PRONAOS was born as a project in the mid eighties. The Far Infrared Space Telescope (now Herschel), a Cornerstone of thehorizon 2000 plan of ESA, was the clear future of submillimetreastronomy, after severalballoon experiments and the tremendous success of the IRAS satellitein the neighbour field of infrared astronomy. Guy Serra and otherFrench scientists proposed to build a learning process in this newfield around the PRONAOS project, a technical and scientific precursorto future submm space missions. This project was fully supported byCNES and by CNRS (IAS and CESR at the first place). PRONAOS (ProjetNational d'AstronOmie Submillimétrique) was based on the use of a twometer telescope aboard a stratospheric balloon-borne gondola. The sizeand the complexity of the instrument made PRONAOS a very unusualballoon-borne payload. Along the 15 years of the project, and threeflights, a lot of learning processes took place. The main goals ofthis enterprise were reached. New and sometimes surprising scientificresults came out from the observations. Teams learned criticalscientific and technical lessons from this pioneering experiment. Somecontributions to Herschel benefit directly from this experience, andthe project Planck, a satellite of ESA, was born inside the PRONAOSteam. This project was also a major professional and human experiencefor young (and also mature) scientists.

The spectroscopic properties in the FIR-mm range of analogue materials forinterstellar refractory grains, such as silicates and carbon material, arereviewed. The mechanisms which determine the absorption and its dependence oncomposition, structure and temperature are discussed.

We have developed a dust emission model in order to reproduce the evolution of the far-infrared (FIR) to sub-millimetre (sub-mm) dust emission properties which is observed in the transition between the diffuse and the dense interstellar medium (ISM). We propose that the coagulation of grains into fluffy aggregates could be the process responsible for the observed dust evolution from the diffuse to the dense ISM in the Galaxy. In dense clouds, dust coagulation can explain the increase in grain size, the decrease in the grain temperature in excess of that predicted by extinction, the sub-mm emissivity excess and the decrease in the small grain abundance.

We have studied the effects of the laboratory-measured, temperature-dependent dust emissivity indices on the FIR/sub-mmemission properties of amorphous silicate and amorphous carbonaceousdust. When the experimentally-determined temperature dependence of thedust emissivity index is taken into account we find temperatures thatcan differ by as much as 1.5 K and emissivity indices that can differby ~15%, with respect to the values obtained using publishedrefractive index data as is to calculate these quantities. Inaddition, we find that the observed diffuse ISM dust temperature of~17.5 K and emissivity index of 2 can seemingly be produced witha mixture of amorphous carbon and silicate grains only iftemperature-dependent dust emissivity indices are adopted. We findthat dust masses determined using the temperature-dependent dustemissivity indices are not significantly different from those derivedusing standard dust properties.

Interstellar gas is thought to be heated by photoelectronsejected by small grains and PAHs. This process is reviewed and evidence for its importance is examined.In particular, the importance of the PAH charge for the photo electric effect and for the spectral characteristics of interstellar PAHs is discussed.

We present in this paper a review of an astrochemistry study concerning interactions between metals and molecules. The possibility of such an organometallic chemistry is based on the metal depletion in the interstellar medium (ISM) and the large abundance expected for polycyclic aromatic molecules (PAHs). Guy Serra was a pioneer of interstellar organometallic chemistry and contributed to promote this theory.

Strong and broad emission bands commonly attributed to polycyclic hydrocarbonsdominate the mid-infrared spectrum of interstellar matter. Early photo- andspectrometry by the ISO and IRTS missions has proved that these bands arewidespread in the diffuse interstellar medium. We have sampled the galacticdistribution of unidentified infrared band emission in the diffuse ISMcomponent. The remarkable insensitivity of the shape of the UIR band spectrum isseen in this sample; little variation with line strength or galactic positionis seen suggesting either a homogeneous history of the UIR band carriers withinthe Milky Way, convergent evolution of grains suppressing any differences seen inyounger populations or an emission mechanism highly insensitive to particle type and physical conditions.

The new astrochemistry experiment PIRENEA is presented.The set-up was designed to approach the physical conditionsthat prevail in the interstellar medium. It enables to performoriginal studies in particular when large molecules andnano-sized particles are involved. The experiment is nowoperational for science.

