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Micro-physical processes on interstellar dust surfaces are tightly connected to dust properties (i.e. dust composition, size, and shape) and play a key role in numerous phenomena in the interstellar medium (ISM). The large disparity in physical conditions (i.e. density and gas temperature) in the ISM triggers an evolution of dust properties.The analysis of how dust evolves with the physical conditions is a stepping stone towards amore thorough understanding of interstellar dust. As physical conditions vary widely and are spatially resolved in nearby photon-dominated regions (PDRs), they are the ideal place to study dust evolution as a function of physical conditions. In this thesis, I focus on how does interstellar dust evolves with physical conditions in nearby PDRs. To this end, I modelled dust emission and scattering using the THEMIS interstellar dust model together withthe 3D radiative transfer code SOC. I used Spitzer and Herschel observations to confront the results of my modelling in order to constrain the dust properties. I found that nano-grains are strongly depleted in the outer irradiated part of the Horsehead and that grains coagulate together to form aggregates in the inner dense part of the Horsehead.I proposed a scenario where in the outer part of the Horsehead nebula, all the nano-grain have not yet had time to re-form completely through photo-fragmentation of aggregates and the smallestof the nano-grain that are sensitive to the radiation field are photo-destroyed. In the inner part of the Horsehead nebula, grains most likely consist of multi-compositional mantled aggregates, as would expected in high density regions.; Les processus microphysiques à la surface des poussières interstellaires sont étroitement liés à leurs propriétés (i.e. la taille et la forme des poussières) et jouent un rôle clé dans de nombreux phénomènes du milieu interstellaire (MIS). La grande disparité des conditions physiques (i.e. la densité et la température du gaz) dans le MIS déclenche une évolution des propriétés des poussières. L'analyse de l'évolution de la poussière en fonction des conditions physiques est un tremplin vers une compréhension plus approfondie de la poussière interstellaire. Étant donné que les conditions physiques varient considérablement et sont spatialement résolues dans les régions photon-dominées (PDRs) proches, elles constituent le lieu idéal pour étudier l'évolution de la poussière en fonction des conditions physiques. Dans cette thèse, je me concentre sur la façon dont la poussière interstellaire évolue en fonction des conditions physiques dans les PDRs proches. À cette fin, j'ai modélisé l'émission et la diffusion de la poussière en utilisant le modèle de poussière interstellaire THEMIS ainsi que le code de transfert radiatif 3D SOC. J'ai utilisé les observations Spitzer et Herschel pour confronter les résultats de ma modélisation afin de contraindre les propriétés de la poussière. J'ai constaté que les nano-grains sont fortement déplétés dans la partie irradiée de la tête de cheval et que les grains coagulent pour former des agrégats dans la partie dense de la tête de cheval. J'ai proposé un scénario dans lequel, dans la partie irradiée de la nébuleuse de la tête de cheval, tous les nano-grains n'ont pas encore eu le temps de se reformer par la photofragmentation des agrégats et les plus petits des nano-grains sensibles au champ de rayonnement sont détruits par le rayonnement. Dans la partie intérieure de la nébuleuse de la Tête de cheval, les grains sont constitués d'agrégats, comme attendu dans les régions à haute densité.

We use new Band-3 CO(1-0) observations taken with the Atacama Large Millimeter/submillimeter Array (ALMA) to study the physical conditions in the interstellar gas of a sample of 27 dusty main-sequence star-forming galaxies at 0.035$\sigma$ in 26 sources. We find an average [CII] to CO(1-0) luminosity ratio of 3500$\pm$1200 for our sample that is consistent with previous studies. Using the [CII], CO and FIR measurements as diagnostics of the physical conditions of the interstellar medium, we compare these observations to the predictions of a photodissociation region (PDR) model to determine the gas density, surface temperature, pressure, and the strength of the incident far-ultraviolet (FUV) radiation field, $G_{0}$, normalised to the Habing Field. The majority of our sample exhibit hydrogen densities of 4 < $\log n/\mathrm{cm}^{3}$ < 5.5 and experience an incident FUV radiation field with strengths of 2 < $\log G_0$ < 3 when adopting standard adjustments. A comparison to galaxy samples at different redshifts indicates that the average strength of the FUV radiation field appears constant up to redshift $z\sim$6.4, yet the neutral gas density increases with redshift by a factor of $\sim$100, that persists regardless of various adjustments to our observable quantities. This evolution could provide an explanation for the observed evolution of the star formation rate density with cosmic time, yet could arise from a combination of observational biases when using different suites of emission lines as diagnostic tracers of PDR gas., Comment: Accepted for publication in A&A. Abstract abridged for arXiv. 13 pages, including 5 figures and 2 tables

