Sergiy Silich

On the energy required to eject processed matter from galaxies

We evaluate the minimum energy input rate that starbursts require for expelling newly processed matter from their host galaxies. Special attention is given to the pressure caused by the environment in which a galaxy is situated, as well as to the intrinsic rotation of the gaseous component. We account for these factors and for a massive dark matter distribution, and we develop a self-consistent solution for the interstellar matter gas distribution. Our results are in excellent agreement with the recent results of Mac Low & Ferrara for galaxies with a flattened, disklike ISM density distribution and a low intergalactic gas pressure (PIGM/k ≤ 1 cm-3 K). However, our solution also requires a much larger energy input rate threshold when one takes into consideration both a larger intergalactic pressure and the possible existence of a low-density, nonrotating, extended gaseous halo component.

WINDS DRIVEN BY SUPER STAR CLUSTERS: THE SELF-CONSISTENT RADIATIVE SOLUTION

Here we present a self-consistent stationary solution for spherically symmetric winds driven by massive star clusters under the impact of radiative cooling. We demonstrate that cooling may drastically modify the distribution of temperature if the rate of injected energy approaches a critical value. We also prove that the stationary wind solution does not exist when the energy radiated away at the star cluster center exceeds ~30% of the energy deposition rate. Finally, we thoroughly discuss the expected appearance of super star cluster winds in the X-ray and visible line regimes. The three solutions found, the quasi-adiabatic, the strongly radiative wind, and the inhibited stationary solution, are then compared to the winds from the Arches cluster, the NGC 4303 central cluster, and to the supernebula in NGC 5253.

Hydrodynamics of the Matter Reinserted within Super Stellar Clusters

We present semianalytic and numerical models that take into account the effect of radiative cooling on the hydrodynamics of the matter reinserted by strong stellar winds and supernovae within the volume occupied by young, massive, and compact super stellar clusters. We first corroborate the location of the threshold line in the plane of mechanical energy input rate versus cluster size found by Silich et al. This line separates clusters that are able to drive a quasi-adiabatic or strongly radiative wind from those in which catastrophic cooling occurs within the cluster volume. Then we show that clusters above the threshold line exhibit a bimodal behavior in which the central, densest zones cool rapidly and accumulate the injected matter to eventually feed further generations of star formation, while the outer zones are still able to drive a stationary wind. The results are presented as a series of universal dimensionless diagrams, from which one can infer the sizes of the two zones, the fraction of the deposited mass that goes into each, and the mechanical luminosity of the resultant winds for clusters of all sizes and energy input rates, regardless the assumed adiabatic terminal speed (VA,∞).

Supergalactic Winds Driven by Multiple Super-Star Clusters

Here we present two-dimensional hydrodynamic calculations of free expanding supergalactic winds, taking into consideration strong radiative cooling. Our main premise is that supergalactic winds are powered by collections of super-star clusters, each of which is a source of a high-metallicity supersonic diverging outflow. The interaction of winds from neighboring super-star clusters is shown here to lead to a collection of stationary oblique shocks and crossing shocks, able to structure the general outflow into a network of dense and cold, kiloparsec long filaments that originate near the base of the outflow. The shocks also lead to extended regions of diffuse soft X-ray emission and, furthermore, to channel the outflow with a high degree of collimation into the intergalactic medium.

EVOLUTION OF SUPER STAR CLUSTER WINDS WITH STRONG COOLING

We study the evolution of super star cluster winds driven by stellar winds and supernova explosions. Time-dependent rates at which mass and energy are deposited into the cluster volume, as well as the time-dependent chemical composition of the re-inserted gas, are obtained from the population synthesis code Starburst99. These results are used as input for a semi-analytic code which determines the hydrodynamic properties of the cluster wind as a function of cluster age. Two types of winds are detected in the calculations. For the quasi-adiabatic solution, all of the inserted gas leaves the cluster in the form of a stationary wind. For the bimodal solution, some of the inserted gas becomes thermally unstable and forms dense warm clumps which accumulate inside the cluster. We calculate the evolution of the wind velocity and energy flux and integrate the amount of accumulated mass for clusters of different mass, radius, and initial metallicity. We also consider conditions with low heating efficiency of the re-inserted gas or mass loading of the hot thermalized plasma with the gas left over from star formation. We find that the bimodal regime and the related mass accumulation occur if at least one of the two conditions above is fulfilled.

