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The large disparity in physical conditions from the diffuse interstellar medium (ISM) to denser clouds such as photon-dominated regions (PDRs) triggers an evolution of the dust properties (i.e. composition, size, and shape). The gas physics and chemistry are tightly connected to these dust properties and are therefore affected by dust evolution and especially the nano-grain depletion in the outer irradiated part of PDRs. We highlight the influence of nano-grain depletion on the gas physics and chemistry in the Horsehead nebula, a prototypical PDR. We used a model for atomic and molecular gas in PDRs, the Meudon PDR code, using diffuse ISM-like dust and Horsehead-like dust to study the influence of nano-grain depletion on the gas physics and chemistry, focusing on the impact on photoelectric heating and H2 formation and, therefore, on the H2 gas lines. We find that nano-grain depletion in the Horsehead strongly affects gas heating through the photoelectric effect and thus the gas temperature and the H2 formation, hence the H -> H2 position. Consequently, the first four pure rotational lines of H2 (e.g. 0-0 S(0), S(1), S(2), and S(3)) vary by a factor of 2 to 14. The 0-0 S(3) line that is often underestimated in models is underestimated even more when taking nano-grain depletion into account due to the decrease in gas heating through the photoelectric effect. This strongly suggests that our understanding of the excitation of H2 and/or of heating processes in the Horsehead, and more generally in PDRs, is still incomplete. Nano-grain depletion in the outer part of the Horsehead has a strong influence on several gas tracers that will be prominent in JWST observations of irradiated clouds. We therefore need to take this depletion into account in order to improve our understanding of the Horsehead, and more generally PDRs, and to contribute to the optimal scientific return of the mission., Accepted in A&A, comments welcome

Unlike microbes known in his time, the first virus (that of tobacco mosaic disease) was discovered by Ivanoski in 1892 because it was not retained by Chamberland's porcelain candles. For more than a century afterward, viruses were equated with this simple property that is still extensively used today (using modern 0,2 µm pore filters) as a practical criterion to delineate the "viral fraction" from other microbes in medical or environmental samples. The first documented exception to the simplistic criterion of particle size came with the discovery of Mimivirus, the viral nature of which was eventually recognized in 2003, following ten years during which it was mistaken for an obligate intracellular bacterium. Thirteen more years later, we now realize that non-filtering "giant viruses" are not rare, probably ubiquitous, and come in a large variety of virion shapes, genome sizes, gene contents, and replication strategies. Following a quick description of the 4 giant virus families known today, we discuss the enigmas, controversies and perspectives of conceptual revolutions that are brought about by this new and booming area of virology.

Context. 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 a more thorough understanding of interstellar dust. Aims. We highlight dust evolution in the Horsehead nebula photon-dominated region. Methods. We used Spitzer/IRAC (3.6, 4.5, 5.8 and 8 μm) and Spitzer/MIPS (24 μm) together with Herschel/PACS (70 and 160 μm) and Herschel/SPIRE (250, 350 and 500 μm) to map the spatial distribution of dust in the Horsehead nebula over the entire emission spectral range. We modelled dust emission and scattering using the THEMIS interstellar dust model together with the 3D radiative transfer code SOC. Results. We find that the nano-grain dust-to-gas ratio in the irradiated outer part of the Horsehead is 6–10 times lower than in the diffuse ISM. The minimum size of these grains is 2–2.25 times larger than in the diffuse ISM, and the power-law exponent of their size distribution is 1.1–1.4 times lower than in the diffuse ISM. In the denser part of the Horsehead nebula, it is necessary to use evolved grains (i.e. aggregates, with or without an ice mantle). Conclusions. It is not possible to explain the observations using grains from the diffuse medium. We therefore propose the following scenario to explain our results. 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 smallest of 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.