We present observations of the dense molecular gas tracers HCN, HNC, and HCO+ in the J=1-0 transition using ALMA. We supplement our datasets with previous observations of CO J=1-0, which traces the total molecular gas content. We separate the Antennae into 7 bright regions in which we detect emission from all three molecules, including the nuclei of NGC 4038 and NGC 4039, 5 super giant molecular complexes in the overlap region and 2 additional bright clouds. We find that the ratio of L(HCN)/L(CO), which traces the dense molecular gas fraction, is greater in the two nuclei (L(HCN)/L(CO) ~ 0.07 - 0.08) than in the overlap region (L(HCN)/L(CO) = 5% - 10% of the PDR surface heating to the total heating budget. Finally, the ratio of L(HCN)/L(HCO+) varies from ~1 in the nucleus of NGC 4038 down to ~0.5 in the overlap region. The lower ratio in the overlap region may be due to an increase in the cosmic ray rate from the increased supernova rate within this region., Accepted to ApJ; 12 pages

Accepted: 2007-12-11, 資料番号: SA1000654000

International audience; Stars form in the densest, coldest, most quiescent regions of molecular clouds. Molecules provide the only probes that can reveal the dynamics, physics, chemistry, and evolution of these regions, but our understanding of the molecular inventory of sources and how this is related to their physical state and evolution is rudimentary and incomplete. The Spectral Legacy Survey (SLS) is one of seven surveys recently approved by the James Clerk Maxwell Telescope (JCMT) Board of Directors. Beginning in 2007, the SLS will produce a spectral imaging survey of the content and distribution of all the molecules detected in the 345 GHz atmospheric window (between 332 and 373 GHz) toward a sample of five sources. Our intended targets are a low-mass core (NGC 1333 IRAS 4), three high-mass cores spanning a range of star-forming environments and evolutionary states (W49, AFGL 2591, and IRAS 20126), and a photodissociation region (the Orion Bar). The SLS will use the unique spectral imaging capabilities of HARP-B/ACSIS (Heterodyne Array Receiver Programme B/Auto-Correlation Spectrometer and Imaging System) to study the molecular inventory and the physical structure of these objects, which span different evolutionary stages and physical environments and to probe their evolution during the star formation process. As its name suggests, the SLS will provide a lasting data legacy from the JCMT that is intended to benefit the entire astronomical community. As such, the entire data set (including calibrated spectral data cubes, maps of molecular emission, line identifications, and calculations of the gas temperature and column density) will be publicly available.

This thesis focus on the study of the Interstellar Medium (ISM) of the Milky Way and consists of two parts: in the first one we present a study of the dust properties in HII regions and their surrounding PDRs. We focus our studies on two compact HII regions: W3(A) and the Orion Nebula (Chapters 2 and 3, respsectively). Using SOFIA/FORCAST observations, we determine the properties of dust in the ionized gas, their surrounding PhotoDissociation Regions and their parent molecular clouds. One of the most important conclusions of this thesis is that the grain growth can affect the dust size distribution in regions of massive star formation. The large amount of archival data available for Orion allow us to study other important phenomena related to dust in this region, such as the photoelectric heating effect and Lyman alpha heating. The second part of this thesis presents theoretical studies of the properties of the cold neutral medium (CNM) using carbon radio recombination lines (CRRL). Chapters 4 and 5 consists on the complete theory of CRRLs from the level population equation to the radiative transfer equation. Using these models and observations of CRRLs the physical parameters of the CNM can be determined.

