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.

The high density and low temperature in the central areas of star-forming cloud cores leads to the efficient depletion of various molecules onto the surfaces of interstellar dust grains. This severely limits the number of available tracer molecules that we can use to obtain information on, e.g., the gas temperature in these conditions. Fortunately, the onset of depletion allows deuterium chemistry to proceed, and indeed the abundances of deuterated molecules tend to peak toward high-density regions where depletion is strong. This provides us with unique observational tools to study the physical properties of star-forming cores, and hence it is highly desirable to develop detailed models of deuterium chemistry in order to understand the observations. Deuterium chemistry is strongly affected by spin-state chemistry, and in fact spin states can play a role in the formation of heavier species, such as ammonia, as well.
In this talk I will discuss in detail the connection of isotope (in particular deuterium) and spin-state chemistry, and will describe how studies of these topics allow us to understand the initial stages of the star-formation process better. I will introduce several past and present modeling efforts in this context, and will also outline future prospects that will further enhance our understanding of interstellar chemistry.

Material in the ISM is subjected to bombardment by several types of ionizing radiation including cosmic rays, stellar winds, x-rays, and gamma-rays from radionuclide decay. It is known that such radiation can have a significant physicochemical impact on interstellar environments, and a large body of experimental work has shown that the interaction between such energetic particles and low-temperatures ices can result in the formation of complex - even prebiotic - molecules. Even so, modeling the chemical effects of cosmic ray collisions with interstellar dust grain ice mantles has proven challenging due to the complexity and variety of the underlying physical processes. In this talk, we review recent work by us on the methods of incorporating such processes into astrochemical models, as well as their effects on cold core simulations.

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.

New receivers and spectrometers installed at millimeter and submillimeter telescopes now have very large bandwidths spanning tens of gigahertz at very high resolutions (~50-200kHz) yielding spectra comprising of hundred of thousands channels. These observing modes are now standard, which means that spectral surveys are now the observations obtained by default. With these instruments, large, several square degree, maps of the ISM are starting to be observed yielding massive hyperspectral datasets combining the emission of tens of species. The observed line intensities are function of both the ISM chemical composition and physical condition; using these molecular datasets it is possible to extract these ISM properties. In the framework of the ORIONB IRAM 30m large program we have developed statistical methods to study the physical conditions of the ISM solely from the observed molecular emission. I will present in particular:
i) the application of a Principal Component Analysis to decompose the maps into regions of low/high density and low/high UV illumination (Gratier et al. 2017),
ii) the application of the MeanShift clustering algorithm to segment the molecular cloud into physically and chemically similar regions (Bron et al. 2018) and,
iii) preliminary results on the application of supervised machine learning to infer the total column density from molecular emission.

The interstellar medium (ISM) is replete with molecules, and high-mass star-formation regions in particular are host to some of the most complex organic molecules yet detected outside of our solar system. Millimeter/sub-millimeter wavelength spectral data from the ALMA telescope allows us to explore the chemistry of such regions in much greater detail than ever before. The ALMA 3mm line survey EMoCA ("Exploring Molecular Complexity with ALMA") of the chemically-rich Galactic Center source Sagittarius B2(N) has not only identified several new molecules in that source, but has led to the identification of new hot cores - a total of five are now known to exist in Sgr B2(N).
I will give a brief overview of the molecular detections made by EMoCA toward Sgr B2(N). I will also present chemical kinetics models of the coupled gas-phase and grain-surface/ice-mantle chemistry occurring in Sgr B2(N) related to these molecules, with an emphasis on the treatment of the recently-detected branched carbon-chain molecule iso-propyl cyanide (i-C3H7CN). I will assess the possibilities for the presence and detectability of other branched carbon-chain molecules in the ISM. I will also present recent work that uses complex molecule abundances to constrain the cosmic-ray ionization rates and chemical timescales within different hot cores.

The initial conditions of star formation are imprinted within the properties of the many interstellar gaseous structures that are involved in the process. Understanding what physical mechanisms drive star formation therefore requires a detailed understanding on what regulates the dynamical evolution of the interstellar gas, from large diffuse clouds to these compact balls of nuclear burning gas. Mostly driven by the powerful combination of recent sensitive sub-millimeter large-scale surveys of star-forming clouds and high-angular resolution follow-up observations of selected sources, significant progress has been made in our understanding of star formation in the past decade. However, despite such progress, fundamental questions regarding, for instance, the pace at which gas flows from large to small scales and the origin of the low star formation efficiency within molecular clouds, still remain highly debated. In this context, I will present recent results on the properties of massive-star-forming molecular clouds from tens of parsec scales down to a few thousand AU scales. More specifically, I will be focusing on three key aspects: the evolution of cloud virial ratios as a function of cloud sizes; gravitational collapse and the role of hub filamentary systems in the formation of massive cores; and the impact of feedback from OB stars on the gas properties of the parent clouds.