Observations Of Young Massive Clusters Research Articles

The loss of the intracluster medium in globular clusters

ABSTRACT Stars in globular clusters (GCs) lose a non-negligible amount of mass during their post-main-sequence evolution. This material is then expected to build up a substantial intracluster medium (ICM) within the GC. However, the observed gas content in GCs is a couple of orders of magnitude below these expectations. Here, we follow the evolution of this stellar wind material through hydrodynamical simulations to attempt to reconcile theoretical predictions with observations. We test different mechanisms proposed in the literature to clear out the gas such as ram-pressure stripping by the motion of the GC in the Galactic halo medium and ionization by UV sources. We use the code ramses to run 3D hydrodynamical simulations to study for the first time, the ICM evolution within discretized multimass GC models including stellar winds and full radiative transfer. We find that the inclusion of both ram pressure and ionization is mandatory to explain why only a very low amount of ionized gas is observed in the core of GCs. The same mechanisms operating in ancient GCs that clear the gas could also be efficient at younger ages, meaning that young GCs would not be able to retain gas and form multiple generations of stars as assumed in many models to explain ‘multiple populations’. However, this rapid clearing of gas is consistent with observations of young massive clusters.

Young and old massive star clusters and their stellar populations - Theoretical challenges for the next decade

AbstractSeveral models presented in the literature compete to explain the origin of multiple stellar populations in globular clusters (GC), but they all fail to reproduce the large variety of present-day characteristics of these systems. In parallel, independent clues on GC early evolution may be derived from observations of young massive clusters (YMC) in the Local Group. But are these two populations of clusters related? And can we reconcile the informations and data concerning GCs and YMCs? Here we summarize some open questions on the nucleosynthetic origin of multiple stellar populations in GCs, on the actual evolution and characteristics of GC low-mass stars, and on early gas expulsion from massive clusters. We propose theoretical paths to be explored in the near future.

MODELING OF THE INITIAL MASS FUNCTION USING THE METROPOLIS−HASTINGS ALGORITHM

A stochastic model has been developed for the hierarchical fragmentation of a molecular cloud. Here, the number of fragments, time between successive fragmentation steps, and mass of a fragment are considered as random variables, and fragment masses are generated using the Metropolis-Hastings algorithm. The resulting mass spectra, computed at different projected distances and taking opacity into consideration, show a signature of mass segregation. The critical mass, mass spectrum, and mass segregation are consistent with the observations of young massive clusters in our Galaxy as well as in external galaxies.

Dynamical Evolution of Globular Clusters in Hierarchical Cosmology

AbstractWe probe the evolution of globular clusters that could form in giant molecular clouds within high-redshift galaxies. Numerical simulations demonstrate that the large and dense enough gas clouds assemble naturally in current hierarchical models of galaxy formation. These clouds are enriched with heavy elements from earlier stars and could produce star clusters in a similar way to nearby molecular clouds. The masses and sizes of the model clusters are in excellent agreement with the observations of young massive clusters. Do these model clusters evolve into globular clusters that we see in our and external galaxies? In order to study their dynamical evolution, we calculate the orbits of model clusters using the outputs of the cosmological simulation of a Milky Way-sized galaxy. We find that at present the orbits are isotropic in the inner 50 kpc of the Galaxy and preferentially radial at larger distances. All clusters located outside 10 kpc from the center formed in the now-disrupted satellite galaxies. The spatial distribution of model clusters is spheroidal, with a power-law density profile consistent with observations. The combination of two-body scattering, tidal shocks, and stellar evolution results in the evolution of the cluster mass function from an initial power law to the observed log-normal distribution.