The near- and mid-IR spectrum of many astronomical objects is dominated by emission bands due to UV-excited polycyclic aromatic hydrocarbons (PAH) and evaporating very small grains (eVSG). Previous studies with the ISO, Spitzer and AKARI space telescopes have shown that the spectral variations of these features are directly related to the local physical conditions that induce a photo-chemical evolution of the band carriers. Because of the limited sensitivity and spatial resolution, these studies have focused mainly on galactic star-forming regions. We discuss how the advent of JWST will allow to extend these studies to previously unresolved sources such as near-by galaxies, and how the analysis of the infrared signatures of PAHs and eVSGs can be used to determine their physical conditions and chemical composition., Comment: To appear in the Proceedings of the annual meeting of the French society of astronomy and astrophysics (SF2A 2015)

We present a far-ultraviolet (PDR) and an X-ray dominated region (XDR) code. We include and discuss thermal and chemical processes that pertain to irradiated gas. An elaborate chemical network is used and a careful treatment of PAHs and H(2) formation, destruction and excitation is included. For both codes we calculate four depth-dependent models for different densities and radiation fields, relevant to conditions in starburst galaxies and active galactic nuclei. A detailed comparison between PDR and XDR physics is made for total gas column densities between similar to 10(20) and similar to 10(25) cm(-2). We show cumulative line intensities for a number of fine-structure lines (e.g., [CII], [OI], [CI], [SiII], [FeII]), as well as cumulative column densities and column density ratios for a number of species ( e. g., CO/H(2), CO/C, HCO(+)/HCN, HNC/HCN). The comparison between the results for the PDRs and XDRs shows that column density ratios are almost constant up to N(H) = 1022 cm(-2) for XDRs, unlike those in PDRs. For example, CO/C in PDRs changes over four orders of magnitude from the edge to NH = 10(22) cm(-2). The CO/C and CO/H(2) ratios are lower in XDRs at low column densities and rise at NH > 1023 cm-2. At most column densities NH > 10(21.5) cm-2, HNC/HCN ratios are lower in XDRs too, but they show a more moderate increase at higher N(H).

We have computed physical parameters such as density, degree of ionization and temperature, constrained by a large observational data set on atomic and molecular species, for the line of sight toward the single cloud HD147889. Diffuse interstellar bands ( DIBs) produced along this line of sight are well documented and can be used to test the PAH hypothesis. To this effect, the charge state fractions of different polycyclic aromatic hydrocarbons (PAHs) are calculated in HD147889 as a function of depth for the derived density, electron abundance and temperature profile. As input for the construction of these charge state distributions, the microscopic properties of the PAHs, e. g., ionization potential and electron affinity, are determined for a series of symmetry groups. The combination of a physical model for the chemical and thermal balance of the gas toward HD147889 with a detailed treatment of the PAH charge state distribution, and laboratory and theoretical data on specific PAHs, allow us to compute electronic spectra of gas phase PAH molecules and to draw conclusions about the required properties of PAHs as DIB carriers. We find the following. 1) The variation of the total charge state distribution of each specific class ( series) of PAH in the translucent cloud toward HD147889 ( and also of course for any other diffuse/ translucent cloud) depends strongly on the molecular symmetry and size ( number of p electrons). This is due to the strong effects of these parameters on the ionization potential of a PAH. 2) Different wavelength regions in the DIB spectrum are populated preferentially by different PAH charge states depending on the underlying PAH size distribution. 3) The PAH size distribution for HD147889 is constrained by the observed DIB spectrum to be Gaussian with a mean of 50 carbon atoms. 4) For the given PAH size distribution it is possible to constrain the total small catacondensed PAH column density along the line of sight to HD147889 to 2.4 x 10(14) cm(-2) by comparing the total observed UV extinction to the strong UV absorptions of neutral PAHs in the 2000-3000 Angstrom region. 5) Catacondensed PAHs with sizes above some 40 C-atoms are expected to show strong DIBS longward of 10 000 Angstrom. Large condensed PAHs in the series, pyrene, coronene, ovalene,...., on the other hand, mainly absorb between 4000 and 10 000 Angstrom but extrapolation to even larger pericondensed PAHs in this series also shows strong absorptions longward of 10 000 Angstrom. 6) Only the weak DIBs in HD147889 could be reproduced by a mix of small catacondensed PAHs (

The chemical and thermal structure of dense gas irradiated by FUV and X-ray photons is explored. The relevance of these so-called PDRs (FUV) and XDRs (X-ray) for AGN and YSOs is discussed. Observational line diagnostics and chemical tracers are summarized.

