Formation Of Massive Stellar Clusters Research Articles

The formation of massive stellar clusters in converging galactic flows with photoionization

ABSTRACT We have performed simulations of cluster formation along two regions of a spiral arm taken from a global Milky Way simulation, including photoionizing feedback. One region is characterized by strongly converging flows, the other represents a more typical spiral arm region. We find that more massive clusters are able to form on shorter time-scales for the region with strongly converging flows. Mergers between clusters are frequent in the case of the strongly converging flows and enable the formation of massive clusters. We compare equivalent clusters formed in simulations with and without ionization. Photoionization does not prevent massive cluster formation, but can be seen to limit the masses of the clusters. On average, the mass is reduced by around 20 per cent, but we see a large spread from ionization having minimal difference to leading to a 50 per cent reduction in mass. Photoionization is also able to clear out the gas in the vicinity of the clusters on Myr time-scales, which can produce clusters with larger radii that are surrounded by more massive stellar haloes. We find that the ionizing feedback has more impact in our second region that is less dense and has less strongly converging flows.

The properties of clusters, and the orientation of magnetic fields relative to filaments, in magnetohydrodynamic simulations of colliding clouds

ABSTRACT We have performed Smoothed Particle Magneto-Hydrodynamics (SPMHD) calculations of colliding clouds to investigate the formation of massive stellar clusters, adopting a timestep criterion to prevent large divergence errors. We find that magnetic fields do not impede the formation of young massive clusters (YMCs), and the development of high star formation rates, although we do see a strong dependence of our results on the direction of the magnetic field. If the field is initially perpendicular to the collision, and sufficiently strong, we find that star formation is delayed, and the morphology of the resulting clusters is significantly altered. We relate this to the large amplification of the field with this initial orientation. We also see that filaments formed with this configuration are less dense. When the field is parallel to the collision, there is much less amplification of the field, dense filaments form, and the formation of clusters is similar to the purely hydrodynamical case. Our simulations reproduce the observed tendency for magnetic fields to be aligned perpendicularly to dense filaments, and parallel to low density filaments. Overall our results are in broad agreement with past work in this area using grid codes.

Cloud–cloud collisions and triggered star formation

Abstract Star formation is a fundamental process for galactic evolution. One issue over the last several decades has been determining whether star formation is induced by external triggers or self-regulated in a closed system. The role of an external trigger, which can effectively collect mass in a small volume, has attracted particular attention in connection with the formation of massive stellar clusters, which in extreme cases may lead to starbursts. Recent observations have revealed massive cluster formation triggered by cloud–cloud collisions in nearby interacting galaxies, including the Magellanic system and the Antennae Galaxies as well as almost all well-known high-mass star-forming regions in the Milky Way, such as RCW 120, M 20, M 42, NGC 6334, etc. Theoretical efforts are going into the foundation for the mass compression that causes massive cluster/star formation. Here, we review the recent progress on cloud–cloud collisions and the triggered star-cluster formation, and discuss future prospects for this area of study.

The formation and evolution of massive stellar clusters in IC4662

We present a multiwavelength study of the formation of massive stellar clusters, their emergence from cocoons of gas and dust, and their feedback on surrounding matter. Using data that span from radio to optical wavelengths, including Spitzer and Hubble Space Telescope ACS observations, we examine the population of young star clusters in the central starburst region of the irregular Wolf–Rayet galaxy IC4662. We model the radio-to-infrared (IR) spectral energy distributions of embedded clusters to determine the properties of their Hii regions and dust cocoons (sizes, masses, densities, temperatures), and use near-IR and optical data with mid-IR spectroscopy to constrain the properties of the embedded clusters themselves (mass, age, extinction, excitation, abundance). The two massive star-formation regions in IC4662 are excited by stellar populations with ages of ∼4 Myr and masses of ∼3×105 M⊙ (assuming a Kroupa initial mass function). They have high excitation and subsolar abundances, and they may actually be comprised of several massive clusters rather than the single monolithic massive compact objects known as ‘super star clusters’ (SSCs). Mid-IR spectra reveal that these clusters have very high extinction values, AV∼20–25 mag, and that the dust in IC4662 is well mixed with the emitting gas, not in a foreground screen.