We present ISOCAM observations (5-18 μm) of illuminated surfaces of molecular clouds. The emission properties of the transiently heated small particles, whichdominate the emission observed byISOCAM, are analysed in relation with the spatial structure of the material. Striking spatial variations of the infrared colour(5-8.5 μm/12-18 μm) are detected. Spectroscopic observations show that they are due to variations of the intensity of the aromatic features(especially at 7.7 μm) relatively to the continuum emission increasing towards long wavelengths. Compared to the intensity of the continuum emission,the intensity of the aromatic features are significantly fainter at the illuminated surfaces of dense structures than in low density regions surrounding these densestructures. This effect could be the signature of photo-chemical evolution, size segregationdue to grain dynamics in uni-directional radiation fields, or abundance variations of very small particles.

Recent CO surveys and measurements of the magnetic field intensity anddirection have renewed the description of molecular clouds. They appear as alarge self-similar hierarchy of structures with a unique power law mass spectrum over 9 orders ofmagnitude in masses. No lower threshold has yet been observed for thishierarchy in non star forming clouds. At the opposite, in clouds which have started to form stars, the self-similarity breaks down at the scale of thedense cores. The wholehierarchy is fractal. Molecular clouds appear to be in virialbalance. The magnitude of the external pressure terms is largein non star forming complexes and very likely due to magneticstresses. The need for an external pressure decreases asthe star formation activity increases and the equality 2T+Ω=0 is reached only for the most massive substructures ofGMCs, on the verge of (or already) forming stars. The gas motions are turbulent and likely trans-Alfvénic, given the observed magneticfield intensities. The whole hierarchy of scales is characterized byan invariant, ρvl3/l, the transfer rate of kinetic energy. Molecular cloud cores appear to be supercritical and a static magneticfield cannot stabilize them against gravitationalinstability. MHD turbulence captures several of the characteristics ofmolecular clouds and is able, as long as it is fed, to prevent cloudsfrom collapsing. Its dissipation, alike in hydrodynamical turbulence,is concentrated in bursts, which are sheetlike structures.

The MidCourse Space eXperiment (MSX) surveyed theGalactic planewithin 5° latitude in four mid-infrared spectral bands. A setof full resolution (20'') 1.5°×1.5° images on 6'' pixelcenters has been created in each spectral band by co-adding all thesurvey data. A lower (1.2') resolution atlas of 10°×10°images provide large-scale panoramas of the plane. A new class ofobjects has been identified in the images, infrared dark clouds, whichare silhouetted against the mid-infrared background emission from theinterstellar medium in the Galactic plane. The IRAS ISSA platesindicate that these clouds are dark out to 100 μm. Submillimeteremission traces the form of the dark cloud and reveals coresindicative of class 0 protostars.

Wide area sub-millimeter mapping of nearby molecularclouds allows for the study of large scale structures such as theIntegral Shaped Filament in the Orion A cloud. Examination of these regions suggests that they are not equilibrium isothermal structures but rather require significant, and radially dependent, non-thermal support such as produced by helical magnetic fields Also observed in the large area maps are dense condensations with masses typical for stars. The mass distribution of these clumps is similar to the stellar initial mass function; however, the clumps appear stable against collapse. The clumps are clustered within the cores of molecular clouds and restricted to those locations where the molecular cloud column density is high (AV > 4). As well, the typical sub-millimeter clump reveals little or no emission from isotopes of CO, likely indicating that the combination of high density and low temperatures within the clumps provides an environment in which these molecules freeze-out onto dust grain surfaces.

Infrared Astronomy Group of the Tata Institute ofFundamental Research (TIFR)has an ongoing programme of high resolution mapping of star forming regionsin two trans-IRAS far infrared bands using a 1 metre balloon-borne telescope. Thetelescope has been flown many times from TIFR Balloon Facility, Hyderabad,India. The photometer consists of two bolometer arrays cooled to 0.3 K by aclosed cycle liquid 3He refrigerator. The effective wavelengths of thetwo bands are ~140 and 200 μm and the angular resolution of theMEM deconvolved maps is ~1'.2. These bands are suitable for the study ofcold sources (temperature ≤30 K). Using this system many Galactic starforming regions have been mapped. Using simultaneous maps in the two bands,maps of dust temperature and optical depth have also been obtained. Our mapsare combined with other available data and radiation transfer calculationshave been made. In this paper we will describe the telescope and give asample of the results obtained for some of the star forming regions inthe Galaxy.