The Pressure-confined Wind of the Massive and Compact Super Star Cluster M82-A1

The observed parameters of the young super star cluster M82-A1 and its associated compact H II region are here shown to indicate a low heating efficiency or immediate loss, through radiative cooling, of a large fraction of the energy injected by stellar winds and supernovae during the early evolution of the cluster. This implies a bimodal hydrodynamic solution, which leads to a reduced mass deposition rate into the ISM, with a much reduced outflow velocity. We also show here that in order to match the observed parameters of the H II region associated with M82-A1, the resulting star cluster wind should be confined by a high-pressure interstellar medium. The cluster wind parameters, together with the location of the reverse shock, its cooling length, and the radius of the standing outer H II region, are derived analytically. All of these properties are then confirmed with a semianalytical integration of the flow equations, which provides also us with the run of the hydrodynamic variables as a function of radius. The impact of the results is discussed and extended to other massive and young super star clusters surrounded by a compact H II region.

HOW SIGNIFICANT IS RADIATION PRESSURE IN THE DYNAMICS OF THE GAS AROUND YOUNG STELLAR CLUSTERS

The impact of radiation pressure on the dynamics of the gas in the vicinity of young stellar clusters is thoroughly discussed. The radiation over the thermal/ram pressure ratio time evolution is calculated explicitly and the crucial roles of the cluster mechanical power, the strong time evolution of the ionizing photon flux, and the bolometric luminosity of the exciting cluster are stressed. It is shown that radiation has only a narrow window of opportunity to dominate the wind-driven shell dynamics. This may occur only at early stages of the bubble evolution and if the shell expands into a dusty and/or a very dense proto-cluster medium. The impact of radiation pressure on the wind-driven shell always becomes negligible after about 3 Myr. Finally, the wind-driven model results allow one to compare the model predictions with the distribution of thermal pressure derived from X-ray observations. The shape of the thermal pressure profile then allows us to distinguish between the energy and the momentum-dominated regimes of expansion and thus conclude whether radiative losses of energy or the leakage of hot gas from the bubble interior have been significant during bubble evolution.

On the Extreme Positive Feedback Star-forming Mode from Massive and Compact Super Star Clusters

The force of gravity acting within the volume occupied by young, compact, and massive super star clusters is here shown to drive in situ all the matter deposited by winds and supernovae into several generations of star formation. These events are promoted by radiative cooling, which drains the thermal energy of the ejected gas causing its accumulation to then rapidly exceed the gravitational instability criterion. A detailed account of the integrated ionizing radiation and mechanical luminosity as a function of time is here shown to lead to a new stationary solution. In this, the mass deposition rate , instead of causing a wind as in the adiabatic solution, turns into a positive feedback star-forming mode equal to the star formation rate. Some of the implications of this extreme positive feedback mode are discussed.

On the X-Ray Emission from Massive Star Clusters and Their Evolving Superbubbles

Here we discuss the X-ray emission properties from the hot thermalized plasma that results from the collisions of individual stellar winds and supernovae ejecta within rich and compact star clusters. We propose a simple analytical way of estimating the X-ray emission generated by super star clusters and derive an expression that indicates how this X-ray emission depends on the main cluster parameters. Our model predicts that the X-ray luminosity from the star cluster region is highly dependent on the star cluster wind terminal speed, a quantity related to the temperature of the thermalized ejecta. We have also compared the X-ray luminosity from the super stellar cluster (SSC) plasma with the luminosity of the interstellar bubbles generated from the mechanical interaction of the high-velocity star cluster winds with the interstellar medium (ISM). We found that the hard (2.0-8.0 keV) X-ray emission is usually dominated by the hotter SSC plasma, whereas the soft (0.3-2.0 keV) component is dominated by the bubble plasma. This implies that compact and massive star clusters should be detected as pointlike, hard X-ray sources embedded into extended regions of soft diffuse X-ray emission. We also compared our results with predictions from the population synthesis models that take into consideration binary systems and found that in the case of young, massive, and compact super star clusters the X-ray emission from the thermalized star cluster plasma may be comparable to or even larger than that expected from the high-mass X-ray binary (HMXB) population.

Super stellar clusters with a bimodal hydrodynamic solution: an approximate analytic approach

Aims. We look for a simple analytic model to distinguish between stellar clusters undergoing a bimodal hydrodynamic solution from those able to drive only a stationary wind. Clusters in the bimodal regime undergo strong radiative cooling within their densest inner regions, which results in the accumulation of the matter injected by supernovae and stellar winds and eventually in the formation of further stellar generations, while their outer regions sustain a stationary wind. Methods. The analytic formulae are derived from the basic hydrodynamic equations. Our main assumption, that the density at the star cluster surface scales almost linearly with that at the stagnation radius, is based on results from semi-analytic and full numerical calculations. Results. The analytic formulation allows for the determination of the threshold mechanical luminosity that separates clusters evolving in either of the two solutions. It is possible to fix the stagnation radius by simple analytic expressions and thus to determine the fractions of the deposited matter that clusters evolving in the bimodal regime blow out as a wind or recycle into further stellar generations.