A gallery of protoplanetary disk formed in numerical simulations
The Universe is known to have had a very simple structure in its early stages, with detailed observations of the cosmic microwave background (CMB) showing a consistent temperature across the sky. Cosmic evolution over the past 13.6 billion years since the Big Bang has been a progression from simplicity to ever-increasing complexity, with emerging structures spanning an enormous range of scales from large galaxy clusters, to molecular clouds, down to protoplanetary disks, stars and planets. These different structures have historically been modelled separately, in isolation from each other, without necessarily paying too much attention to the feedback and interactions between them at different spatial scales. For example,
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detailed models of planetary-forming disks have been developed by some research groups, without considering the material that continuously flows onto this structure from the scale immediately above. This does not reflect the current understanding of how galaxies and the structures within them form and evolve. While previously it was thought that structures in the Milky Way evolve only slowly, over extended timescales, evidence gathered over recent years shows that it is in fact a highly dynamic process, with gravity, turbulence and magnetic fields all contributing significantly to the dynamic evolution of our Galaxy. Our intuitive picture of how a galaxy works typically starts at larger scales, cascading down to structures like spiral
arms and molecular clouds, and eventually arriving at stars and planets. While stars are relatively insignificant in size compared to the Galaxy as a whole, they still have a dramatic impact on how the rest of the Galaxy evolves. Collective feedback from stars – for example radiation, stellar winds, cosmic rays and supernova explosions – will ultimately have an influence at larger scales and on the processes of structure formation.
ECOGAL project The team behind the ERC-backed ECOGAL project are pursuing a more comprehensive approach to modelling the Milky Way, linking together a hierarchical distribution of structures in an integrated approach. This encompasses the entire Galactic disk, to molecular clouds, to ever denser
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Credit: ALMA (ESO/NAOJ/NRAO)/S. Facchini et al.
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The next step in interpreting observational data is often to collaborate with theorists, who attempt to reproduce the observations using numerical simulations. A rigorous theoretical framework is essential to interpreting observed data and placing it in a wider context, yet at the same time a theory without any relationship to observations is almost certainly incorrect. So there is typically an iterative process of modifying theoretical frameworks on the basis of observations.
Numerical simulations This approach provides solid foundations for the development of numerical simulations, which are designed to allow researchers to zoom in and probe specific
details within cosmic structures at different scales, such as a spiral arm or an individual star-forming region. It is a highly complex task, as a very large, dynamical range needs to be resolved, and researchers need to make sure that all the data from local, individual structures can be reproduced and understood. A second important consideration with the numerical simulations is to account for the individual physical processes that are relevant in different regions of the Galaxy, or in the Galaxy as a whole. For instance, it is essential to include gravitational forces, magnetic field and chemical evolution when simulating the entire Milky Way, while different physical processes come into play at smaller scales, such as certain diffusion and stellar feedback processes. Two separate but complementary approaches to performing these simulations are being followed within the project, using the RAMSES and AREPO simulation codes. One approach involves starting with consistent, large simulations - of the full Galaxy or a piece of it - and then zooming into specific regions of interest in those simulations, in order to gather statistics. One aim in the project is to produce an ensemble of these self-consistent, hierarchical simulations.
Background image by Marc Sendra Martorell on UnSplash.
Patrick Hennebelle, Ralf Klessen, Sergio Molinari, and Leonardo Testi, the team behind the ERC-backed ECOGAL project are developing the first ever predictive model of the Galactic ecosystem, aiming to link different structures in the Galaxy together through an integrated approach. This work will open up new insights into the conditions under which stars and planets form.
structures, right down to individual starforming regions with protoplanetary disks, in which planets are formed. This research brings together groups from across Europe, including specialists in observational astronomy, numerical astrophysics, instrument development and astroinformatics, aiming to break new ground by developing the first predictive model of star and planet formation in the Galaxy. A wide variety of tools are being applied in this work, with researchers building on observational data and numerical simulations to develop a unified model. The project’s work will open up new insights into the conditions under which stars and planets are formed, a topic of fundamental interest in modern science, and will provide the first comprehensive understanding of our Galaxy as a star formation machine. While stars have been forming continuously in our Galaxy over billions of years, the ECOGAL project team are focusing largely on the ongoing star and planet formation process to understand through a variety of new models and observational datasets the processes that govern present-time structure formation. These include data gathered from systematic surveys of our Galaxy, including from the Herschel Space Observatory and the ALMAGAL survey, the largest programme executed so far on the ALMA telescope, located at high altitude in Chile’s Atacama desert. The ALMAGAL survey observed more than 1,000 high-mass star-forming regions across the Galaxy at different stages of their evolution, providing a rich source of data for researchers to tap into. At smaller scales, the team is also investigating the process that leads to the formation of single stars and protoplanetary disks, by means of dedicated surveys with the European Southern Observatory facilities ALMA and Very Large Telescope, and with the NOEMA observatory of IRAM. These surveys have produced extensive data on single pre-stellar cores, and proto-planetary disks in which planets are currently being formed. Protoplanetary disks appear inhomogeneous, with bright rings and gaps, suggesting that planetesimals start forming right from the very early stages of star formation, rather than towards the end, which is why the study of disk formation and the initial conditions for their evolution is of paramount importance.
A zoom toward the Galactic center in a numerical simulation by Tress et al.
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