We review the submillimetric PRONAOS observations in the direction of the IC 1318a nebular located at the edge of an ionized region in the Cygnus X complex. The extended emission measured in the PRONAOS four broadphotometric bands extending from 200 up to 1100 μm enables a fullcharacterization of the interstellar dust big grains component radiating atthermal equilibrium, leading to a temperature and column density mapping.

The ISOGAL survey discovered a population of narrow(down to 10 arcsec in size) absorbing filaments and globules which appear as dark features inthe 7 μm and 15 μm plates. These infrared dark clouds turn out to be the highly condensed parts of Giant Molecular Clouds (GMC's) kiloparsecs away from the Sun. We report here follow-up observations at radio-mm wavelengths conducted at the IRAM 30-m telescope, and present the first results of the physico-chemical analysis of the gas associated to the dense dust revealed in mid-IR.

The maximum luminosity of protostars forming in molecular clouds isinvestigated as a function of the parent cloud mass on the basis of arich cloud sample searched for in the literature. In total, we gathered499 molecular clouds among the published data, out of which 243clouds are found to be associated with candidates for protostars selectedfrom the IRAS point source catalog. A diagram of the maximum stellarluminosity in each cloud and the parent cloud mass shows that theyoung stars in the clouds associated withregions are apparentlymore luminous than those in clouds away from HIIregions over theentire cloud mass range investigated (1 < MCL/M⊙ < 106).In addition, wefound that there are well-defined upper and lower limits in themaximum stellar luminosity distribution with the lower limit having asteeper dependence on the cloud mass(LMAX ∝ MCL1.5) than the upperone (LMAX ∝ MCL0.8). All these featurescould be naturally accounted forif we assume that the luminosity function of protostars is controlled by thecloud mass and the external pressure imposed on the cloud surface. Weintroduce a simple model for the stellar luminosity function as afunction of these quantities.

L134N is a cold, starless cloud, very high above the galactic plane,close to us and well delineated in continuum dust emission maps. Thiscloud is considered to be representative of oxygen rich dark clouds(with the presence of SO, SO2, NO, ...). It is thus a goodreference together with TMC-1 to test astrochemical models. Thanks toISO, SCUBA and near IR wide field cameras, the detailed study of thedust has become possible in such cold and dark clouds. In parallel,progress in radiotelescope receiver sensitivity now allows to map weaklines on large surfaces. We have thus started a project to study bothdust and a few gaseous key species (CO, CS, SO and N2H+) toaddress several questions. We want to assess the quantity of dust andgas all over the cloud, study possible C18O and/or C17Odepletion towards dense cores, evaluate the structure of the gas, theabundance of CS and SO to possibly estimate the chemical age of thecloud (time dependent models show that the CS/SO ratio diminishes withtime) and evaluate the rare isotope abundances, especially 17Oand 34S in a first step. To constrain the molecular abundanceswith the highest possible confidence, we have observed severaltransitions for each species and each isotopomer. Though we haveobserved far less species than Dickens et al. [1], wehave done it on a larger area, including thus the strongest C18Opeak and two other peaks, with a better signal-to-noise ratio. Most ofthe data are already acquired. We present here preliminary results.

We provide insight into the origin of the far-IR to mm emission from the Galaxyby presenting a decomposition of the emission where we separate thecontributions from dust in the atomic gas and dust associated with quiescent molecular gas, as a function ofGalacto-centric distance. We find that the sub-mm dust emissivity per hydrogen change by afactor 3 between the diffuse ISM and molecular clouds, first observedon localized clouds in the Solar Neighborhood, applies on large scaleto the Molecular Ring. The decomposition leaves an emission excess inthe sub-mm with a very cold effective temperature, concentratedin the plane but with a brightness independent of Galacticlongitude. This emission might be associated with interstellar matterin the outer Galaxy not traced by H I nor CO emission.