We search for variations in the disk of Centaurus A of the emission from atomic fine structure lines using Herschel PACS and SPIRE spectroscopy. In particular we observe the [C II](158 $\mu$m), [N II](122 and 205 $\mu$m), [O I](63 and 145 $\mu$m) and [O III](88 $\mu$m) lines, which all play an important role in cooling the gas in photo-ionized and photodissociation regions. We determine that the ([C II]+[O I]$_{63}$)/$F_{TIR}$ line ratio, a proxy for the heating efficiency of the gas, shows no significant radial trend across the observed region, in contrast to observations of other nearby galaxies. We determine that 10 - 20% of the observed [C II] emission originates in ionized gas. Comparison between our observations and a PDR model shows that the strength of the far-ultraviolet radiation field, $G_0$, varies between $10^{1.75}$ and $10^{2.75}$ and the hydrogen nucleus density varies between $10^{2.75}$ and $10^{3.75}$ cm$^{-3}$, with no significant radial trend in either property. In the context of the emission line properties of the grand-design spiral galaxy M51 and the elliptical galaxy NGC 4125, the gas in Cen A appears more characteristic of that in typical disk galaxies rather than elliptical galaxies., Comment: Accepted for publication in the Astrophysical Journal. 22 pages, 10 figures, 5 tables

Recent observations with the Submillimeter Wave Astronomy Satellite (SWAS) indicate abundances of gaseous H2O and O-2 in dense molecular clouds that are significantly lower than those found in standard homogeneous chemistry models. We present here results for the thermal and chemical balance of inhomogeneous molecular clouds exposed to ultraviolet radiation in which the abundances of H2O and O-2 are computed for various density distributions, radiation field strengths, and geometries. It is found that an inhomogeneous density distribution lowers the column densities of H2O and O-2 compared with the homogeneous case by more than an order of magnitude at the same A(nu). O-2 is particularly sensitive to the penetrating ultraviolet radiation, more so than H2O. The S140 and rho Ophiuchi clouds are studied as relevant test cases of star-forming and quiescent regions. The SWAS results of S140 can be accommodated naturally in a clumpy model with a mean density of 2 x 10 cm(-3) and an enhancement of I-UV = 140 compared with the average interstellar radiation field, in agreement with observations of [C I] and (CO)-C-13 of this cloud. Additional radiative transfer computations suggest that this diffuse H2O component is warm, similar to 60- 90 K, and can account for the bulk of the 1(10) -1(01) line emission observed by SWAS. The rho Oph model yields consistent O-2 abundances but too much H2O, even for, [C]/[O] = 0.94, if I-UV

Recent observations with the Submillimeter Wave Astronomy Satellite (SWAS) indicate abundances of gaseous H2O and O-2 in dense molecular clouds that are significantly lower than those found in standard homogeneous chemistry models. We present here results for the thermal and chemical balance of inhomogeneous molecular clouds exposed to ultraviolet radiation in which the abundances of H2O and O-2 are computed for various density distributions, radiation field strengths, and geometries. It is found that an inhomogeneous density distribution lowers the column densities of H2O and O-2 compared with the homogeneous case by more than an order of magnitude at the same A(nu). O-2 is particularly sensitive to the penetrating ultraviolet radiation, more so than H2O. The S140 and rho Ophiuchi clouds are studied as relevant test cases of star-forming and quiescent regions. The SWAS results of S140 can be accommodated naturally in a clumpy model with a mean density of 2 x 10 cm(-3) and an enhancement of I-UV = 140 compared with the average interstellar radiation field, in agreement with observations of [C I] and (CO)-C-13 of this cloud. Additional radiative transfer computations suggest that this diffuse H2O component is warm, similar to 60- 90 K, and can account for the bulk of the 1(10) -1(01) line emission observed by SWAS. The rho Oph model yields consistent O-2 abundances but too much H2O, even for, [C]/[O] = 0.94, if I-UV

Infrared Space Observatory (ISO) CC nl 158 mu m, [O I] 63 mu m, and H-2 9 and 17 mu m observations are presented of the reflection nebula Ced 201, which is a photon-dominated region (PDR) illuminated by a B9.5 star with a color temperature of 10,000 K (a cool PDR). In combination with ground-based [C I] 609 mu m, CO, (CO)-C-13, CS, and HCO+ data, the carbon budget and physical structure of the reflection nebula are constrained. The obtained data set is the first one to contain all important cooling lines of a cool PDR and allows a comparison to be made with classical PDRs, To this effect one- and three-dimensional PDR models are presented that incorporate the physical characteristics of the source and are aimed at understanding the dominant heating processes of the cloud. The contribution of very small grains to the photoelectric heating rate is estimated from these models and is used to constrain the total abundance of polycyclic aromatic hydrocarbons and small grains. Observations of the pure rotational H-2 lines with ISO, in particular the S(3) line, indicate the presence of a small amount of very warm similar to 330 K molecular gas. This gas cannot be accommodated by the presented models.

