indirect chemisorption where a physisorbed H-atom scans the grain surface before recombining with a chemisorbed H-atom is most promising in PDRs. Moreover small grains which dominate the total grain surface and spend most of their time at relatively low (below 30 K for X < 3000) temperatures may be the most promising surface for forming H 2 in PDRs.

Dense Molecular Clumps in the Orion Bar Photon-dominated Region

We present high angular resolution observations of the Orion Bar photon-dominated region (PDR) in optically thin H13CN and H13CO+ (1-0) lines, obtained using the IRAM Plateau de Bure interferometer. At least 10 spatially resolved molecular condensations are identified in the H13CN image with virial masses in the range 0.5-1.5 M☉. The median value of their H2 volume density, ~6 × 106 cm-3, is a factor of ~4 higher than the estimate based on previous PDR modeling of the main isotopomers of HCN and HCO+. Since optically thin H13CN emission is likely to trace the densest gas in the clump interiors, as compared to the main isotopomer, the H13CN clumps appear to be close to virial equilibrium. The H13CN fractional abundance is a factor of ~8 lower than that in the Orion ridge, well shielded from the far-ultraviolet (FUV) photons (~1 × 10-10). The H13CN condensations can be described in the framework of models of photoevaporating clumps exposed to an intense flux of FUV photons. The derived clump parameters are consistent with models of clumps of turbulent origin that evolve, so that their column densities are equal to the critical value determined by the incident FUV field. In this case, the column densities of the H13CN clumps seem high enough so that gravitational collapse can be triggered by the FUV-driven shock wave compression. The clumps may thus be collapsing to form low-mass stars. The observed H13CN clump parameters are also consistent with pressure-confined clump models. However, in this case the clumps would not be virialized and susceptible to gravitational collapse.

A multiwavelength study of the S 106 region

The O star S 106 IR powers a bright, spatially extended 10 0 3 0 (1:75 0:5 pc at a distance of 600 pc) photon domi- nated region (PDR) traced by our observations of FIR fine structure lines and submm molecular transitions. The (C II) 158m, (C I) 609 and 370m, CO 7!6, and CO 4!3 measurements probe the large scale (1.2 pc) PDR emission, whereas (O I )6 3m, CN N = 3!2, and CS J = 7!6 observations are focused on the immediate (1 0 (0.2 pc)) environment of S 106 IR. A hot (T> 200 K) and dense (n> 3 10 5 cm 3 ) gas component (emission peaks of (C II) 158m, CO 7!6, and CO 4!3) is found at S 106 IR. Cooler gas associated with the bulk emission of the molecular cloud is characterized by two emission peaks (one close (20 00 east) to S 106 IR and one 120 00 to the west) seen in the (C I )a nd low-J (Jup 500 K), resulting in substantial population of the energetically higher 3 P2 state; the analysis of the mid- and high- J CO excitation con- firms the higher temperature at S 106 IR. At this position, the (O I )6 3m line is the most important cooling line, followed by other atomic FIR lines ((O III )5 2m, (C II) 158m) and high-J CO lines, which are more ecient coolants compared to (C I )2 ! 1a nd 1!0. We compare the observed line ratios to plane-parallel PDR model predictions and obtain consistent re- sults for UV fluxes spanning a range from 10 2 to 10 3:5 G0 and densities around 10 5 cm 3 only at positions away from S 106 IR. Towards S 106 IR, we estimate a density of at least 3 10 5 at temperatures between 200 and 500 K from non-LTE modelling of the CO 16!15/14!13 ratio and the CO 7!6 intensity. Our new observations support the picture drawn in the first part of this serie of papers that high-density ( n> 10 5 cm 3 ) clumps with a hot PDR surface are embedded in low- to medium density gas (n 10 4 cm 3 ).

Ci] 492 GHz Mapping Observations of the High‐Latitude Translucent Cloud MCLD 123.5+24.9

We present the first map of the [C I] 3P1 → 3P0 fine-structure transition of neutral carbon made toward a translucent molecular cloud (MCLD 123.5+24.9, located in the Polaris Flare). The [C I] observations were made with the Submillimeter Wave Astronomy Satellite and are supplemented by ground-based observations of 12CO and 13CO rotational transitions. We find that the [C I] emission is spatially extended following the region bright in 12CO. The [C I] to CO line ratios observed throughout the MCLD 123.5+24.9 cloud are relatively low, and the [C I] line flux density is only ~50% of the emission by the three lowest CO rotational transitions. However, the ratios are still within the range observed along selected lines of sight toward other diffuse and translucent molecular clouds. Assuming LTE conditions for the neutral atomic carbon with an excitation temperature of 8 K derived from the 12CO spectra, we derive a total carbon column density of (0.25-1) × 1017 cm-2 and a C to CO column density ratio between 0.2 and 1.1. Comparison with a photon-dominated region model shows that the model consistently would require uncomfortably high values for the gas volume density in order to reproduce the low [C I] to CO line ratios observed (n > 105 cm-3), unless we assume that the line-emitting clumps are embedded in an interclump medium with a density of n < 103 cm-3. The low-density interclump medium does not significantly contribute to the observed [C I] and CO line emission, but the molecular hydrogen in the gas provides an effective shielding for the CO in the embedded clumps by blocking the FUV photons at the frequencies of CO line transition to the predissociation states. This reduces the photodissociation of CO and, thus, the abundance of neutral and ionized carbon in the denser clumps.

D/HD transition in Photon Dominated Regions (PDR)

We present the basic features of a steady state chemical model of Photon Dominated Regions (PDR), where the deuterium chemistry is explicitly introduced. The model is an extension of a previous PDR model (Abgrall et al. [CITE]; Le Bourlot et al. [CITE]; Le Bourlot [CITE]) in which the microscopic processes relative to HD have been incorporated. The J-dependent photodissociation probabilities have been calculated and included in the statistical equilibrium of the rotational levels of HD where the latest collision molecular data are also introduced. The thermal balance is calculated from the equilibrium between the different heating and cooling processes. We introduce a standard model of density = 500 cm-3 embedded in the Interstellar Standard Radiation Field (ISRF) from which we derive the main properties of HD in PDR. The D/HD transition does not depend only on the density, radiation field but also on the chemical processes and especially on the dust formation efficiency. In standard radiation field conditions, the D/HD transition occurs in a narrow range of visual extinctions as long as density is less than 1000 cm-3 and HD is formed through the D+ + H2 reaction. At higher densities a logarithmic dependence of the location of the transition is derived. The model is applied both to ultraviolet absorption observations from the ground rotational state of HD performed in diffuse and translucent clouds and infrared emission detectable at high densities and for high ultraviolet radiation fields coming from the bright surrounding stars.