Evolution des poussières interstellaires : apport des données de l'observatoire spatial Herschel

Abstract : Interstellar dust grains are nanometre to micrometer-sized particles. Although a weak proportion of the total interstellar mass is at solid state, dust plays a fundamental role in the evolution of the interstellar medium (ISM) and of the galaxy itself. Grains can be observed in the UV and visible wavelength through extinction whereas their emission is in the infrared to sub-millimetre range. Astrophysical observations combined to numerical models and laboratory studies of dust analogues improve our comprehension of the nature and the physics of interstellar grains. For example, evidence of dust evolution in the interstellar medium are now numerous, even if the physical processes responsible of this evolution are still poorly understood. Understanding how grains evolve with physical conditions requires observations of various environments. Photodissociation regions (PDRs) are zones of the ISM where the radiation field and the local density vary on short spatial scales (~10- 20 arcsec). Moreover the many gas tracers offer the opportunity to constraint efficiently the physical conditions within PDRs. Past missions such as ISO and Spitzer allow to study the evolution of dust in the near-Infrared range. At longer wavelengths, where the grains at thermal equilibrium with the radiation dominate the emission, instruments rarely resolved the spatial emission in PDRs. PACS and SPIRE instruments onboard Herschel Space Observatory provide spectro-photometric data between 70 and 500 µm. Their high spatial resolution (from 5 to 35 arcmin) makes these observations ideal for the study of dust evolution in PDRs. We present here an analysis of Herschel observations of three PDRs: the Orion Bar, the Horsehead and NGC 7023 East, characterized by different physical conditions. By combining these data with shorter wavelength observations from Spitzer, we can study the dust emission spectrum from 3.6 to 500 µm at different positions within the PDR. Intensity profiles are extracted along the PDR at each wavelength and spatially compared. We highlight a shift between the position of the emission peak: the longest the wavelength, the furthest the peak from the exciting star. This is a consequence of the radiative transfer in the PDR as shown from a plane parallel transfer code coupled with a dust model. The comparison between the observed and the modelled profiles computed with typical diffuse dust abundances and properties shows differences linked to dust evolution in each studied PDR. These discrepancies between the data and the model indicate a lower Polycyclic Aromatic Hydrocarbon (PAH, the smallest dust component) abundance in the PDR than in the diffuse medium suggesting photo-destruction and/or PAH sticking on larger grains. This could be accompanied by an increase of big grain emissivity linked to coagulation. Herschel's observations of PDR also offer the chance to probe the variations of the grains at thermal equilibrium with the radiation through PDRs. A modified blackbody fit allows to compute an optical depth, a temperature and a dust spectral emissivity index. Those two last parameters are clearly anticorrelated, which confirms previous works. However, comparing the temperature and emissivity index dependence in different regions shows various behaviours, which excludes a universal law between these parameters. This result opens new perspectives in the study of the dust evolution in the interstellar medium.
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Heddy Arab. Evolution des poussières interstellaires : apport des données de l'observatoire spatial Herschel. Autre. Université Paris Sud - Paris XI, 2012. Français. ⟨NNT : 2012PA112199⟩. ⟨tel-00829096⟩

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