Very Small Dust Particles and Chemistry in Photodissociation Regions: from ISO to Spitzer C. Joblin 1, A. Abergel 2, J.-P. Bernard 1, O. Berne 1, F. Boulanger 2, J. Cernicharo 3, M. Compiègne 2, Y. Deville 4, M. Gerin 5, J.R. Goicoechea 5, E. Habart 2, J. Le Bourlot 6, J.-P. Maillard 7, M. Rapacioli 1, W. Reach 8, A. Simon 1, J.-D. Smith 9, D. Teyssier 3, 10, L. Verstraete 2 1 CESR, CNRS et Univ. Paul Sabatier, OMP, Toulouse, France 2 IAS, CNRS et Univ. Paris Sud, Orsay, France 3 DAMIR, IEM, CSIC, Madrid, Spain 4 LATT, CNRS et Univ. Paul Sabatier, OMP, Toulouse, France Introduction Since the IRAS mission, it is well known that there exists a population of very small particles which are transiently heated by the absorption of single UV photons and emit in the mid-IR range. Polycyclic aromatic hydrocarbons (PAHs) were proposed to account for the well-known Aromatic IR Bands (AIBs) between 3.3 and 12.7  m. Another component, called very small grains (VSGs; Désert et al. 1990, A&A 237, 215 ), was introduced as the carrier of the emission in the 25  m IRAS band. The very small dust particles (PAHs and VSGs) are now considered to play a major role in the physics and chemistry of the interstellar medium. They dominate the heating of interstellar gas through photoelectric effect. They are also suspected to play a major role in the formation of H 2 in particular in Photo-Dissociation Regions (PDRs) where UV photons heat the dust making inefficient the physisorption and diffusion of H atoms at the surface of grains, the processes which are usually invoked to form H 2 ( Habart et al. 2004, A&A 414, 531 ). In PDRs, the UV field varies as a function of depth within the cloud, and spatial studies give a unique opportunity to follow the evolution of the dust populations and the molecular content as a function of the excitation conditions. Spectro-imagery in the mid-IR (5-16  m) was possible with the CAM camera on board the Infrared Space Observatory (ISO). A striking result from these observations is the presence of continuum emission in PDRs far from exciting sources ( Abergel et al. 2002, A&A 389, 239 ). This reveals the presence of very small dust particles transiently heated by single UV photons. The band-to-continuum intensity ratio is highly variable showing that the continuum is not carried by PAHs but rather by VSGs. Further analysis of the data ( Rapacioli et al. 2005, A&A 429, 193 ) allowed to extract three spectra : two with band emission only (attributed to PAH cations and neutrals) and one with broad band and continuum emission (attributed to VSGs). The authors found a drop in the VSG emission at the cloud edge which is correlated with the increase in the PAH emission. They proposed that VSGs could be aggregates of PAHs which photoevaporate at the surface of clouds. This work can now continue thanks to the unique spectro-imagery capabilities of the Spitzer Space Telescope ( ; ; see Special Issue of ApJ Supp. 154, 2004 ). This poster describes the work which is in progress in our team and reports the very first results of the SPECPDR programme.  Selection of 10 galactic PDRs (G 0 ~ ; T eff =10,000-37,000 K) + 2 extragalactic sources : M31, SMC (low UV field, low metallicity)  Use the 3 instruments on board Spitzer: - the IRS slit-spectrometer in its spectral mapping mode at low resolution (SL and LL sub-modules:  m; R~100) and high res. (SH and LH sub-modules:  m; R~600) - the IRAC camera (4 filters: 3.6, 4.5, 5.8 and 8.0  m) - the MIPS photometer in the 24.0  m filter and in the SED mode (55-95  m, very low-res. spectroscopy: R~10). Observations Data reduction 8 IPAC, Caltech, Pasadena CA, USA 9 Steward Obs., Univ Arizona, Tucson AZ, USA 10 SRON, Groningen, The Netherlands Two softwares were used independently for the data reduction 1- the IAS software based on tools developed for the ISOCAM cvf data processing ( Boulanger et al. 2005, A&A 436, 1151 ), 2- the CUBISM software specifically developed in the frame of the SINGS legacy programme ( Kennicutt et al. 2003, PASP 115, 928 ). The main steps are:  reading of the BCD files which