[Context] The methylidyne cation (CH) and hydroxyl (OH) are key molecules in the warm interstellar chemistry, but their formation and excitation mechanisms are not well understood. Their abundance and excitation are predicted to be enhanced by the presence of vibrationally excited H or hot gas (~500-1000 K) in photodissociation regions (PDRs) with high incident far-ultraviolet (FUV) radiation field. The excitation may also originate in dense gas (>10 cm) followed by nonreactive collisions with H, H, and electrons. Previous observations of the Orion Bar suggest that the rotationally excited CH and OH correlate with the excited CO, which is a tracer of dense and warm gas, and that formation pumping contributes to CH excitation., [Aims] Our goal is to examine the spatial distribution of the rotationally excited CH and OH emission lines in the Orion Bar to establish their physical origin and main formation and excitation mechanisms., [Methods] We present spatially sampled maps of the CHJ = 3-2 transition at 119.8 μm and the OH Λ doublet at 84 μm in the Orion Bar over an area of 110″× 110″ with Herschel/PACS. We compare the spatial distribution of these molecules with those of their chemical precursors, C, O and H, and tracers of warm and dense gas (high-J CO). We assess the spatial variation of the CHJ = 2-1 velocity-resolved line profile at 1669 GHz with Herschel/HIFI spectrometer observations., [Results] The OH and especially CH lines correlate well with the high-J CO emission and delineate the warm and dense molecular region at the edge of the Bar. While notably similar, the differences in the CH and OH morphologies indicate that CH formation and excitation are strongly related to the observed vibrationally excited H. This, together with the observed broad CH line widths, indicates that formation pumping contributes to the excitation of this reactive molecular ion. Interestingly, the peak of the rotationally excited OH 84 μm emission coincides with a bright young object, proplyd 244-440, which shows that OH can be an excellent tracer of UV-irradiated dense gas., [Conclusions] The spatial distribution of CH and OH revealed in our maps is consistent with previous modeling studies. Both formation pumping and nonreactive collisions in a UV-irradiated dense gas are important CHJ = 3-2 excitation processes. The excitation of the OH Λ doublet at 84 μm is mainly sensitive to the temperature and density., J. Bernard-Salas wishes to acknowledge the support of a Career Integration Grant within the 7th European Community Framework Program, FP7-PEOPLE-2013-CIG-630861-FEASTFUL. J. R. Goicoechea thanks Spanish MINECO for funding support under grants CSD2009-00038 and AYA2012-32032. He also thanks the ERC for support under grant ERC-2013-Syg-610256-NANOCOSMOS. M. Gerin and B. Godard thank the French PCMI program for funding.

18 páginas, 17 figuras.-- Planck Collaboration: et al., Planck has observed the entire sky from 30 GHz to 857GHz. The observed foreground emission contains contributions from different phases of the interstellar medium (ISM). We have separated the observed Galactic emission into the different gaseous components (atomic, molecular and ionised) in each of a number of Galactocentric rings. This technique provides the necessary information to study dust properties (emissivity, temperature, etc.), as well as other emission mechanisms as a function of Galactic radius. Templates are created for various Galactocentric radii using velocity information from atomic (neutral hydrogen) and molecular (12CO) observations. The ionised template is assumed to be traced by free-free emission as observed by WMAP, while 408 MHz emission is used to trace the synchrotron component. Gas emission not traced by the above templates, namely “dark gas”, as evidenced using Planck data, is included as an additional template, the first time such a component has been used in this way. These templates are then correlated with each of the Planck frequency bands, as well as with higher frequency data from IRAS and DIRBE along with radio data at 1.4 GHz. The emission per column density of the gas templates allows us to create distinct spectral energy distributions (SEDs) per Galactocentric ring and in each of the gaseous tracers from 1.4 GHz to 25 THz (12μm). The resulting SEDs allow us to explore the contribution of various emission mechanisms to the Planck signal. Apart from the thermal dust and free-free emission, we have probed the Galaxy for anomalous (e.g., spinning) dust as well as synchrotron emission. We find the dust opacity in the solar neighbourhood, τ/NH = 0.92 ± 0.05 × 10-25 cm2 at 250 μm, with no significant variation with Galactic radius, even though the dust temperature is seen to vary from over 25 K to under 14 K. Furthermore, we show that anomalous dust emission is present in the atomic, molecular and dark gas phases throughout the Galactic disk. Anomalous emission is not clearly detected in the ionised phase, as free-free emission is seen to dominate. The derived dust propeties associated with the dark gas phase are derived but do not allow us to reveal the nature of this phase. For all environments, the anomalous emission is consistent with rotation from polycyclic aromatic hydrocarbons (PAHs) and, according to our simple model, accounts for (25 ± 5)% (statistical) of the total emission at 30 GHz.

