InterStellar Medium (ISM) :

1. Statistical Study of Dust Emissionin Molecular Clouds A Sakura PAI CNRS/JSPS program InterStellar Medium (ISM) : Objectives : Fig. 1 : Average diffuse ISM spectrum scaled to a column density of 4.e20 H/cm2 (Av=0.2 mag). The ISOCAM and FIRAS spectra are shown as black curves and the DIRBE broad band measurements as black triangles. The sensitivities obtainable with Spitzer are shown by the pink and blue boxes and the red curve. The spectral range covered by ASTRO-F (green) and ground measurements of SCUBA (grey) are also shown. The different components of the diffuse matter in the InterStellar Medium (ISM) in our Galaxy include :  Gas : - neutral hydrogen (H) - molecules (H2, CO, CS,…)  Dust : - PAHs (Polycyclic Aromatic Hydrocarbon) - VSGs (Very Small Grains) - BGs (Big Grains) The diffuse matter is illuminated by the Interstellar Radiation Field (ISRF) produced by the many stars in the Galaxy. In the most diffuse regions of the ISM, the gas is in the atomic form, because of the fast molecular dissociations caused by the ISRF. Under the combined effect of gravitation and magnetic field, some regions of the ISM can contract. The radiation field is then shielded by the dust and the cooling of the gas accelerates the contraction. The molecules are then self-protected against photo-dissociation and so the dense medium is essentially molecular. Dust grains in the ISM play a major role in the chemistry and the structure of the ISM. In particular, the most abundant molecules in the gas phase of the ISM, like molecular Hydrogen (H2), are formed at the surface of grains. The dust also partially controls the thermal behaviour of the gas through photo-electric effect and cooling. The "Statistical Study of Dust Emission in Molecular Clouds" is a research program currently supported by CNRS/JSPS under the PAI (Programmes d'Actions Integrées) of the French ministery of foreign affaires. The main objectives of this research program are: 1/ Determine the dust properties and its role in the physics of star forming regions. 2/ Identify the physical processes at the origin of the IR color variations, in and out molecular clouds, in relation with the nature and temperature of the grains and the dynamics of the molecular gas. This study will be conducted using the Spitzer data (essentially the publically available legacy survey data), the NANTEN molecular survey, as well as large-scale extinction maps obtained from star counts. We also plan to use the Astro-F data, when it becomes available. The comparison will be carried over most the nearby star forming regions with an angular resolution of few arc minutes, limited by the molecular data resolution. Dust Emission: Dust in the ISM absorbs the star-light, causing significant obscuration in the UV and visible (fig2). The absorbed energy is reradiated by dust in the IR and far-IR. The dust present in the ISM can be evidenced through its thermal emission. The emission from Big Grains ressembles a grey-body law with a temperature of about 20K and a peak of the emission located in far-IR (see Fig 1). This temperature varies significantly from regions to regions, from less than 10 K in the densest and most shielded regions to 100K or more near star forming regions. Unlike the Big grains, the small dust particulesare not in thermal equilibrium with the ISRF, and are heated to high vibrational temperatures (700-1000K) by single UV photons. Thus, they undergo strong thermal fluctuations. The emission in the Near and Mid IR is dominated by small dust particules whereas the Big Grains emission dominates the far IR emission (see Fig. 1). The PAHsplay an important role in the heating of the gas via the photo-electric effect. They may actually be formed in the diffuse medium, perhaps following collisions between bigger dust particules. The VSGs nature is very badly constrained because of the lack of observations between 10 and 100 microns. The Big Grains are probably composed of graphite and amorphous silicate. They contribute efficiently to the ISM cooling in the densest regions, leading to star formation. Spitzer image Fig. 2 (left) : Map of the dark molecular globule IC 