M. Margulis

Bio: M. Margulis is an academic researcher. The author has an hindex of 1, co-authored 1 publications receiving 7 citations.

The Formation and Early Dynamical Evolution of Bound Stellar Systems

Abstract: We present the results of numerical N-body calculations which simulate the dynamical evolution of young clusters as they emerge from molecular clouds. We follow the evolution of initially virialized stellar systems of 50 and, in some cases, 100 stars from the point in time immediately after the stars have formed in a cloud until a time long after all the residual star-forming gas has been dispersed from the system. By varying the star formation efficiency and the gas dispersal time for each model, we determined the combination of these parameters which result in the production of bound stellar groups after the gas not used in star formation is completely dispersed.

Gas expulsion and the destruction of massive young clusters

Abstract: We examine the luminosity and dynamical mass estimates for young massive stellar clusters. For many young (<50 Myr) clusters, the luminosity and dynamical mass estimates differ by a significant amount. We explain this as being due to many young clusters being out of virial equilibrium (which is assumed in dynamical mass estimates) because the clusters are undergoing violent relaxation after expelling gas not used in star formation. We show that, if we assume that luminous mass estimates are correct (for a standard IMF), at least 50 per cent of young clusters for which dynamical masses are known are likely to be destroyed within a few 10s Myr of their formation. Even clusters which will retain a bound core may lose a large fraction of their stellar mass. We also show that the core radius and other structural parameters change significantly during the violent relaxation that follows gas expulsion and that they should be considered instantaneous values only, not necessarily reflecting the final state of the cluster. In particular we note that the increasing core radii observed in young LMC/SMC clusters can be well explained as an effect of rapid gas loss.

The Genesis of Super Star Clusters as Self-Gravitating HII Regions

Abstract: We examine the effects of ionization, radiation pressure and main sequence winds from massive stars on self-gravitating, clumpy molecular clouds, thereby modeling the formation and pre-supernova feedback of massive star clusters. We find the process of ``turbulent mass loading'' is effective in confining HII regions. Extrapolating typical Galactic high-mass star forming regions to greater initial gas cloud masses and assuming steady star formation rates, we determine the timescales for cloud disruption. We find that a dense n_c ~ 2 x 10^5 cm^-3 cloud with initial mass M_c ~ 4 x 10^5 M_sun is able to form ~2 x 10^5 M_sun of stars (50% efficiency) before feedback disperses the gas after ~3 Myr. This mass and age are typical of young, optically visible super star clusters (SSCs). The high efficiency permits the creation of a bound stellar system.

Turbulent, Molecular Clouds Regulated by Radiation Feedback

Abstract: Radiation feedback from stellar clusters is expected to play a key role in setting the rate and efficiency of star formation in giant molecular clouds (GMCs) and across whole galaxies. In particular, stellar radiation may quench star formation by driving outflows and unbinding stellar clusters. To investigate how radiation forces influence realistic clouds, we have conducted a series of simulations employing the Hyperion radiation hydrodynamics solver, considering the regime that is optically thick to ultraviolet and optically thin to infrared radiation. Our model clouds cover initial surface densities between Σcl,0 ∼ 10 − 300 M pc−2, with varying initial turbulence and magnetic field strength (Bz,0). We follow them through turbulent, self-gravitating collapse, formation of star clusters, and cloud dispersal by stellar radiation. All our models display a lognormal distribution of gas surface density Σ as seen by both the observer and the central cluster. For an initial virial parameter αvir,0 = 2, the lognormal standard deviation is σlnΣ = 1−1.5 and the star formation rate (SFR) coefficient εff,ρ̄ = 0.3 − 0.5, both of which are sensitive to turbulence, and magnetic fields, but not radiation feedback. Embedded stars are more centrally concentrated than the gas so that above Σcl,0 ∼ 60 M pc−2, the star cluster remains intact even when surrounding gas is dispersed. The net star formation efficiency depends primarily on the distribution of Eddington ratios in the cloud and therefore increases with Σcl,0 and decreases with both αvir,0 and Bz,0. This also has implications for outflows, since low surface density regions may be driven outwards to nearly 10 times their initial escape speed (vesc). However, the overall efficiency of momentum injection to the gas is reduced because much of the radiation escapes and irrespective of Σcl,0, the mean outflow velocity is approximately twice vesc. Unless GMCs are highly magnetized and turbulent, the lognormal structure of gas modulates the effect of radiative feedback in disrupting clouds, so that it cannot alone explain the low observed galactic SFR.

Numerical simulations on the formation of massive star clusters

Abstract: Die Entstehung von massereichen gebundenen Sternhaufen in Molekulwolken ist ein Wettrennen zwischen effizienter Sternentstehung und energetischen Ruckkopplungsprozessen massereicher Sterne. Diese unterbinden den Sternentstehungsprozes und treiben das Restgas aus dem System. Der Einflus des Gasausstoses auf die dynamische Entwicklung von Sternentstehungsregionen wurde untersucht. Bei kollisionsfreien numerischen N-Korperrechnungen wurde der Gasausstos aus einem gebundenen System aus Sternen und umgebenden Gas durch ein externes Potential beschrieben. Die Sternhaufen bleiben nur dann gebunden, wenn entweder die Sternentstehungseffizienz wesentlich hoher ist als die in der Galaxis typische, wenn die Zeitskala des Gaausstoses ein mehrfaches der dynamischen Zeitskala betragt oder wenn sich das System anfangs nicht im virialen Gleichgewicht befand. Deswegen wurde auserdem die Entstehung von Sternhaufen aus anfanglich kalten, turbulenten Molekulwolken untersucht. Zur numerischen Simulation des Gases wurde 'Smoothed Particle Hydrodynamics' mit einer idealisierenden Beschreibung der Sternentstehung verwendet. Die Sterne selbst werden wieder mittels kollisionsfreier N-Korperrechnung beschrieben, die Ruckkopplung wird durch thermisches Heizen des umgebenden Gases simuliert. Die Sternentstehungseffizienz der kollabierenden und fragmentierenden Molekulwolke und damit die Gebundenheit des resultierenden Systems wird wesentlich beeinflust von der Zeitverzogerung zwischen der Bildung der Sterne und dem Einsetzen der Ruckkopplung, sowie von dem gewahlten globalen Dichtekriterium, das bestimmt zu welchem Zeitpunkt sich die Sterne wahrend des Kollapses der Wolke bilden.

Open clusters in a dispersing molecular cloud

Abstract: Star clusters are formed in molecular clouds which are believed to be the birth places of most stars. From recent observational data, Lada & Lada(2003) estimated that only 4 to 7% of the proto-clusters have survived. Many factors could cause this high infant mortality. Galactic tidal forces, close encounters with molecular clouds and shock heating are among the possible causes but they have a longer timescale than typical lifetime of molecular clouds. Another possible reason is mass loss in very beginning of cluster evolution in the form of UV radiation, stellar winds or supernova explosions. Mass loss is the main factor we study in this work by using N-body simulations. We find that most proto-clusters survive for more than 40 Myr even when the mass loss rate is high.