Context. With Herschel, we can for the first time observe a wealth of high-J CO lines in the interstellar medium with a high angular resolution. These lines are specifically useful for tracing the warm and dense gas and are therefore very appropriate for a study of strongly irradiated dense photodissocation regions (PDRs). Aims. We characterize the morphology of CO J = 19–18 emission and study the high-J CO excitation in a highly UV-irradiated prototypical PDR, the Orion Bar. Methods. We used fully sampled maps of CO J = 19–18 emission with the Photoconductor Array Camera and Spectrometer (PACS) on board the Herschel Space Observatory over an area of ~110′′ × 110′′ with an angular resolution of 9′′. We studied the morphology of this high-J CO line in the Orion Bar and in the region in front and behind the Bar, and compared it with lower-J lines of CO from J = 5–4 to J = 13–12 and 13CO from J = 5–4 to J = 11–10 emission observed with the Herschel Spectral and Photometric Imaging Receiver (SPIRE). In addition, we compared the high-J CO to polycyclic aromatic hydrocarbon (PAH) emission and vibrationally excited H2. We used the CO and 13CO observations and the RADEX model to derive the physical conditions in the warm molecular gas layers. Results. The CO J = 19–18 line is detected unambiguously everywhere in the observed region, in the Bar, and in front and behind of it. In the Bar, the most striking features are several knots of enhanced emission that probably result from column and/or volume density enhancements. The corresponding structures are most likely even smaller than what PACS is able to resolve. The high-J CO line mostly arises from the warm edge of the Orion Bar PDR, while the lower-J lines arise from a colder region farther inside the molecular cloud. Even if it is slightly shifted farther into the PDR, the high-J CO emission peaks are very close to the H/H2 dissociation front, as traced by the peaks of H2 vibrational emission. Our results also suggest that the high-J CO emitting gas is mainly excited by photoelectric heating. The CO J = 19–18/J = 12–11 line intensity ratio peaks in front of the CO J = 19–18 emission between the dissociation and ionization fronts, where the PAH emission also peak. A warm or hot molecular gas could thus be present in the atomic region where the intense UV radiation is mostly unshielded. In agreement with recent ALMA detections, low column densities of hot molecular gas seem to exist between the ionization and dissociation fronts. As found in other studies, the best fit with RADEX modeling for beam-averaged physical conditions is for a density of 106 cm−3 and a high thermal pressure (P∕k = nH × T) of ~1–2 × 108 K cm−3. Conclusions. The high-J CO emission is concentrated close to the dissociation front in the Orion Bar. Hot CO may also lie in the atomic PDR between the ionization and dissociation fronts, which is consistent with the dynamical and photoevaporation effects.

International audience; Mid-IR (5-17~mum) Spectro-Imaging observations towards several hundred sky positions were obtained with the Infrared Space Observatory Camera (ISOCAM), and a Circular Variable Filter (CVF) that provided spectral resolution R = lambda/Delta lambda ~ 45 over a 3' × 3' field (for 6, and 12´´ pixels, proportionally smaller for the observations carried with the 3, and 1.5´´ pixels). The wavelength range includes dust bands - in particular several of the aromatic carbon bands - fine structure lines from ionized gas, and H2 rotational lines. The observed fields comprise nearby and distant galaxies, Galactic and Extragalactic star forming regions, the Galactic diffuse emission, nearby molecular clouds, infrared cirrus, young stellar objects, evolved stars, and Solar System targets. We present the final data reduction procedure that improves on the standard pipeline reduction in several ways, mainly the subtraction of zodiacal light, that of stray light associated with the uniform, most often dominant, emission component and the correction of a pixel dependent wavelength shift. We also propose a correction of the largest astrometry errors introduced by the optics. We have processed most of the ISOCAM CVF observations and made the results available on the ISO archive for public use. The processed data may be routinely used over the full sensitivity range of the instrument down to a brightness as faint as a few % of the zodiacal emission. For extended emission, the CVF observations represent a data base with a lasting value which can serve many scientific goals and motivate follow-up observations in particular with the Spitzer Space Telescope spectrometer.