The interstellar medium of galaxies is the reservoir out of which stars are born and into which stars inject newly created elements as they age. The physical properties of the interstellar medium are governed in part by the radiation emitted by these stars. Far-ultraviolet (6 eV

The interstellar medium of galaxies is the reservoir out of which stars are born and into which stars inject newly created elements as they age. The physical properties of the interstellar medium are governed in part by the radiation emitted by these stars. Far-ultraviolet (6 eV less than h(nu) less than 13.6 eV) photons from massive stars dominate the heating and influence the chemistry of the neutral atomic gas and much of the molecular gas in galaxies. Predominantly neutral regions of the interstellar medium in which the heating and chemistry are regulated by far ultraviolet photons are termed Photo-Dissociation Regions (PDRs). These regions are the origin of most of the non-stellar infrared (IR) and the millimeter and submillimeter CO emission from galaxies. The importance of PDRs has become increasingly apparent with advances in IR and submillimeter astronomy. The IR emission from PDRs includes fine structure lines of C, C+, and O; rovibrational lines of H2, rotational lines of CO; broad middle features of polycyclic aromatic hydrocarbons; and a luminous underlying IR continuum from interstellar dust. The transition of H to H2 and C+ to CO occurs within PDRs. Comparison of observations with theoretical models of PDRs enables one to determine the density and temperature structure, the elemental abundances, the level of ionization, and the radiation field. PDR models have been applied to interstellar clouds near massive stars, planetary nebulae, red giant outflows, photoevaporating planetary disks around newly formed stars, diffuse clouds, the neutral intercloud medium, and molecular clouds in the interstellar radiation field-in summary, much of the interstellar medium in galaxies. Theoretical PDR models explain the observed correlations of the [CII] 158 microns with the COJ = 1-0 emission, the COJ = 1-0 luminosity with the interstellar molecular mass, and the [CII] 158 microns plus [OI] 63 microns luminosity with the IR continuum luminosity. On a more global scale, MR models predict the existence of two stable neutral phases of the interstellar medium, elucidate the formation and destruction of star-forming molecular clouds, and suggest radiation-induced feedback mechanisms that may regulate star formation rates and the column density of gas through giant molecular clouds.

The abundances of interstellar CH+ and SH+ are not well understood as their most likely formation channels are highly endothermic. Using data from Herschel, we study the formation of CH+ and SH+ in a typical high UV-illumination photon-dominated region (PDR), the Orion Bar. Herschel/HIFI provides velocity-resolved data of CH+ 1-0 and 2-1 and three hyperfine transitions of SH+. Herschel/PACS provides information on the excitation and spatial distribution of CH+ (up to J=6-5). The widths of the CH+ 2-1 and 1-0 transitions are of ~5 km/s, significantly broader than the typical width of dense gas tracers in the Orion Bar (2-3 km/s) and are comparable to the width of tracers of the interclump medium such as C+ and HF. The detected SH+ transitions are narrower compared to CH+ and have line widths of 3 km/s, indicating that SH+ emission mainly originates in denser condensations. Non-LTE radiative transfer models show that electron collisions affect the excitation of CH+ and SH+, and that reactive collisions need to be taken into account to calculate the excitation of CH+. Comparison to PDR models shows that CH+ and SH+ are tracers of the warm surface region (AV, Accepted for publication in A&A, 12 pages, 11 figures, abstract shortened

