Stellar clusters
Scientific context
Today there is an exceptional revival of the research field on stellar clusters, following new observational results obtained in particular with the VLT, the Hubble Space Telescope, and the Herschel satellite, but also due to theoretical developments and cutting-edge numerical simulations. For instance, the results of the Herschel mission have lead to a new paradigm for stellar formation in clusters, while the discovery of multiple stellar populations in globular clusters is recognized as one of the most spectacular results of the last decade.
Furthermore, several infrared surveys (GLIMPSE, VVV) have allowed to discover many massive clusters that were unknown up to now, and that can contain the more massive stars ever discovered. Indeed, recent studies indicate that such clusters could harbor objects of mass of the order of 300 solar masses, which considerably modifies our understanding of the initial mass function as well as the formation process leading to massive stars. Such clusters are observed in different environments in the local Universe, but are presently spatially unresolved. They will be priority targets for future satellites and telescopes like the JWST and E-ELT.
Moreover, the study of the formation, dynamics, and evolution of clusters is crucial for understanding the origin and the properties of stellar populations in the Milky Way and more generally in galaxies, which is one of the main goals of the Gaia mission launched by ESA in December 2013. Indeed, even if most stars are formed in clusters which initially contain a few dozens to several millions stars, only the more massive clusters remain gravitationally bound on lifetimes larger than the Hubble time, while the so-called open clusters and stellar associations quickly dissolve into their host galaxy.
Clusters are also cornerstones for understanding stellar physics and for constraining increasingly sophisticated models of stellar structure and evolution. In the modeling, it is becoming possible and necessary to account for various processes related to interactions between neighbor stars and with their environment (disc, planets, interstellar medium). These interactions, which strongly depend on the cluster density, affect the early evolution of stars by modifying their mass during accretion phases, their multiplicity rate, their disc, and, in fine, planetary formation and the initial mass function.
It is very important to update our knowledge on stellar clusters, both close ones and clusters at high redshift, in order to have an overall picture and to be able to make the link between different populations. This is a key step to make to get prepared to the development of the theoretical, numerical, and observational tools, which are essential for maximizing the scientific return of the Gaia and JWST satellites as well as of ground-based instruments (ALMA, NOEMA, and in the future SKA and E-ELT).
Objectives
Within this framework, the EES2015 aims at training the community in this field, in order to prepare the exploitation and interpretation of available and future data. The Herschel, Gaia, and JWST missions as well as the NOEMA and ALMA instruments, and later SKA and E-ELT, are improving (and will improve) our understanding of stellar clusters and their contribution to the different stellar populations in galaxies. This concerns very different and complementary astrophysical fields including the formation and initial mass function of stars and planets up to chemical and dynamical evolution of galaxies, but also the structuration of interstellar matter on different scales. Herschel also produced detailed maps of stellar formation regions in the Galaxy. Gaia data (which will be available in 2016) will revolutionize our understanding of clusters and associations by probing their spatial and kinematical structures (6D). ALMA and then SKA, as well as JWST and later E-ELT, will offer first possibilities to spatially resolve the stellar population of young supermassive clusters in the local group, and to relate them to globular clusters.
To take the full benefit of these high quality observations, numerical simulations of the dynamical evolution of clusters are essential. What are their present predictions, and how can we test them? What kind of new simulations will be necessary? One of the goals of the school is to initiate the reflection of the participants on these topics, and to make strong links between observers and modelers.
The aim of the school is also to favor the interactions between concerned actors working in different fields like stellar physics, physics of galaxies, and interstellar medium physics. The school
will allow the participants to share their knowledge and skills, their analysis tools, and to precise their needs. This will help to structure and organize research studies focused on clusters.
To ensure a wider audience, lectures will be given in English and the proceedings of the School will be published within the Publications Series of the European Astronomical Society.
