Interstellar dust within the life cycle of the interstellar medium

Abstract

Cosmic dust is omnipresent in the Universe. Its presence influences the evolution of the astronomical objects which in turn modify its physical and chemical properties. The nature of cosmic dust, its intimate coupling with its environment, constitute a rich field of research based on observations, modelling and experimental work. This review presents the observations of the different components of interstellar dust and discusses their evolution during the life cycle of the interstellar medium. Interstellar dust grains are found everywhere in the Universe: in the Solar System, around stars at all evolutionary stages, in interstellar clouds of all kind, in galaxies and in the intergalactic medium. Cosmic dust is intimately mixed with the gas-phase and represents about 1% of the gas (in mass) in our Galaxy. The interstellar extinction and the emission of diffuse interstellar clouds is reproduced by three dust components: a population of large grains, the BGs (Big Grains, ∼10-500nm) made of silicate and a refractory mantle, a population of carbonaceous nanograins, the VSGs (Very Small Grains, 1-10nm) and a population of macro-molecules the PAHs (Polycyclic Aromatic Hydrocarbons) (1). These three components are more or less abundant in the diverse astrophysical environments reflecting the coupling of dust with the environment and its evolution according to the physical and dynamical conditions. The life cycle of cosmic matter is closely related to the cycle of the interstellar medium (ISM) and of our Galaxy. Dust is mainly form within the atmosphere and circumstellar shells surrounding dying stars. It is expelled in the ISM by stellar outflows and transit in the different phases of the ISM where it is submitted to various physical and chemical processes that either destroy it or change its nature (size, structure, composition, porosity...). When a molecular cloud collapse to form new stars, the interstellar dust grains are incorporated into the protostellar object in which the survival grains pursue their evolution, into proto-planetary disks and stellar systems. At their turn, these stars, at the end of their life will produce new dust grains. Interstellar dust was extensively studied via the observations from the instruments on-board the space-missions IRAS (InfraRed Astronomical Satellite, 1983), ISO (Infrared Space Observatory, 1995-1998) and Spitzer (2003-2005), free from the atmosphere opacity in the infrared, which gave tremendous amount of data. While these missions were observing in the mid- and far-infrared spectral range (3-200 m), the satellite Herschel, launched in May 2009, opens the still unexplored spectral range between infrared and ground-based radio observations (60-672 m, 5THz-480GHz). The spectrophotometers on-board these satellites allow to map with an increasing angular resolution the spatial distribution of the different dust components, at the Galaxy scale, but also in specific objects.