contain the flux along the slit for a set of wavelengths,  construction of cubes for each sub-module (SL1, SL2, LL1, LL2) including identification and removal of bad pixels and photometric calibration,  reprojection to build the full cube. The main problems which are encountered concern: - the correction for the bad (rogue) pixels which cannot be performed because of the lack of sky backgrounds in our observations, - the flux calibration, in particular a proper correction for diffraction losses for extended sources, is not available yet. Ground-based observations Fundamental studies and models Our team is involved in several actions related to the scientific interpretation of the SPECPDR programme, in particular:  fundamental studies are performed on PAHs and on carbonaceous VSG candidates such as PAH clusters and PAH-Fe complexes,  modelling to combine dust models and radiative transfer codes,  modelling of the H 2 formation and excitation mechanisms in PDRs. First results 5-8 µm = SL µm = SL µm = LL µm = LL1 The Horsehead nebula Map 1: spectral mapping of the Horsehead filament with IRS: slit positions superposed on the 2.12  m H 2 image observed with SOFI at NTT ( Habart et al. 2005, A&A 437, 177 ), Spectrum 1 : full IRS spectrum. Maps 2 and 3 : images at specific wavelengths. Ced 201 and NGC 7023 East Two PDRs with no associated ionized gas and two different geometries: shell /filament (maps 4-5). The IR intensity is comparable as shown by spectrum 2. A blind source separation method (FastICA algorithm) was applied to the data leading to the extraction of two characteristic spectra : one PAH spectrum (sp. 3) with bands only and one VSG spectrum (sp. 4) with continuum. Both spectra are centred and normalized. The PAH emission is located at the edge of the PDR (map 6). This nicely extends the work of Rapacioli et al. ( 2005, A&A ). Perspectives Current plans consist in :  improving the data reduction and analysis with blind source separation methods in order to reveal new bands at longer wavelengths which could provide information on the chemical identity of their carriers. The 16.4  m band was identified as one of the AIB by Moutou et al. ( 2000, A&A 354, L17 ). New bands of unknown origin were reported in the Spitzer data of NGC 7023 at 17.4 and 19.0  m ( Werner et al. 2004, ApJ Supp. 154, 309 ). Other bands could be present at longer wavelengths but are more difficult to extract because of the presence of strong continuum emission.  constraining and improving dust models. The observations point to the need of changing the abundance of the dust populations as a function of the depth inside the cloud. For instance, free PAHs are abundant only at the illuminated edges of clouds,  getting some insight into the H 2 formation mechanism(s) and the ortho- to para- conversion mechanism (see Le Bourlot 2000, A&A 360, 656 ). The ultimate goal is to describe in an unified model both dust components and gas species as evolving chemical entities under the action of UV photons and collisions. Spectrum 2: intensity averaged over the brightest pixels. Maps 4-5: images at 20  m (2.8’  2.3’, no reprojection) of Ced201 (top) and NGC7023E (bottom). Spectra 3 and 4 Extracted PAH and VSG spectra Map 6: spatial distributions of the PAH (contours) and VSG (colours) emissions. In addition to the SPECPDR programme, observations are performed at ground-based observatories (30m IRAM, 20m Onsala telescopes). The main objectives are:  to get more insight into the carbon chemistry by studying the spatial distribution of the small hydrocarbons,  to complete the information on the dust populations by mapping the emission of the coldest grains at 1.2 mm (map 7). This also provides important information on the geometry and the amount of dust along the line of sight. Map 7: emission at 1.2 mm measured by MAMBO at IRAM in NGC 7023 (the bright northern PDR and the East PDR are clearly seen). 5 LERMA, Observatoire de Paris et Ecole Normale Supérieure, Paris, France 6 LUTH, Observatoire de Paris, Meudon, France 7 IAP, CNRS et Univ. P. & M. Curie, Paris, France Map 1