Star Particles Research Articles

Geophysical and Atmospheric Evolution of Habitable Planets

The evolution of Earth-like habitable planets is a complex process that depends on the geodynamical and geophysical environments. In particular, it is necessary that plate tectonics remain active over billions of years. These geophysically active environments are strongly coupled to a planet's host star parameters, such as mass, luminosity and activity, orbit location of the habitable zone, and the planet's initial water inventory. Depending on the host star's radiation and particle flux evolution, the composition in the thermosphere, and the availability of an active magnetic dynamo, the atmospheres of Earth-like planets within their habitable zones are differently affected due to thermal and nonthermal escape processes. For some planets, strong atmospheric escape could even effect the stability of the atmosphere.

Random packings of spiky particles: Geometry and transport properties

Spiky particles are constructed by superposing spheres and oblate ellipsoids. The resulting star particles (but nonconvex) are randomly packed by a sequential algorithm. The geometry, the conductivity, and the permeability of the resulting packings are systematically studied. Overall correlations are proposed to approximate these properties when the geometry of the particle is known.

What makes a planet habitable?

This work reviews factors which are important for the evolution of habitable Earth-like planets such as the effects of the host star dependent radiation and particle fluxes on the evolution of atmospheres and initial water inventories. We discuss the geodynamical and geophysical environments which are necessary for planets where plate tectonics remain active over geological time scales and for planets which evolve to one-plate planets. The discoveries of methane–ethane surface lakes on Saturn’s large moon Titan, subsurface water oceans or reservoirs inside the moons of Solar System gas giants such as Europa, Ganymede, Titan and Enceladus and more than 335 exoplanets, indicate that the classical definition of the habitable zone concept neglects more exotic habitats and may fail to be adequate for stars which are different from our Sun. A classification of four habitat types is proposed. Class I habitats represent bodies on which stellar and geophysical conditions allow Earth-analog planets to evolve so that complex multi-cellular life forms may originate. Class II habitats includes bodies on which life may evolve but due to stellar and geophysical conditions that are different from the class I habitats, the planets rather evolve toward Venus- or Mars-type worlds where complex life-forms may not develop. Class III habitats are planetary bodies where subsurface water oceans exist which interact directly with a silicate-rich core, while class IV habitats have liquid water layers between two ice layers, or liquids above ice. Furthermore, we discuss from the present viewpoint how life may have originated on early Earth, the possibilities that life may evolve on such Earth-like bodies and how future space missions may discover manifestations of extraterrestrial life.

High pT correlations with strange particles in STAR

We present the highlights of the current identified strange particles Δϕ and Δη correlations analyses, including system-size and trigger-p⊥ of the jet and ridge, jet, ridge and away-side meson/baryon ratios, and the current state of the multi-strange baryon analysis. We see clear azimuthal peaks of comparable strength for all strange baryons and K0S mesons. We see no observable species dependence on the same-side jet or ridge yields as a function of p⊥. However, while the away side and the ridge have the Λ0 to K0S ratio similar to that of the bulk, the jet-only ratio is similar to that in p+p. The implications of these findings on current in-medium jet theoretical explanations are discussed.

Supermassive black hole binaries in gaseous and stellar circumnuclear discs: orbital dynamics and gas accretion

The dynamics of two massive black holes in a rotationally supported nuclear disc of mass Mdisc= 108 M⊙ is explored using N-body/smoothed particle hydrodynamics simulations. Gas and star particles are copresent in the disc. Described by a Mestel profile, the disc has a vertical support provided by turbulence of the gas, and by stellar velocity dispersion. A primary black hole of mass 4 × 106 M⊙ is placed at the centre of the disc, while a secondary black hole is set initially on an eccentric corotating orbit in the disc plane. Its mass is in a 1:1, 1:4, and 1:10 ratio, relative to the primary. With this choice, we mimic the dynamics of black hole pairs released in the nuclear region at the end of a gas-rich galaxy merger. It is found that, under the action of dynamical friction, the two black holes form a close binary in ∼10 Myr. The inspiral process is insensitive to the mass fraction in stars and gas present in the disc and is accompanied by the circularization of the orbit. We detail the gaseous mass profile bound to each black hole that can lead to the formation of two small Keplerian discs, weighing ≈2 per cent of the black hole mass, and of size ∼0.01 pc. The mass of the tightly (loosely) bound particles increases (decreases) with time as the black holes spiral into closer and closer orbits. Double active galactic nucleus activity is expected to occur on an estimated time-scale of ≲10 Myr, comparable to the inspiral time-scale. The double nuclear point-like sources that may appear during dynamical evolution will have typical separations of ≲10 pc.

