arXiv subjects

Jan Palous

Publications and source records attributed to Jan Palous.

25 records · Page 2Linked to original sources

arXiv2009

The fragmentation of expanding shells I: Limitations of the thin--shell approximation

We investigate the gravitational fragmentation of expanding shells in the context of the linear thin--shell analysis. We make use of two very different numerical schemes; the FLASH Adaptive Mesh Refinement code and a version of the Benz Smoothed Particle Hydrodynamics code. We find that the agreement between the two codes is excellent. We use our numerical results to test the thin--shell approximation and we find that the external pressure applied to the shell has a strong effect on the fragmentation process. In cases where shells are not pressure--confined, the shells thicken as they expand and hydrodynamic flows perpendicular to the plane of the shell suppress fragmentation at short wavelengths. If the shells are pressure--confined internally and externally, so that their thickness remains approximately constant during their expansion, the agreement with the analytical solution is better.

astro-ph.GA↗

arXiv2008

Spatial motion of the Magellanic Clouds. Tidal models ruled out?

Recently, Kallivayalil et al. derived new values of the proper motion for the Large and Small Magellanic Clouds (LMC and SMC, respectively). The spatial velocities of both Clouds are unexpectedly higher than their previous values resulting from agreement between the available theoretical models of the Magellanic System and the observations of neutral hydrogen (HI) associated with the LMC and the SMC. Such proper motion estimates are likely to be at odds with the scenarios for creation of the large-scale structures in the Magellanic System suggested so far. We investigated this hypothesis for the pure tidal models, as they were the first ones devised to explain the evolution of the Magellanic System, and the tidal stripping is intrinsically involved in every model assuming the gravitational interaction. The parameter space for the Milky Way (MW)-LMC-SMC interaction was analyzed by a robust search algorithm (genetic algorithm) combined with a fast restricted N-body model of the interaction. Our method extended the known variety of evolutionary scenarios satisfying the observed kinematics and morphology of the Magellanic large-scale structures. Nevertheless, assuming the tidal interaction, no satisfactory reproduction of the HI data available for the Magellanic Clouds was achieved with the new proper motions. We conclude that for the proper motion data by Kallivayalil et al., within their 1-sigma errors, the dynamical evolution of the Magellanic System with the currently accepted total mass of the MW cannot be explained in the framework of pure tidal models. The optimal value for the western component of the LMC proper motion was found to be pm_w(LMC) > -1.3 mas/yr in case of tidal models. It corresponds to the reduction of the Kallivayalil et al. value for pm_w(LMC) by approx. 40% in its magnitude.

astro-ph↗

arXiv2006

On the hydrodynamics of the matter reinserted within superstellar clusters

We present semi-analytical and numerical models, accounting for the impact of radiative cooling on the hydrodynamics of the matter reinserted as strong stellar winds and supernovae within the volume occupied by young, massive and compact superstellar clusters. First of all we corroborate the location of the threshold line in the mechanical energy input rate vs the cluster size plane, found by Silich et al. (2004). Such a line separates clusters able to drive a quasi-adiabatic or a strongly radiative wind from clusters in which catastrophic cooling occurs within the star cluster volume. Then we show that the latter, clusters above the threshold line, undergo 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 into a series of universal dimensionless diagrams from which one can infer: the size of the two zones, the fraction of the deposited mass that goes into each of them and the luminosity of the resultant winds, for clusters of all sizes and energy input rates, regardless the assumed adiabatic terminal speed V_A.

astro-ph↗

arXiv2004

Galaxy Collisions, Gas Stripping and Star Formation in the Evolution of Galaxies

A review of gravitational and hydrodynamical processes during formation of clusters and evolution of galaxies is given. Early, at the advent of N-body computer simulations, the importance of tidal fields in galaxy encounters has been recognized. Orbits are crowded due to tides along spiral arms, where the star formation is enhanced. Low relative velocity encounters lead to galaxy mergers. The central dominating galaxies in future clusters form before the clusters in a merging process in galaxy groups. Galaxy clusters are composed in a hierarchical scenario due to relaxation processes between galaxies and galaxy groups. As soon as the overall cluster gravitational potential is built, high speed galaxy versus galaxy encounters start to play a role. These harassment events gradually thicken and shorten spiral galaxy disks leading to the formation of S0 galaxies and ellipticals. Another aspect of the high speed motion in the hot and diluted Intracluster Medium (ICM) is the ram pressure exerted on the Interstellar Matter (ISM) leading to stripping of the ISM from parent spirals. The combinations of tides and ram pressure stripping efficiently removes the gas from spirals, quenching the star formation in galactic disks, while triggering it in the tidal arms and at the leading edge of gaseous disk. Gas stripping from disks transports the metals to the ICM. In some cases, the gas extracted from the galactic disks becomes self-gravitating forming tidal dwarf galaxies.

astro-ph↗

arXiv2003

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 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. The self-contamination resultant from the continuous generation of stars is shown to lead to a large metal spread in massive ($\sim$ 10$^6$ M$_\odot$) clusters, while clusters with a mass similar to 10$^5$ M$_\odot$ 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.

astro-ph↗

arXiv2002

Environmental Dependencies for Star Formation Triggered by Expanding Shell Collapse

Criteria for gravitational collapse of expanding shells in rotating, shearing galaxy disks were determined using three-dimensional numerical simulations in the thin shell approximation. The simulations were run over a grid of 7 independent variables, and the resultant probabilities for triggering and unstable masses were determined as functions of 8 dimensionless parameters. When the ratio of the midplane gas density to the midplane total density is small, an expanding shell reaches the disk scale height and vents to the halo before it collapses. When the Toomre instability parameter Q, or a similar shear parameter, Q_A, are large, Coriolis forces and shear stall or reverse the collapse before the shell accumulates enough mass to be unstable. With large values of C=c_sh/(GL)^0.2, for rms velocity dispersion c_sh in the swept-up matter and shell-driving luminosity L, the pressure in the accumulated gas is too large to allow collapse during the expansion time. Considering ~5000 models covering a wide range of parameter space, the common properties of shell collapse as a mechanism for triggered star formation are: (1) the time scale is 4*sqrt(C/2 pi G rho) for ambient midplane density rho, (2) the total fragment mass is ~2x10^7 Msun, of which only a small fraction is likely to be molecular, (3) the triggering radius is ~2 times the scale height, and the triggering probability is ~0.5 for large OB associations. Star formation triggered by shell collapse should be most common in gas-rich galaxies, such as young galaxies or those with late Hubble types.

astro-ph↗

arXiv2001

Continuous stellar mass-loss in N-body models of galaxies

We present an N-body computer code - aimed at studies of galactic dynamics - with a CPU-efficient algorithm for a continuous (i.e. time-dependent) stellar mass-loss. First, we summarize available data on stellar mass-loss and derive the long-term (20 Gyr) dependence of mass-loss rate of a coeval stellar population. We then implement it, through a simple parametric form, into a particle-mesh code with stellar and gaseous particles. We perform several tests of the algorithm reliability and show an illustrative application: a 2D simulation of a disk galaxy, starting as purely stellar but evolving as two-component due to gradual mass-loss from initial stars and due to star formation. In a subsequent paper we will use the code to study what changes are induced in galactic disks by the continuous gas recycling compared to the instantaneous recycling approximation, especially the changes in star formation rate and radial inflow of matter.

astro-ph↗