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 decomposed the MIR PAH emission into that of neutral (PAH^0) and positively ionized (PAH^+) species to derive the fraction of the positively charged PAHs, and compare it to the photoelectric heating efficiency. The heating efficiency traced by ([OI]63um+[OI]145um+[CII]158um) / TIR, ranges between 0.1% and 0.9% in different sources, and the fraction of PAH^+ relative to (PAH^0 + PAH^+) spans from 0(+11)% to 87(+/-10)%. All positions with a high PAH^+ fraction show a low heating efficiency, and all positions with a high heating efficiency have a low PAH^+ fraction, supporting the scenario in which a positive grain charge results in a decreased heating efficiency. Theoretical estimates of the photoelectric heating efficiency show a stronger dependence on the charging parameter gamma=G_0 T^{1/2}/n_e than the observed efficiency reported in this study, and the discrepancy is significant at low gamma. The photoelectric heating efficiency on PAHs, traced by ([OI]63um+[OI]145um+[CII]158um) / (PAH+[OI]63um+[OI]145um+[CII]158um), shows a much better match between the observations and the theoretical estimates. The good agreement of the photoelectric heating efficiency on PAHs with a theoretical model indicates the dominant contribution of PAHs to the photoelectric heating. (abridged for arXiv), Comment: 14 pages with 10 figures, accepted for publication in A&A

Aims. Molecules that trace the high-density regions of the interstellar medium may be used to evaluate the changing physical and chemical environment during the ongoing nuclear activity in (ultra-) luminous infrared galaxies (ULIRGs).Methods. The changing ratios of the HCN (1-0), HNC (1-0), HCO(+) (1-0), CN (1-0) and CN (2-1), and CS (3-2) transitions were compared with the HCN (1-0)/CO (1-0) ratio, which is proposed to represent the progression time scale of the starburst. These diagnostic diagrams were interpreted using the results of theoretical modeling with a large physical and chemical network to describe the state of the nuclear ISM in the evolving galaxies.Results. Systematic changes are seen in the line ratios as the sources evolve from early stage for the nuclear starburst (ULIRGs) to later stages. These changes result from changing environmental conditions and particularly from the lowering of the average density of the medium. A temperature rise due to mechanical heating of the medium by feedback explains the lowering of the ratios at later evolutionary stages. Infrared pumping may affect the CN and HNC line ratios during early evolutionary stages.Conclusions. Molecular transitions display a behavior that relates to changes of the environment during an evolving nuclear starburst. Molecular properties may be used to designate the evolutionary stage of the nuclear starburst. The HCN(1-0)/CO(1-0) and HCO(+)(1-0)/HCN(1-0) ratios serve as indicators of the time evolution of the outburst.

The molecular gas in the DR21 massive star formation region is known to be affected by the strong UV field from the central star cluster and by a fast outflow creating a bright shock. The relative contribution of both heating mechanisms is the matter of a long debate. By better sampling the excitation ladder of various tracers we provide a quantitative distinction between the different heating mechanisms. HIFI observations of mid-J transitions of CO and HCO+ isotopes allow us to bridge the gap in excitation energies between observations from the ground, characterizing the cooler gas, and existing ISO LWS spectra, constraining the properties of the hot gas. Comparing the detailed line profiles allows to identify the physical structure of the different components. In spite of the known shock-excitation of H2 and the clearly visible strong outflow, we find that the emission of all lines up to > 2 THz can be explained by purely radiative heating of the material. However, the new Herschel/HIFI observations reveal two types of excitation conditions. We find hot and dense clumps close to the central cluster, probably dynamically affected by the outflow, and a more widespread distribution of cooler, but nevertheless dense, molecular clumps., Accepted for publication by A&A

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., Comment: Published in A&A 498, 161-165 (2009); v2: updated with correct line strengths after corrigendum in A&A 510, C1 (2010)

We present a far-ultraviolet (PDR) and an X-ray dominated region (XDR) code. We include and discuss thermal and chemical processes that pertain to irradiated gas. An elaborate chemical network is used and a careful treatment of PAHs and H(2) formation, destruction and excitation is included. For both codes we calculate four depth-dependent models for different densities and radiation fields, relevant to conditions in starburst galaxies and active galactic nuclei. A detailed comparison between PDR and XDR physics is made for total gas column densities between similar to 10(20) and similar to 10(25) cm(-2). We show cumulative line intensities for a number of fine-structure lines (e.g., [CII], [OI], [CI], [SiII], [FeII]), as well as cumulative column densities and column density ratios for a number of species ( e. g., CO/H(2), CO/C, HCO(+)/HCN, HNC/HCN). The comparison between the results for the PDRs and XDRs shows that column density ratios are almost constant up to N(H) = 1022 cm(-2) for XDRs, unlike those in PDRs. For example, CO/C in PDRs changes over four orders of magnitude from the edge to NH = 10(22) cm(-2). The CO/C and CO/H(2) ratios are lower in XDRs at low column densities and rise at NH > 1023 cm-2. At most column densities NH > 10(21.5) cm-2, HNC/HCN ratios are lower in XDRs too, but they show a more moderate increase at higher N(H).