An introduction to dynamical friction in stellar systems

An introductory exposition of Chandrasekhar’s gravitational dynamical friction, appropriate for an undergraduate class in mechanics, is presented. This friction results when a heavy particle moving through a “sea” of much lighter star particles experiences a retarding force due to the exchange of energy and momentum. The general features of dynamical friction are presented, both in an elementary and in a more elaborate way using a hyperbolic two-body interaction. The orbital decay of a heavy particle in a homogeneous gravitational system is solved analytically, which leads to damped harmonic oscillator motion. Numerical integration of the equation of motion in a more realistic case is done. The results are compared to those of an N-body computer simulation. Several problems and projects are suggested for further study.

The importance of mergers for the origin of intracluster stars in cosmological simulations of galaxy clusters

We study the origin of the diffuse stellar component (DSC) in 117 galaxy clusters extracted from a cosmological hydrodynamical simulation. We identify all galaxies present in the simulated clusters at 17 output redshifts, starting with z= 3.5, and then build the family trees for all the z= 0 cluster galaxies. The most massive cluster galaxies show complex family trees, resembling the merger trees of dark matter haloes, while the majority of other cluster galaxies experience only one or two major mergers during their entire life history. Then, for each diffuse star particle identified at z= 0, we look for the galaxy to which it once belonged at an earlier redshift, thus linking the presence of the DSC to the galaxy formation history. The main results of our analysis are as follows. (i) On average, half of the DSC star particles come from galaxies associated with the family tree of the most massive galaxy (bright cluster galaxy – hereafter BCG), one quarter comes from the family trees of other massive galaxies and the remaining quarter from dissolved galaxies. That is, the formation of the DSC is parallel to the build-up of the BCG and other massive galaxies. (ii) Most DSC star particles become unbound during mergers in the formation history of the BCGs and of other massive galaxies, independent of cluster mass. Our results suggest that the tidal stripping mechanism is responsible only for a minor fraction of the DSC. (iii) At cluster radii larger than 250 h−1 kpc, the DSC fraction from the BCG is reduced and the largest contribution comes from the other massive galaxies; in the cluster outskirts, galaxies of all masses contribute to the DSC. (iv) The DSC does not have a preferred redshift of formation: however, most DSC stars are unbound at z < 1. (v) The amount of DSC stars at z= 0 does not correlate strongly with the global dynamical history of clusters, and increases weakly with cluster mass.

Need for purely laboratory-based axionlike particle searches

The PVLAS signal has led to the proposal of many experiments searching for light bosons coupled to photons. The coupling strength probed by these near-future searches is, however, far from the allowed region, if astrophysical bounds apply. But the environmental conditions for the production of axionlike particles in stars are very different from those present in laboratories. We consider the case in which the coupling and the mass of an axionlike particle depend on environmental conditions such as the temperature and matter density. This can relax astrophysical bounds by several orders of magnitude, just enough to allow for the PVLAS signal. This creates exciting possibilities for a detection in near-future experiments.

Formation of Star-Like and Core-Shell AuAg Nanoparticles during Two- and Three-Step Preparation in Batch and in Microfluidic Systems

Regular dendrit-like metal nanoparticles and core-shell nanoparticles were formed by the reduction of mixtures of tetrachloroaurate and silver nitrate solutions with ascorbic acid at room temperature in two- and three-step procedures. The formation of these particles was found in batch experiments as well as in micro flow-through processes using static micromixers. The characteristic diameters of 4-branched star particles were in the range between 60 and 100 nm. The typical particles consist of four metal cores which are embedded in a common shell. Additionally, particles with five and more metallic cores were formed, to some extent, and aggregates of the 4-branched particles also were formed. Larger aggregates and network-like structures of connected star particles were formed after sedimentation. The properties of the formed particles are dependent on the educt concentrations as well as on the order of mixing steps and on the time interval between them. Obviously, the relation of nucleation and particle growth in relation to the concentrations of metal ions determines the composition and the properties of formed nanoparticles. So, star-like particles are observed in case of nucleation of Au in absence of silver ions but with silver deposition after short nucleation time. Spherical core shell particles are formed in case of silver salt addition after complete reduction of tetrachloroaurate in flow-through experiments with sufficient residence time between both mixing steps. Polymer layers are always found in the form of a second outer shell even if the polymer solutions are added in an early stage of particle formation.