A New Scenario for the Formation of Massive Stellar Clusters

AbstractA new scenario is proposed, in which the continuous creation of stars results into a highly concentrated massive (globular cluster-like) stellar system. We assume that the collapse of a massive cloud leads to the formation of first stars. Their winds and terminal supernova explosions create a standing, small radius, cold and dense shell, where the next stars form. The shell is in steady state location, which is due to a detailed balance established between the ram pressure from the collapsing cloud, the gravitational force exerted on the shell and the ram pressure of the stellar wind from the forming cluster. In this contribution we show that the standing shell remains stable against Rayleigh -Taylor instability, and discuss future prospects of this work.

On the formation of massive stellar clusters

Here we model a star forming factory in which the continuous creation of stars results in a highly concentrated, massive (globular cluster-like) stellar system. We show that under very general conditions a large-scale gravitational instability in the ISM, which triggers the collapse of a massive cloud, leads with the aid of a spontaneous first generation of massive stars, to a standing, small-radius, cold and dense shell. Eventually, as more of the collapsing matter is processed and incorporated, the shell becomes gravitationally unstable and begins to fragment, allowing the formation of new stars, while keeping its location. This is due to a detailed balance established between the ram pressure from the collapsing cloud which, together with the gravitational force exerted on the shell by the forming cluster, acts against the mechanical energy deposited by the collection of new stars. We present a full analysis of feedback and show how the standing shell copes with the increasing mechanical energy generated by an increasing star-formation rate. The latter also leads to a rapidly growing number of ionizing photons, and we show that these manage to ionize only the inner skin of the standing star-forming shell. We analyze the mass spectrum of fragments that result from the continuous fragmentation of the standing shell and show that its shape is well approximated at the high mass end by a power law with slope -2.25, very close to the value that fits the universal IMF. Furthermore, it presents a maximum near to one solar mass and a rapid change towards a much flatter slope for smaller fragments. The self-contamination resultant from the continuous generation of stars is shown to lead to a large metal spread in massive (~10) clusters, while clusters with a mass similar to 10 or smaller, simply reflect the initial metalicity of the collapsing cloud. This is in good agreement with the data available for globular clusters in the Galaxy. Other observables such as the expected IR luminosity and the Hα equivalent width caused by the forming clusters are also calculated.

Formation of Globular Clusters in Turbulent Molecular Clouds

The formation of massive stellar clusters in turbulent molecular clouds is investigated. We include artificial star formation and energy feedback of newly born stars. The obtained systems are not likely to survive. Case studies to determine conditions necessary for forming bound clusters will be done in the future.

UV-Observations of Young Populous Clusters in the Large Magellanic Cloud

The young populous star clusters give evidence for the ‘explosive’ star formation in the Magellanic Clouds which took place in the time interval 5·106 yrs to < 108 yrs agoe. They are also key objects for the understanding of the formation of massive stellar clusters, because they are still situated close to their ‘birthplace’ in the parent galaxy and are dynamically not relaxed (Geyer et al. 1979). Their HRD-morphology shows most of the member stars in the upper Main Sequence range with only a few massive yellow and red supergiants. The lower massive stars are still in the pre-main-sequence evolution phase (‘T-Tauri state’), which cannot be observed at the MC's distances. Thus in the uv-spectral range the blue stars with (B-V) < 0.1 on the upper MS contribute to the uv-fluxes. In the optical spectral regions the bright ‘blue’ globular clusters seem not be embedded in remanent interstellar matter, though neighbouring loose stellar aggregates of similar age are in many cases surrounded by dense HII-regions. This rises the questions wether the starformation process in such massive clusters was so efficient that no remanent matter was left over, or was this material blown away by the uv-radiation of the numerous OB-member stars?