1396 obtained by the SPITZER satellite (IRAC) showing a spectacular contrast between the IR emission (Spitzer image) and the opaque cloud seen in extinction in visible light (Optival image). Courtesy of the SSC, W. Reach. Optical image Galactic Latitude (degrees) Fig. 3 (right) : Nagoya/NANTEN survey has revealed the complexity of the molecular emission along the Galactic plane, bet also towards high latitude regions closer to earth. Galactic Longitude (degrees) The Spitzer Space Telescope (formerly SIRTF, the Space Infrared Telescope Facility) was launched into space by a Delta rocket from Cape Canaveral, Florida on 25 August 2003. Spitzer is the largest infrared telescope ever launched into space. During its 2.5-year mission, Spitzer will obtain images and spectra by detecting the infrared energy, or heat, radiated by objects in space between wavelengths of 3 and 180 microns. SPITZER is composed of 3 science instruments : - The Infrared Array Camera (IRAC) which provides imaging capabilities at near- and mid-infrared wavelengths -The Infrared Spectrograph (IRS) which provides both high- and low-resolution spectroscopy at mid-infrared wavelengths. - The Multiband Imaging Photometer for Spitzer (MIPS) which provides imaging and limited spectroscopic data at far-infrared wavelengths. SPITZER NANTEN NANTEN is a 4m millimeter-submillimeter wave telescope installed at the Las Campanas Observatory in Chile. Today, the molecular survey obtained using the NANTEN telescope is the most complete. It covers 2/3 of the galactic plane and encompasses most of the nearby star forming regions at a resolution of a few arc minutes. Since 1996, the Nagoya University have carried out surveying molecular clouds, mainly in the J=1-0 transitions of the CO isotopes of 12CO, 13CO, and 18CO at a beam size of 2.7 arcmin toward more than a million points (See Fig. 3). This survey succeeded to reveal the CO distribution in the Galactic plane with higher spatial resolution and sensitivity than the Colombia survey. In the near future, its surface will be improved, allowing observations of the high frequency CO lines (345-500 GHz). These observations will bring new information about strongly excited molecular gas. Fig. 4 : The NANTEN telescope ASTRO-F will be Japan’s first infrared-ray astronomical satellite to perform "survey observation," an all-sky survey at infrared wavelengths, with greater sensitivity and higher resolution than those achieved by the Infrared Astronomy Satellite (IRAS). ASTRO-F will make a second generation survey which meets current astronomer's expectations. ASTRO-F will perform the following observations : - An unbiased all-sky survey at wavelengths from 50 to 200 microns. - High sensitivity imaging and spectroscopic observations covering more than several tens of square degrees at wavelengths from 2 to 200 microns. Observational instruments on board the ASTRO-F are the Far Infrared Surveyor (FIS) and the Infrared Camera (IRC) for low- to mid-range resolution spectroscopy. ASTRO-F EXTINCTION The determination of dust emissivity requires a good determination of the total colum density (HI, H2, molecular). For this purpose, we will use the molecular data combined with dust extinction maps obtained from the star counts and stellar colors (see Fig. 5). Morever, the use of extinction maps will allow to evidence regions surrounding the CO molecular clouds, where the H2 molecule survives photodissociation, but not CO. This information is uniquely provided by extinction maps, since H2 is not directly observable. Fig. 5 : Large scale Galactic maps of the extinction by interstellar dust, based on the digitized sky survey I (Dobashi et al. 2004). Y. Fukui(1), A. Kawamura(1), A. Mizuno(1), N. Mizuno(1), T. Onishi(1), H. Sasago (1), H. Shibai(1) K. Dobashi (5), J.P. Bernard(2), C. Joblin (2), C. Meny (2), R. Paladini (2), D. Paradis (2), I. Ristorcelli (2) A. Abergel (3), F. Boulanger(3), G. Lagache(3), N. Bot (4), L. Cambresy (4), A. Coulais(6) • Nagoya University, Nagoya, Japan, (2) CESR, Toulouse, France, (3) IAS, Orsay, France • (4) Obs. Strasbourg, Strasbourg, France, (5) Tokyo Gakugei University, Tokyo, Japan, (6) LERMA, Paris, France