Properties of the galaxy population in hydrodynamical simulations of clusters

We present a study of the galaxy population predicted by hydrodynamical simulations of galaxy clusters. These simulations, which are based on the gadget-2tree + sph code, include gas cooling, star formation, a detailed treatment of stellar evolution and chemical enrichment, as well as supernova energy feedback in the form of galactic winds. As such, they can be used to extract the spectrophotometric properties of the simulated galaxies, which are identified as clumps in the distribution of star particles. Simulations have been carried out for a representative set of 19 cluster-sized haloes, having mass M200 in the range 5 × 1013–1.8 × 1015 h−1 M⊙. All simulations have been performed for two choices of the stellar initial mass function (IMF), namely using a standard Salpeter IMF with power-law index x= 1.35, and a top-heavy IMF with x= 0.95. In general, we find that several of the observational properties of the galaxy population in nearby clusters are reproduced fairly well by simulations. A Salpeter IMF is successful in accounting for the slope and the normalization of the colour–magnitude relation for the bulk of the galaxy population. In contrast, the top-heavy IMF produces too red galaxies, as a consequence of their exceedingly large metallicity. Simulated clusters have a relation between mass and optical luminosity, which generally agrees with observations, both in normalization and in slope. Also in keeping with observational results, galaxies are generally bluer, younger and more star forming in the cluster outskirts. However, we find that our simulated clusters have a total number of galaxies which is significantly smaller than the observed one, falling short by about a factor of 2–3. We have verified that this problem does not have an obvious numerical origin, such as lack of mass and force resolution. Finally, the brightest cluster galaxies are always predicted to be too massive and too blue, when compared to observations. This is due to gas overcooling, which takes place in the core regions of simulated clusters, even in the presence of the rather efficient supernova feedback used in our simulations.

Feedback and metal enrichment in cosmological smoothed particle hydrodynamics simulations — I. A model for chemical enrichment

We discuss a model for treating chemical enrichment by Type II and Type Ia supernova (SNII and SNIa) explosions in simulations of cosmological structure formation. Our model includes metal-dependent radiative cooling and star formation in dense collapsed gas clumps. Metals are returned into the diffuse interstellar medium by star particles using a local smoothed particle hydrodynamics (SPH) smoothing kernel. A variety of chemical abundance patterns in enriched gas arise in our treatment owing to the different yields and lifetimes of SNII and SNIa progenitor stars. In the case of SNII chemical production, we adopt metal-dependent yields. Because of the sensitive dependence of cooling rates on metallicity, enrichment of galactic haloes with metals can in principle significantly alter subsequent gas infall and the build-up of the stellar components. Indeed, in simulations of isolated galaxies we find that a consistent treatment of metal-dependent cooling produces 25 per cent more stars outside the central region than simulations with a primordial cooling function. In the highly enriched central regions, the evolution of baryons is however not affected by metal cooling, because here the gas is always dense enough to cool. A similar situation is found in cosmological simulations because we include no strong feedback processes which could spread metals over large distances and mix them into unenriched diffuse gas. We demonstrate this explicitly with test simulations which adopt suprasolar cooling functions leading to large changes both in the stellar mass and in the metal distributions. We also find that the impact of metallicity on the star formation histories of galaxies may depend on their particular evolutionary history. Our results hence emphasize the importance of feedback processes for interpreting the cosmic metal enrichment.

N-Body Growth of a Bahcall-Wolf Cusp around a Black Hole

We present a clear N-body realization of the growth of a Bahcall-Wolf f ~ E^0.25 (rho ~ 1/r^1.75) density cusp around a massive object (black hole) at the center of a stellar system. Our N-body algorithm incorporates a novel implementation of Mikkola-Aarseth chain regularization to handle close interactions between star and black hole particles. Forces outside the chain were integrated on a GRAPE-6A/8 special-purpose computer with particle numbers up to N=0.25 x 10^6. We compare our N-body results with predictions of the isotropic Fokker-Planck equation and verify that the time dependence of the density (both configuration and phase-space) predicted by the Fokker-Planck equation is well reproduced by the N-body algorithm, for various choices of N and of the black hole mass. Our results demonstrate the feasibility of direct-force integration techniques for simulating the evolution of galactic nuclei on relaxation time scales.

