Planet-forming circumstellar disks are a fundamental part of the star formation process. Since stars form in a hierarchical fashion in groups of up to hundreds or thousands, the UV radiation environment that these disks are exposed to can vary in strength by at least six orders of magnitude. This radiation can limit the masses and sizes of the disks. Diversity in star forming environments can have long lasting effects in disk evolution and in the resulting planetary populations. We perform simulations to explore the evolution of circumstellar disks in young star clusters. We include viscous evolution, as well as the impact of dynamical encounters and external photoevaporation. We find that photoevaporation is an important process in destroying circumstellar disks: in regions of stellar density ρ ∼ 100 M pc −3 around 80% of disks are destroyed before 2 Myr of cluster evolution. Our findings are in agreement with observed disk fractions in young star forming regions and support previous estimations that planet formation must start in timescales < 0.1 − 1 Myr.Key words: key 1 key 2 1 G 0 is the FUV field measured in units of the Habing flux, 1.6 × 10 −3 erg s −1 cm −2 (Habing 1968).

Circumstellar discs are the precursors of planetary systems and develop shortly after their host star has formed. In their early stages these discs are immersed in an environment rich in gas and neighbouring stars, which can be hostile for their survival. There are several environmental processes that affect the evolution of circumstellar discs, and external photoevaporation is arguably one of the most important ones. Theoretical and observational evidence point to circumstellar discs losing mass quickly when in the vicinity of massive, bright stars. In this work we simulate circumstellar discs in clustered environments in a range of stellar densities, where the photoevaporation mass-loss process is resolved simultaneously with the stellar dynamics, stellar evolution, and the viscous evolution of the discs. Our results indicate that external photoevaporation is efficient in depleting disc masses and that the degree of its effect is related to stellar density. We find that a local stellar density lower than 100 stars pc−2 is necessary for discs massive enough to form planets to survive for 2.0Myr. There is an order of magnitude difference in the disc masses in regions of projected density 100 stars pc−2 versus 104 stars pc−2. We compare our results to observations of the Lupus clouds, the Orion Nebula Cluster, the Orion Molecular Cloud-2, Taurus, and NGC 2024, and find that the trends observed between region density and disc masses are similar to those in our simulations.

Stars with circumstellar disks may form in environments with high stellar and gas densities which affects the disks through processes like truncation from dynamical encounters, ram pressure stripping, and external photoevaporation. Circumstellar disks also undergo viscous evolution which leads to disk expansion. Previous work indicates that dynamical truncation and viscous evolution play a major role in determining circumstellar disk size and mass distributions. However, it remains unclear under what circumstances each of these two processes dominates. Here we present results of simulations of young stellar clusters taking viscous evolution and dynamical truncations into account. We model the embedded phase of the clusters by adding leftover gas as a background potential which can be present through the whole evolution of the cluster, or expelled after 1 Myr. We compare our simulation results to actual observations of disk sizes, disk masses, and accretion rates in star forming regions. We argue that the relative importance of dynamical truncations and the viscous evolution of the disks changes with time and cluster density. Viscous evolution causes the importance of dynamical encounters to increase in time, but the encounters cease soon after the expulsion of the leftover gas. For the clusters simulated in this work, viscous growth dominates the evolution of the disks.

We present an account of the state of the art in the fields explored by the research community invested in "Modeling and Observing DEnse STellar systems". For this purpose, we take as a basis the activities of the MODEST-17 conference, which was held at Charles University, Prague, in September 2017. Reviewed topics include recent advances in fundamental stellar dynamics, numerical methods for the solution of the gravitational N-body problem, formation and evolution of young and old star clusters and galactic nuclei, their elusive stellar populations, planetary systems, and exotic compact objects, with timely attention to black holes of different classes of mass and their role as sources of gravitational waves.Such a breadth of topics reflects the growing role played by collisional stellar dynamics in numerous areas of modern astrophysics. Indeed, in the next decade many revolutionary instruments will enable the derivation of positions and velocities of individual stars in the Milky Way and its satellites, and will detect signals from a range of astrophysical sources in different portions of the electromagnetic and gravitational spectrum, with an unprecedented sensitivity. On the one hand, this wealth of data will allow us to address a number of long-standing open questions in star cluster studies; on the other hand, many unexpected properties of these systems will come to light, stimulating further progress of our understanding of their formation and evolution.

The evolution of circumstellar discs is influenced by their surroundings. The relevant processes include external photoevaporation due to nearby stars and dynamical truncations. The impact of these processes on disc populations depends on the star-formation history and on the dynamical evolution of the region. Since star-formation history and the phase-space characteristics of the stars are important for the evolution of the discs, we start simulating the evolution of the star cluster with the results of molecular cloud collapse simulations. In the simulation, we form stars with circumstellar discs, which can be affected by different processes. Our models account for the viscous evolution of the discs, internal and external photoevaporation of gas, external photoevaporation of dust, and dynamical truncations. All these processes are resolved together with the dynamical evolution of the cluster, and the evolution of the stars. An extended period of star formation, lasting for at least 2 Myr, results in some discs being formed late. These late-formed discs have a better chance of survival because the cluster gradually expands with time, and a lower local stellar density reduces the effects of photoevaporation and dynamical truncation. Late formed discs can then be present in regions of high UV radiation, solving the proplyd lifetime problem. We also find a considerable fraction of discs that lose their gas content but remain sufficiently rich in solids to be able to form a rocky planetary system.