Collisions Of Stars Research Articles

Phenomenological gravitational radiation from rotating stellar collapse to an intermediate mass Kerr black hole

Intermediate mass black holes may be formed through repeated coalescences of compact objects or through the direct collapse of a hypermassive star formed through runaway collisions of main sequence stars. The gravitational wave signature of these two formation scenarios will be different. Here we present an initial study of the waveform generated during the direct axisymmetric collapse of a hypermassive star in order to facilitate searches for this source. We approximate the collapse of the core as an axisymmetric Newtonian free-fall of a rotating relativistic degenerate iron core. The collapse waveform can be reasonably well modeled by an exponential growth.

Star Cluster Migration Near the Galactic Center

AbstractWe performed a self-consistent N-body simulation of star clusters in the Galactic center (GC), taking into account the collisions of stars and formation of an intermediate-mass black hole (IMBH). We find that if an IMBH forms in the cluster, it carries young stars to the GC by a 1:1 resonance.

Irradiation of dust in molecular clouds. I. UV doses

The fluxes of anomalous cosmic rays inside typical molecular clouds produced during collisions of stars with clouds are calculated. The charged particles formed in the cloud in the neighborhood of the star are accelerated in a shock front in the astrosphere by known statistical mechanisms to energies on the order of a few 100 MeV. It is shown that protons and α-particles with energies in the 1 keV ≤ E ≤ 10 GeV range penetrate deeply enough that, over the time of 1-5 hundred thousand years a star is passing through a cloud, they produce sufficient irradiation doses for the ice mantle of dust particles such that the cumulative effect owing to multiple passages would exceed a threshold value on the order of 0.1-1 eV/amu. The possible use of these results for astrophysical interpretation of laboratory experiments on the irradiation of H2O:CH3OH:NH3:CO ice mixtures is discussed. The complex organic substances formed by radiation-chemical transformation may play an important role in the prebiological evolution of the dust component of molecular clouds.

Study of jet fragmentation in p+p collisions at 200 GeV in the STAR experiment

The measurement of jet fragmentation functions in p+p collisions at 200 GeV is of great interest because it provides a baseline to study jet quenching in heavy-ion collisions. It is expected that jet quenching in nuclear matter modifies the jet energy and multiplicity distributions, as well as the jet hadrochemical composition. Therefore, a systematic study of the fragmentation functions for charged hadrons and identified particles is a goal both in p+p and Au+Au collisions at RHIC. Studying fragmentation functions for identified particles is interesting in p+p by itself because it provides a test of NLO calculations at RHIC energies. We present a systematic comparison of jet energy spectra and fragment distributions using different jet-finding algorithms in p+p collisions in STAR. Fragmentation functions of charged and neutral strange particles are also reported for different jet energies.

ϒ Production in p+p and Au+Au collisions in STAR

The study of quarkonium production in relativistic heavy-ion collisions provides an insight into the properties of the produced medium. The lattice studies show a sequential suppression of quarkonia states when compared to normal nuclear matter; which further affirms that a full spectroscopy including bottomonium can provide us with a better thermometer for the matter produced under extreme conditions in relativistic heavy-ion collisions. With the completion of the STAR electromagnetic calorimeter and with the increased luminosity provided by RHIC in Runs 6 and 7, the study of ϒ production via the di-electron channel becomes possible. We present preliminary results on ϒ measurements in p+p collisions (from Run 6) along with preliminary results from Au+Au collisions (in Run 7) at from the STAR experiment.

Orbital dynamics of binary boson star systems

We extend our previous studies of head-on collisions of boson stars by considering orbiting binary boson stars. We concentrate on equal-mass binaries and study the dynamical behavior of boson/boson and boson/antiboson pairs. We examine the gravitational wave output of these binaries and compare with other compact binaries. Such a comparison lets us probe the apparent simplicity observed in gravitational waves produced by black hole binary systems. In our system of interest however, there is an additional internal freedom which plays a significant role in the system's dynamics, namely, the phase of each star. Our evolutions show rather simple behavior at early times, but large differences occur at late times for the various initial configurations.

Thermodynamic Description of Inelastic Collisions in General Relativity

We discuss head-on collisions of neutron stars and disks of dust ("galaxies") following the ideas of equilibrium thermodynamics, which compares equilibrium states and avoids the description of the dynamical transition processes between them. As an always present damping mechanism, gravitational emission results in final equilibrium states after the collision. In this paper we calculate selected final configurations from initial data of colliding stars and disks by making use of conservation laws and solving the Einstein equations. Comparing initial and final states, we can decide for which initial parameters two colliding neutron stars (non-rotating Fermi gas models) merge into a single neutron star and two rigidly rotating disks form again a final (differentially rotating) disk of dust. For the neutron star collision we find a maximal energy loss due to outgoing gravitational radiation of 2.3% of the initial mass while the corresponding efficiency for colliding disks has the much larger limit of 23.8%.