We present ISOCAM observations (5-18 µm) of the Horsehead nebula, together with observations of the (J = 1−0) and (J = 2−1) transitions of 12 CO, 13 CO and C 18 O taken at the IRAM 30-m telescope. The Horsehead nebula presents a typical photodissociation region illuminated by the O9.5 V system σ Ori. The ISOCAM emission is due to very small particles transiently heated to high temperature each time they absorb a UV photon. A very sharp filament (width: ∼10 �� or ∼0.02 pc) is detected by ISOCAM at the illuminated edge of the nebula. This filament is due to a combined effect of steep increase of the column density and extinction of incident radiation, on typical sizes below ∼0.01 pc. Both the three-dimensional shape and the local density of the illuminated interface are strongly constrained. The dense material forming the edge of the Horsehead nebula appears illuminated edge-on by σ Ori, and the particles located beyond the border should not be affected by the incident radiation field. This structure may be due to dense filaments in the parental cloud which have shielded the material located in their shadow from the photo-dissociating radiations. The measurement of the penetration depth of the incident radiation from the infrared data (∼0.01 pc) gives a density of a few 10 4 cm −3 just behind the bright filament. This value is comparable to the estimate of the density beyond the edge and deduced from our molecular observations, and also to the density behind the ionization front calculated in the stationary case. The material behind the illuminated edge could also be non-homogeneous, with clump sizes significantly smaller than the observed penetration depth of ∼0.01 pc. In that case no upper limit on the average density just behind the illuminated edge can be given.

We present submillimetre observations obtained using the balloon-borne experiment PRONAOS/SPM, from 200 to 600 μm with an angular resolution of 2-3.5', of a quiescent dense filament (typically A V ∼ 4) in the Taurus molecular complex. This filament, like many other molecular clouds, presents a deficit in its IRAS I 6 0 μ m /I 1 0 0 μ m flux ratio in comparison with the diffuse interstellar medium. We show, from the combination of the PRONAOS/SPM and IRAS data, that, inside the filament, there is no evidence for emission from the transiently heated small particles responsible for the 60 μm emission, and that the temperature of large grains in thermal equilibrium with the radiation field is reduced in the inner parts of the filament. The temperature is as low as 12.1 + 0 . 2 - 0 . 1 K with β = 1.9 ′ 0.2 (or 12.0 + 0 . 2 - 0 . 1 K using β = 2) toward the filament centre. These phenomena are responsible for the IRAS colour ratio observed toward the filament. In order to explain this cold temperature, we have developed a model for the emission from the filament using star counts from the 2MASS catalog as an independent tracer of the total column density and a radiative transfer code. We first use the optical properties of the dust from the standard model of Desert et al. (1990). The computed brightness profiles fail to reproduce the data inside the filament, showing that the dust properties change inside the filament. An agreement between data and model can be found by removing all the transiently heated particles from the densest parts of the filament, and multiplying the submillimetre emissivity by a significant factor, 3.4 + 0 . 3 - 0 . 7 (for typically n H > 3 ′ 1 × 10 3 cm - 3 , A V > 2.1 ′ 0.5), We show that grain-grain coagulation into fluffy aggregates may occur inside the filament, explaining both the deficit of small grain abundance and the submillimetre emissivity enhancement of the large grains.

We present imaging and spectroscopic observations of dust and gas (H2) emission, obtained with ISO, from the western edge of the ρ Ophiuchi molecular cloud illuminated by the B2 star HD147889 (χ ∼ 400). This photodissociation region (PDR) is one of the nearest PDRs to the Sun (d = 135 ± 15 pc from the stellar parallax) and the layer of UV light penetration and of H2 emission is spatially resolved. It is therefore an ideal target to test the prediction of models on the integrated fluxes but also on the spatial distribution. The emission from dust heated by the external UV radiation, from collisionally excited and fluorescent H2 are observed to coincide spatially. The spectroscopic data, obtained with ISO-SWS, allows us to estimate the gas temperature to be 300-345 K in the H2 emitting layer, in which the ortho-to-para H2 ratio is about 1 or significantly smaller than the equilibrium ratio (∼3 at that temperature). We interpret this data with an equilibrium PDR model. In this low excitation PDR, the gas heat budget is dominated by the contribution of the photoelectric heating from very small grains and polycyclic aromatic hydrocarbons (PAHs). With the standard PAH abundance ((C/H)PAH � 5 × 10 −5 ), we find that the H2 formation rate Rf must be high in warm gas (∼6 times the rate derived by Jura, 1975), in order to account for the observed H2 emission. This result and the spatial coincidence between the PAHs and H2 emission suggest that H2 forms efficiently by chemisorption on the PAHs surface. If the latter interpretation is correct, the enhancement in Rf may also result from an increased PAH abundance: assuming that Rf scales with the PAH abundance, the observed H2 excitation is well explained with Rf � 1 × 10 −16 cm 3 s −1 at Tgas = 330 K (∼3 times the rate derived by Jura 1975) and (C/H)PAH � 7.5 × 10 −5 .

