Winter Term 2018/19

Stars and star clusters form by gravitational collapse in regions of high density in the multi-phase interstellar medium. The process of stellar birth is controlled by the complex interplay between the self-gravity of the star-forming gas and various opposing agents, such as supersonic turbulence, magnetic fields, cosmic ray and gas pressure, or stellar feedback in form of radiation, winds, and supernovae. Turbulence plays a dual role. On global scales it provides support, while at the same time it can promote local collapse. This process is modified by the thermodynamic response of the gas, which is determined by the balance between various heating and cooling processes, which in turn depend on the chemical composition of the material. I will review the current status of the field and discuss a few examples of recent progress.

The images of the interstellar clouds, obtained observing molecular lines or dust continuum emission, exhibit a certain degree of self-similarity, generally ascribed to internal turbulence. The fractal geometry is therefore invoked to provide a quantitative description of the cloud morphology. As a further development, the multifractal approach is based on a continuous spectrum of exponents rather than on a single descriptor (the fractal dimension). It is more suitable to describe sets of intertwined fractals, showing a deviation from a strict self-similarity, which is typical of natural (stochastic) fractals, including clouds. However, the multifractal geometry remained an underexploited approach to describe and quantify the large-scale structure of interstellar clouds. I'll describe how multifractal analysis was applied to Herschel far-infrared (70-500 ?m) dust continuum maps, representing an ideal case of study. In particular, I computed the so-called multifractal spectrum and generalized fractal dimension of six Hi-GAL regions in the third Galactic quadrant, measuring systematic variations of the spectrum at increasing wavelength, which generally correspond to an increasing complexity of the image. Furthermore, I found a peculiar behaviour for each investigated field, strictly related to the presence of high-emission regions, which in turn are connected to star formation activity. The same analysis was also applied to synthetic column density maps, generated from numerical turbulent molecular cloud models and from fractal Brownian motion, allowing for the confrontation of the observations with models with well controlled physical parameters. I will discuss the results for both these classes of images. Finally, the link between mono-fractal parameters and multifractal indicators was investigated: I will describe common trends, but also differences highlighting the fact that multifractal analysis can offer a more extended and complete characterization of the cloud structure.

Extremely low densities leading to rare collisions between molecules and low temperatures in the majority of the interstellar space make it a very unfriendly environment for any kind of chemistry. Thus, the more complex molecules in the space have to be formed either in barrier-less ion-molecule reactions or reactions on ice/dust particles, which can adsorb reactant molecules on surfaces for long times.
In our laboratory, we can investigate the reactions on ice nanoparticles in a complex molecular-beam apparatus. Beams of different ice nanoparticles are produced in supersonic expansions. The nanoparticles fly isolated in vacuum and cool down by evaporation to typical temperatures of 10-100 K. Thus they represent good proxy for astronomical ices. As they pass through several pick-up cells, the nanoices can pick-up various gas-phase molecule on their surfaces. Then reactions can be triggered by photons (UV or IR) or electrons (fast, ~100 eV, causing ionization or slow, 0-10 eV, leading to the electron attachment), and we observe the reactions by mass spectrometry, velocity map imaging and other techniques.
Several examples of such investigations will be presented: pickup and coagulation of simple molecules on pure water-ice nanoparticles, generation of biologically relevant hydrogen bonds between molecules adsorbed on ices, ionization of doped ammonia ices, reactions of methanol and formic acid on ices, etc.

Carbon and heavier atoms are born in the centre of stars. At the end of their life, stars spread their inner material into the diffuse InterStellar Medium (ISM), yielding at the same time the formation of interstellar dust refractory cores. This diffuse medium (where photo-dissociation dominates the chemistry) gets locally denser in the form of dark clouds whose innermost part is shielded from the external UV field by the dust, allowing for molecules to grow and get more complex. As dust grains start to be covered by molecules, they also undergo coagulation. Gravitational collapse occurs inside these dense clouds forming protostars and their surrounding disks, and eventually planetary systems like (or unlike) our own solar system. Throughout all these stages, chemistry keeps evolving far from the equilibrium/steady-state set by physical conditions.

In recent years, our knowledge on the composition of the ISM and the process of star formation has entered a new area with the powerful capabilities (in term of sensitivity, spatial resolutions and/or new wavelength windows) of recent ground based and space observatories (Spitzer, Herschel, ALMA, NOEMA, and the new EMIR receiver on the IRAM 30m). These new instruments have allowed for the detection of new molecules (in the gas-phase or in the ices), either simple ones such as HCl+ or complex ones such as CH3NCO. In addition to revealing the richness of the ISM chemistry, they have revealed the non-uniform distribution of their abundances from object to object or even within a certain type of object (with similar physical conditions). One of the key questions astrochemists want to answer is: how do these molecules in the gas-phase or in the ices form and can we reproduce their variability in abundance?

In this presentation, I will describe the tools and methodology used to study the chemical composition of the interstellar medium and what we think we currently know about it.