The 112th ICREA Colloquium ‘Silicate stardust: from the nanoscale to galactic relevance’

ABSTRACT

Although stars are the most eye-catching component of galaxies, the space between stars inside galaxies is not empty. Instead, this interstellar medium (ISM) is sparsely filled with gas and dust. Denser parts of the ISM – so-called molecular clouds – may form stars from gravitational contraction. Stars, as they age,  produce increasingly heavier atoms (e.g. C, O, Mg, Si) from fusion reactions in their highly compressed interiors. Ultimately, old dying stars shed parts of their outer layers back into the ISM, enriched with the products of these reactions. Indeed, stars are the source of virtually all elements that we see around us today, with the exception of hydrogen and helium, which were formed during the Big Bang.

While many of these heavier elements are mostly present in the gas phase, many become condensed into solids, often in the form of sub-micron-sized dust grains. These dust particles absorb and emit radiation in the ISM and thus play an important role in the energy balance of a galaxy. As such, at galactic length scales, they can regulate the process of star formation and indeed galaxy evolution itself.  At much smaller nanoscale length scales, dust grains also provide a catalytic surface for essential chemical reactions to take place, potentially including those relevant to the origins of life. Ultimately, these tiny grains also provide the building blocks for forming planets like our own Earth. Thus, even though dust takes up only 1% of the mass of the ISM, and even a smaller fraction of the mass of galaxies as a whole, understanding its nature, formation and evolution is an important quest in understanding the evolution of galaxies, astrochemical processes and the formation of planetary systems.

With a share of about 75% of the total galactic dust mass, silicates (i.e. solids based on mixtures of Si, O and Mg/Fe) form the main constituent of interstellar dust. We will discuss the life cycle of silicate dust from its production in evolved stars, through processing in the interstellar medium to its demise in star forming regions in our own Milky Way and in other galaxies. To understand the nature of silicate dust at the nanoscale we will highlight how quantum chemical computer modelling can provide otherwise difficult to obtain insights that can be used to interpret experiments and observations. Here, we will focus on the properties and formation of silicate nanograins, and the role of silicate dust in catalysing the  formation of important chemical species. From a larger scale observational perspective we will explore the observational evidence of the presence of silicates in different astrophysical environments, and the different forms in which it may be present.  Here we will particularly highlight new results from the James Webb Space Telescope, which is ideally suited to study the properties of astronomical silicates with unprecedented detail.