Emission and extinction of dust
Interstellar dust grains play an important role in the physics and chemistry of the interstellar medium. Indeed, the photoelectric effect on dust grains is the primary heating source for the gas and
molecular hydrogen can only be formed at the surface of grains. During the lifecycle of interstellar matter from the dense phases (nH > 10^4 cm-3) to the diffuse medium (nH < 100 cm-3), dust grains are processed by stellar photons and gas dynamics (shocks, turbulence). Such processing primarily affects the size distribution of grains through fragmentation, erosion and coagulation. In addition, the structure and optical properties of grains also change.
The properties of interstellar dust grains are constrained with observational data from the various phases of the interstellar medium. Over the years, our group has developed a strong expertise in the data processing and scientific analysis of the IR-to-submillimeter (submm) emission of dust grains. Spectroscopic data from the ISO and now Spitzer missions have allowed us to deduce the optical properties of the smallest grains called Polycyclic Aromatic Hydrocarbons or PAHs (Flagey et al. 2006; Verstraete et al. 2001). These results have been incorporated in a new dust model which describes simultaneously the extinction by and emission from grains while respecting the abundance constraints. This model is based on Cloudy which now includes a detailed treatment of grain physics as well as a Mie code to compute opacities of a variety of dust candidates.
Model of interstellar dust emission
On the next figure (left), the results of this model are compared to spectral and broadband data in the case of the diffuse medium representative of the interstellar medium at large scales. The carbon based grains (PAHs and graphitic spheres) have a total abundance of 180 ppm with 72 ppm in particles of radius less than 5 nm. The silicates contain most interstellar Si (33 ppm). Noteworthy facts are: (i) PAHs contain less than 50 C-atoms (ii) nanometric silicates are required to explain the linear component in the extinction curve (Fitzpatrick & Massa 1988). The figure on the right shows our dust model compared to an ISO-SWS spectrum in the case of a denser medium heated by nearby massive stars, the Orion Bar. In this case, PAHs are efficiently photofragmented (abundance reduced by a factor 3 with respect to the diffuse medium). Moreover, nanometric silicates contribute notably to the 10 microns flux as ISOCAM-CVF data has already shown (Cesarsky et al. 2000).
|Model of interstellar dust emission in the diffuse interstellar medium (left) and in the Orion nebula (right).|
Observations at submm wavelength range allow to constrain the properties of big grains, ~0.1 microns in size. The analysis of data from the balloon-born instrument PRONAOS-SPM on the emission of big grains between 200 and 600 microns clearly shows the coagulation of small, nanometric grains onto the bigger grains (Stepnik et al. 2003, Abergel et al. 2006). To interpret these results, our group has developed a model based on the Discrete Dipole Approximation (DDA, Draine 1988) to compute the optical properties and submm emission of coagulated grains of various composition, structure and shape which will be very valuable to characterize the evolution of dust grains throughout the lifecycle of interstellar matter. With the short advent of the Herschel/Planck space missions which will observe the submm-to-cm sky, our group is further involved in the development of scientific tools to analyse the dust emission in this wavelength range.
Anomalous microwave emission
Within the framework of a coordinated research program including funding and in partnership with the CESR (Toulouse), we are developing dust emission models which include the spcecific physics of big grains at low temperature, low internal energy (Agladze et al. 1996, Boudet et al. 2005). The long wavelength data will also bring original constraints on small grains. Indeed, a centimetric emission called "anomalous component" is observed at large scale in the Galaxy and is related to the dust. The emission of spinning PAHs is so far the most convincing explanation but other explanations involving big grains must also considered (see refs. in Hinshaw et al. 2006). This is the topic of the PhD thesis of Nathalie Ysard. The tools she develops to model the rotation of PAHs are also applicable to the alignment of big grains and the polarization of the dust emission, the dominant foreground to the CMB polarization. The recent discovery by the Astrochemistry and Origins group at IAS that silicates may form metallic iron balls during their evolution certainly bears important consequence for the grain alignment.
People involved : A. Abergel, F. Boulanger, N. Flagey, E. Habart, A. Jones, M.-A. Miville-Deschênes, L. Verstraete, N. Ysard