We present ISOCAM observations (5 18 m) of the southern part of Orion B, including the reflection nebula NGC 2023 and the Horsehead nebula illuminated by the B star HD 37903 and the O star Orionis, respectively. Due to the limited radiation eld, the emission in these wavelengths is due to very small particles which are heated each time they absorb a UV photon. A lamentary structure is detected at small angular scales (down to the angular resolution of 6 00 ) on top of a smoother background. The particular case of the Horsehead nebula suggests that the laments in general result from the illuminated surfaces of dense structures, while the smoother background comes from lower density matters probably ionised. Striking spatial variations of the infrared colour (5 8.5 m/12{18 m) are also detected. Spectroscopic observations show that they are due to variations of the intensity of the aromatic features (especially at 7:7 m) relative to a continuum emission increasing in intensity towards longer wavelengths. The contribution of the continuum relative to the aromatic features appears signicantly higher at the illuminated surfaces of dense structures than in lower density matter. This eect could be the signature of the evolution of the very small particles from shielded molecular material to photo-dissociated and photo-ionised matter. We also show that size segregation due to grain dynamics in uni-directional radiation elds may play a major role.

We present new data processing techniques that allow to correct the main instrumental effects that degrade the images obtained by ISOCAM, the camera on board the Infrared Space Observatory (ISO). Our techniques take advantage of the fact that a position on the sky has been observed by several pixels at different times. We use this information (1) to correct the long term variation of the detector response, (2) to correct memory effects after glitches and point sources, and (3) to refine the deglitching process. Our new method allows the detection of faint extended emission with contrast smaller than 1% of the zodiacal background. The data reduction corrects instrumental effects to the point where the noise in the final map is dominated by the readout and the photon noises. All raster ISOCAM observations can benefit from the data processing described here. These techniques could also be applied to other raster type observations (e.g. ISOPHOT or IRAC on SIRTF)., 13 pages, 10 figures, to be published in Astronomy and Astrophysics Supplement Series

The detector of the Long Wavelength chan- nel of the ISOCAM camera on-board the Infrared Space Observatory (ISO) presents systematic memory eects of the response which can bias the photometry by a factor as high as 40%. We show that, for uniform illumination, the non linear model developed by Fouks and Schubert (1995) for the detector of PHT-S channel of the ISOPHOT in- strument allows us to describe the LW-ISOCAM response to upward and downward steps of flux with an accuracy better than 1%. For each pixel of the detector, only two parameters have to be used. These parameters are con- stant over the whole life of ISO. On the other hand, spatial variations from pixel to pixel are signicant. We present an inversion method of the Fouks-Schubert model. Without any tting and for low contrasted illumi- nation, data are corrected with a photometric accuracy of 1%, which is typically ten times better than previous methods. We suggest that the same approach could be used for others ISO detectors, and in the future for other experiments.

Nous etudions deux problemes de controle optimal se rapportant aux equations de Navier-Stokes stationnaires. L'etat du systeme est le champ de vitesses dans le fluide, et les controles sont, soit les forces volumiques, soit le flux de chaleur au bord; dans le second cas, les equations de Navier-Stokes sont couplees avec l'equation de convection-diffusion pour la temperature, dans l'approximation de Boussinesq. On cherche a minimiser une fonctionnelle caracterisant l'etat de la turbulence a l'interieur du fluide, eventuellement sous certaines contraintes portant sur les controles

We present far-infrared spectra and maps of the DR21 molecular cloud core between 196 and 671 μm, using the Herschel-SPIRE spectrometer.\ud Nineteen molecular lines originating from CO, 13CO, HCO+ and H2O, plus lines of [N ii] and [CI] were recorded, including several transitions not\ud previously detected. The CO lines are excited in warm gas with Tkin ∼ 125 K and nH2\ud ∼ 7×104 cm−3, CO column density N(CO) ∼ 3.5×1018 cm−2\ud and a filling factor of ∼12%, and appear to trace gas associated with an outflow. The rotational temperature analysis incorporating observations\ud from ground-based telescopes reveals an additional lower excitation CO compoment which has a temperature ∼78 K and N(CO) ∼ 4.5×1021 cm−2.