The Diffuse Light in Simulations of Galaxy Clusters

We study the properties of the diffuse light in galaxy clusters forming in a large hydrodynamical cosmological simulation of the Λ cold dark matter cosmology. The simulation includes a model for radiative cooling, star formation in dense cold gas, and feedback by Type II supernova explosions. We select clusters having mass M > 1014 h-1 M☉ and study the spatial distribution of their star particles. While most stellar light is concentrated in gravitationally bound galaxies orbiting in the cluster potential, we find evidence for a substantial diffuse component, which may account for the extended halos of light observed around central cD galaxies. We find that more massive simulated clusters have a larger fraction of stars in the diffuse light than the less massive ones. The intracluster light is more centrally concentrated than the galaxy light, and the stars in the diffuse component are on average older than the stars in cluster galaxies, supporting the view that the diffuse light is not a random sampling of the stellar population in the cluster galaxies. We thus expect that at least ~10% of the stars in a cluster may be distributed as intracluster light, largely hidden thus far because of its very low surface brightness.

Iron abundances and heating of the intracluster medium in hydrodynamical simulations of galaxy clusters

Results from a large set of hydrodynamical smoothed particle hydrodynamics (SPH) simulations of galaxy clusters in a flat ΛCDM cosmology are used to investigate the metal enrichment and heating of the intracluster medium (ICM). The physical modelling of the gas includes radiative cooling, star formation, energy feedback and metal enrichment which follow from the explosions of supernovae of type II and Ia. The metallicity dependence of the cooling function is also taken into account. The gas is metal-enriched from star particles according to the SPH prescriptions. The simulations have been performed to study the dependence of final metal abundances and heating of the ICM on the numerical resolution and the model parameters. For a fiducial set of model prescriptions the results indicate radial iron profiles in broad agreement with observations; global iron abundances are also consistent with data. It is found that the iron distribution in the intracluster medium is critically dependent on the shape of the metal deposition profile. At large radii the radial iron abundance profiles in the simulations are steeper than those in the data, suggesting a dynamical evolution of simulated clusters different from those observed. For low-temperature clusters simulations yield iron abundances below the allowed observational range, unless a minimum diffusion length of metals in the ICM is introduced. The simulated emission-weighted radial temperature profiles are in good agreement with data for cooling flow clusters, but at very small distances from the cluster centres (∼2 per cent of the virial radii) the temperatures are a factor of ∼2 higher than the measured spectral values. The luminosity–temperature relation is in excellent agreement with the data; cool clusters (TX∼ 1 keV) have a core excess entropy of ∼200 keV cm2 and their X-ray properties are unaffected by the amount of feedback energy that has heated the ICM. The findings support the model proposed recently by Bryan, where the cluster X-ray properties are determined by radiative cooling. The fraction of hot gas fg at the virial radius increases with TX, and the distribution obtained from the simulated cluster sample is consistent with the observational ranges.

Shape factors and interaction parameters in equations of state Part I. Repulsion phenomena in rigid particle systems

An equation of state for the fluid phase of nonattracting rigid particles of arbitrary shape is presented. A particle shape factor is defined and its dependence on the packing fraction is evaluated from Monte Carlo and molecular dynamics simulation results for a variety of nonattracting asymmetric rigid particles, e.g., rigid-sphere polymers, ellipsoids, star particles, spherocylinders.

Residual gas expulsion from young globular clusters

The results of N-body simulations of the expulsion of residual gas from young globular clusters are presented. Globular clusters with a variety of initial masses, Galactocentric radii, concentration and initial mass function slope with star formation efficiencies (SFEs) below 50% were investigated. The residual gas in the clusters was simply treated as an external potential acting upon the star particles which was reduced in a number of ways to simulate the loss of the gas. The states of the clusters after ~50 Myr was compared to the results of Chernoff & Shapiro (1987) in order to estimate if the clusters would be able to survive for a Hubble time or would be disrupted. The rapid expulsion of large amounts of a cluster's initial mass is found to considerably effect the structures of clusters, but they may be able to survive with SFEs far lower than 50%. It is suggested that the Galactic globular cluster population could have formed from a proto-cluster populations with SFEs ~40% and central densities approximately the same as those found in giant molecular clouds.