The chemical and thermal structure of dense gas irradiated by FUV and X-ray photons is explored. The relevance of these so-called PDRs (FUV) and XDRs (X-ray) for AGN and YSOs is discussed. Observational line diagnostics and chemical tracers are summarized.

Recent observations with the Submillimeter Wave Astronomy Satellite indicate abundances of gaseous H2O and O2 in dense molecular clouds which are significantly lower than found in standard homogeneous chemistry models. We present here results for the thermal and chemical balance of inhomogeneous molecular clouds exposed to ultraviolet radiation in which the abundances of H2O and O2 are computed for various density distributions, radiation field strengths and geometries. It is found that an inhomogeneous density distribution lowers the column densities of H2O and O2 compared to the homogeneous case by more than an order of magnitude at the same A_V. O2 is particularly sensitive to the penetrating ultraviolet radiation, more so than H2O. The S140 and rho Oph clouds are studied as relevant test cases of star-forming and quiescent regions. The SWAS results of S140 can be accommodated naturally in a clumpy model with mean density of 2x10^3 cm-3 and enhancement I_UV=140 compared with the average interstellar radiation field, in agreement with observations of [CI] and 13CO of this cloud. Additional radiative transfer computations suggest that this diffuse H2O component is warm, ~60-90 K, and can account for the bulk of the 1_10-1_01 line emission observed by SWAS. The rho Oph model yields consistent O2 abundances but too much H2O, even for [C]/[O]=0.94, if I_UV, To be published in ApJL

ISO [C II] 158 micron, [O I] 63 micron, and H_2 9 and 17 micron observations are presented of the reflection nebula Ced 201, which is a photon-dominated region illuminated by a B9.5 star with a color temperature of 10,000 K (a cool PDR). In combination with ground based [C I] 609 micron, CO, 13CO, CS and HCO+ data, the carbon budget and physical structure of the reflection nebula are constrained. The obtained data set is the first one to contain all important cooling lines of a cool PDR, and allows a comparison to be made with classical PDRs. To this effect one- and three-dimensional PDR models are presented which incorporate the physical characteristics of the source, and are aimed at understanding the dominant heating processes of the cloud. The contribution of very small grains to the photo-electric heating rate is estimated from these models and used to constrain the total abundance of PAHs and small grains. Observations of the pure rotational H_2 lines with ISO, in particular the S(3) line, indicate the presence of a small amount of very warm, approximately 330 K, molecular gas. This gas cannot be accommodated by the presented models., 32 pages, 7 figures, in LaTeX. To be published in ApJ

Context: The emission line of [CII] at 158 micron is one of the strongest cooling lines of the interstellar medium (ISM) in galaxies. Aims: Disentangling the relative contributions of the different ISM phases to [CII] emission, is a major topic of the HerM33es program, a Herschel key project to study the ISM in the nearby spiral galaxy M33. Methods: Using PACS, we have mapped the emission of [CII] 158 micron, [OI] 63 micron, and other FIR lines in a 2'x2' region of the northern spiral arm of M33, centered on the HII region BCLMP302. At the peak of H-alpha emission, we have observed in addition a velocity resolved [CII] spectrum using HIFI. We use scatterplots to compare these data with PACS 160 micron continuum maps, and with maps of CO and HI data, at a common resolution of 12 arcsec or 50 pc. Maps of H-alpha and 24 micron emission observed with Spitzer are used to estimate the SFR. We have created maps of the [CII] and [OI] 63 micron emission and detected [NII] 122 micron and NIII 57 micron at individual positions. Results: The [CII] line observed with HIFI is significantly broader than that of CO, and slightly blue-shifted. In addition, there is little spatial correlation between [CII] observed with PACS and CO over the mapped region. There is even less spatial correlation between [CII] and the atomic gas traced by HI. Detailed comparison of the observed intensities towards the HII region with models of photo ionization and photon dominated regions, confirms that a significant fraction, 20--30%, of the observed [CII] emission stems from the ionized gas and not from the molecular cloud. The gas heating efficiency, using the ratio between [CII] and the TIR as a proxy, varies between 0.07 and 1.5%, with the largest variations found outside the HII region., Comment: Accepted for publication in A&A. 12 pages, 12 figures