NANOCOSMOS · The cosmic nanoworldAstrophysics at the interface between the molecular and the macroscopic

The cosmic nanoworld Astrophysics at the interface between the molecular and the macroscopic

The "free" baryonic content of galaxies pervades the space between stars, and is usually dubbed Interstellar Medium (ISM). It is mainly composed of tenuous hydrogen and helium gas with a contribution from heavier elements which can be present in the gas phase (in atomic and molecular form) or in the solid state. The solid state component of the ISM is identified with the generic term of “interstellar dust”. Extinction measurements and studies of the mid-infrared emission of the galaxy indicates that interstellar dust is composed by macromolecules, mainly Polycyclic Aromatic Hydrocarbons (PAHs) and fullerenes C60 and C70, and sub-microscopic amorphous carbon and silicate grains. The size of the particles ranges from ~ 0.5 nm for macromolecules and smallest particles, to > 50 nm (up to 500 nm) for the largest particles. While the largest grains can be essentially treated as bulk materials, macromolecules and smaller interstellar particles have a truly nanoscale nature and legitimately populate the "cosmic nanoworld". This world exists at the interface between the molecular and the macroscopic worlds. Its constituents are small enough to exhibit characteristics reminiscent of molecules, but large enough to have properties typical of solid state materials. Because of the “hybrid” nature of macromolecules and nanoparticles, an appropriate way to characterise their physical and chemical properties is still missing. Although very scarce (1% of the total mass of the ISM), dust is the key agent that drives and regulate the core physical and chemical processes responsible for star formation and galaxy evolution (e.g., control of the temperature of the gas in the ISM through photo-electric heating, formation of H2 via surface reactions). The focus and global objective of this proposal has been therefore the characterisation of macromolecules and nanoparticles, the main components of dust. To study the cosmic nanoworld, the researcher has developed a specific theoretical framework which she has then used to tackle a variety of important astrophysical problems. In particular, the researcher has used this framework to study the effects of the irradiation with high-energy photons of the smallest interstellar dust nano-particles (radius less than 3 nm). The researcher has modified the formalism that she has previously developed to model the energy transfer into PAH molecules due to collisions with ions and electrons and the following dissociation. The formalism has been adapted to treat interactions with photons and applied to amorphous carbon nanoparticles. These particles have a 3D structure where carbon atoms are arranged into hexagonal rings having different orientations in space, opposite to “classical” PAHs where the carbon atoms are arranged into a single layer of hexagonal rings fused together. This study is crucial to characterise PAHs and nanoparticles in terms of their response to photon absorption, which is one of the most common phenomena occurring in space. The results have shown indeed that, to fully understand this phenomenon, it is necessary to fully consider the specific structure of nanoparticles, which is very different with respect to the flat geometry of PAHs. In terms of astrophysical implications, the dissociation induced by irradiation with photons could explain the variations in the observed mid-infrared spectra of galaxies as a function of the interstellar radiation field. These results are important to better understand the evolution of nanoparticles in the interstellar medium. In terms of characterisation of cosmic macromolecules, the researcher has studied the stability of C60 and C70, the largest molecules detected so far in space. Because of their peculiar structure and physical properties, fullerenes are promising candidates as building blocks of larger cosmic nanoparticles, and they could contribute to the mid-infrared emission of galaxies. To do so, fullerenes must be able to survive long enough under the harsh conditions of the ISM. The researcher has tested their stability against collisions with ions in the interstellar medium. While fullerenes are very resistant molecules, the results of this study show that the physical conditions in space are consistent with the dissociation of these molecules, induced by ion bombardment. Relevant findings have come from the study of the possible formation of different molecular species (including H2) from PAHs damaged by ionic collisions. The results show that this is indeed a viable formation route for molecular hydrogen and have important astrophysical implications, due to the lack of a complete understanding of the H2 formation mechanism in the interstellar medium. Because of the importance of the topic to the scientific community, the researcher has also developed a complete model of the shock-induced processing of nanoparticles in the cavity of supernova remnants, using the well studied supernova remnant Cassiopeia A (Cas A) as a test-case. Recent observations of have shown that supernovae are efficient dust factories. From an evolutionary point of view, they could therefore represent a viable source for the large quantity of dust detected at high red-shift, where the conventional sources (evolved stars) are not effective. To quantify the net contribution of supernovae to the observed dust population, it is necessary to evaluate how much of the fresh dust produced in the ejecta survives the processing by the reverse shock in the cavity of the supernova and reaches the ISM. Such a study requires models for dust processing coupled with models for the evolution of the remnants and this has been provided by the researcher. This work is relevant for different research areas (and corresponding scientific communities): for the study of interstellar dust (infrared astronomy/low-energy astrophysics) through the characterisation of the physical response of dust nanoparticles when processed by ions in the supernova ejecta, for the study of supernova remnants (high-energy astrophysics) through investigation of the impact of dust on the evolution of supernova remnants, and for cosmology through the research of a viable source of dust in the early Universe.

Data: CORDIS, © European Union