A low noise amplifier-shaper with tail correction for the STAR detector

A 16 channel low noise amplifier shaper has been designed for the STAR particle detector of the RHIC accelerator. The STAR Amplifier-Shaper (SAS) includes a pole/zero network which cancels the long tail of the Time Projection Chamber (TPC) signal. The tail correction can be adjusted depending on the type of gas used in the TPC. The SAS equivalent noise charge is 900e/sub RMS/ with 25 pf detector capacitance (the test board having 7.7 pF of parasitic capacitance), and with 80 ns shaping time (step response). The measured noise slope is 13.7e/sub RMS//pF The shaper pulse FWHM is adjusted at 180 ns (detector response) with +/-4% variation over the entire dynamic range. The shaping time and the tail correction are adjusted with external voltages using MOS resistors. The gain is 16 mV/fC with a linearity of 4%. The crosstalk is about 0.36% which have a negligible effect on the position resolution. The circuit also includes an on-chip calibration system in which the test input charge is controlled by a DC voltage. The output buffer drives a 2 V swing on 50 pF output load for a total power consumption of less than 750 mW (+/-5 Volt supply). On-chip protection diodes have also been integrated. The full custom chip has been integrated in the CMOS ORBIT 1.2 /spl mu/m technology with double polysilicon capacitors.

Star-forming galaxy models: Blending star formation into TREESPH

We have incorporated star-formation algorithms into a hybrid N-body/smoothed particle hydrodynamics code (TREESPH) in order to describe the star forming properties of disk galaxies over timescales of a few billion years. The models employ a Schmidt law of index n approximately 1.5 to calculate star-formation rates, and explicitly include the energy and metallicity feedback into the Interstellar Medium (ISM). Modeling the newly formed stellar population is achieved through the use of hybrid SPH/young star particles which gradually convert from gaseous to collisionless particles, avoiding the computational difficulties involved in creating new particles. The models are shown to reproduce well the star-forming properties of disk galaxies, such as the morphology, rate of star formation, and evolution of the global star-formation rate and disk gas content. As an example of the technique, we model an encounter between a disk galaxy and a small companion which gives rise to a ring galaxy reminiscent of the Cartwheel (AM 0035-35). The primary galaxy in this encounter experiences two phases of star forming activity: an initial period during the expansion of the ring, and a delayed phase as shocked material in the ring falls back into the central regions.

Weakly interacting, massive particles in horizontal-branch stars

view Abstract Citations (19) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Weakly Interacting, Massive Particles in Horizontal-Branch Stars Spergel, David N. ; Faulkner, John Abstract Weakly interacting, massive particles (WIMPs or cosmions) can be an important energy transport mechanism in stars. In previous papers, the authors have shown that WIMPs can isothermalize the core of the Sun and solve the solar neutrino problem. This Letter reconsiders the role of cosmions in horizontal-branch stars. The authors demonstrate that WIMPs either rapidly evaporate from horizontal-branch stars or are inefficient at energy transport. They have no effect on the later stages of stellar evolution. Publication: The Astrophysical Journal Pub Date: August 1988 DOI: 10.1086/185227 Bibcode: 1988ApJ...331L..21S Keywords: Dark Matter; Horizontal Branch Stars; Particle Interactions; Particle Mass; Stellar Interiors; Weak Energy Interactions; Energy Transfer; Nuclear Reactions; Solar Neutrinos; Stellar Evolution; Astrophysics; DARK MATTER; NUCLEAR REACTIONS; STARS: HORIZONTAL-BRANCH; STARS: INTERIORS full text sources ADS |

Bounds on exotic-particle interactions from SN1987A.

The observation of a neutrino pulse from the supernova SN 1987A constrains the production of light exotic particles in the protoneutron star. A new bound on the axion decay constant is derived, f(a) of greater than about 10 to the 10th GeV. If right-handed (RH) neutrinos exist, 'RH Fermi constant' is G(RH) of less than about 10 exp-4 G(F), two orders of magnitude below laboratory bounds. The Dirac mass of hv sub m can be constrained below laboratory limits. The Dirac mass of the mu-neutrino can be constrained below laboratory limits.