A comparison of hydrodynamic techniques for modelling collisions between main-sequence stars

An Eulerian total variation diminishing (TVD) code and a Lagrangian smoothed particle hydrodynamics (SPH) code are used to simulate the off-axis collision of equal-mass main-sequence stars in order to address the question of whether stellar mergers can produce a remnant star where the interior has been replenished with hydrogen due to significant mixing. Each parent main-sequence star is chosen to be found near the turn-off, with hydrogen depleted in the core, and is modelled with a M = 0.8 M ⊙ realistic stellar model and as a n = 3 polytrope. An ideal fluid description with adiabatic index y = 5/3 is used for all hydrodynamic calculations. We found good agreement between the simulations for the polytropic case, with the remnant showing strong, non-local mixing throughout. In the interior quarter of the mass, ∼35 per cent is mixed in from larger radii and on average the remnant is ∼50 per cent fully mixed. For the realistic model, we found less mixing, particularly in the interior and in the SPH simulation. In the inner quarter, ∼20 per cent of the contained mass in the TVD case, but only ∼3 per cent in the SPH one is mixed in from outside. The simulations give consistent results for the overall profile of the merger remnant and the amount of mass-loss, but the differences in mixing suggests that the intrinsic difference between grid and particle based schemes remains a possible artefact. We conclude that both the TVD and SPH schemes can be used equally well for problems that are best suited to their strengths and that care should be taken in interpreting results about fluid mixing.

Hypervelocity collisions of binary stars at the Galactic Centre

Recent surveys have identified seven hypervelocity stars (HVSs) in the halo of the Milky Way. Most of these stars may have originated from the breakup of binary star systems by the nuclear black hole SgrA*. In some instances, the breakup of the binary may lead to a collision between its member stars. We examine the dynamical properties of these collisions by simulating thousands of different binary orbits around SgrA* with a direct N-body integration code. For some orbital parameters, the two stars collide with an impact velocity lower than their escape velocity and may therefore coalesce. It is possible for a coalescing binary to have sufficient velocity to escape the galaxy. Furthermore, some of the massive S-stars near Sgr A* might be the merger remnants of binary systems, however this production method can not account for most of the S-stars.

Critical Phenomena in Head-On Collisions of Neutron Stars

We find type I critical collapses of compact objects modeled by a polytropic equation of state (EOS) with a polytropic index Gamma=2 without the ultrarelativistic assumption. The object is formed in head-on collisions of neutron stars. Further, we show that the critical collapse can occur due to a change of the EOS, without fine-tuning of initial data. This opens the possibility that a neutron-star-like compact object, not just those formed in a collision, may undergo a critical collapse in processes which slowly change the EOS, such as cooling.

Head-on collisions of boson stars

We study head-on collisions of boson stars in three dimensions. We consider evolutions of two boson stars which may differ in their phase or have opposite frequencies but are otherwise identical. Our studies show that these phase differences result in different late time behavior and gravitational wave output.

Femtoscopy in p+p vs. heavy ion collisions in STAR

The geometric substructure of the particle-emitting source has been characterized via two-particle interferometry by the STAR collaboration for several energies and colliding systems at RHIC. In heavy ion collisions the mT dependence of femtoscopic radii has been observed for all particle types by several experiments at different collision energies. This dependence has been thought to arise from space-momentum correlations generated by collective flow. On the other hand, there are several reports of a similar mT dependence by experiments measuring elementary particle collisions. Here, quite different physical mechanisms – including resonances, strings, and uncertainty arguments – have been proposed to explain the dependence. Determining the differences or similarities in the space-time physics driving the signal in heavy ion versus p+p collisions requires a direct comparison of mT dependence of the radii. Such a comparison has, until now, been sorely lacking. STAR data allow, for the first time, such a direct comparison between A+A, d+A, and p+p collisions, at the same energy, measured in the same detector, and using the same analysis techniques. Surprisingly, our preliminary results indicate an mT-independent scaling of the femtoscopic radii with overall system size. Possible physics implications of these observations will be discussed, and the importance of long-range non-femtoscopic correlations for low multiplicity collisions will be emphasized.

Strangelets: who is looking and how?

It has been over 30 years since the first suggestion that the true ground state of cold hadronic matter might be not nuclear matter but rather strange quark matter (SQM). Ever since, searches for stable SQM have been proceeding in various forms and have observed a handful of interesting events but have neither been able to find compelling evidence for stable strangelets nor to rule out their existence. I will survey the current status and near future of such searches with particular emphasis on the idea of SQM from strange star collisions as part of the cosmic ray flux.

Open charm production from d + Au collisions in STAR

Charmed hadrons are interesting observables in heavy ion collisions. They are becoming more accessible to experimental scrutiny at RHIC energies due to the increased production cross-section of charm with the larger centre-of-mass energy available at RHIC compared to SPS. One source of interest in charm production is due to the fact that gluon fusion dominates the charm production cross-section at high energy. Hence, a measurement of charm hadrons is directly sensitive to the gluon distributions of the colliding particles. In addition, any measurement of production at RHIC, and more importantly any observed suppression, must be compared to the overall production of pairs. A systematic study of charmed hadrons in all collision systems available at RHIC is therefore an invaluable experimental tool in the characterization of the matter produced at RHIC. In particular, d + Au collisions are a necessary step for the comparison of any possible modification of charm production in Au + Au collisions. We present preliminary results on D meson production from d + Au collisions in STAR at = 200 .

K0S, Λ and measurements from 62.4 GeV Au+Au collisions in STAR

We present preliminary measurements by the STAR experiment of Λ, and K0S in Au+Au collisions at . Transverse mass spectra and integrated yield, dN/dy, at mid-rapidity are shown for seven different event centrality classes. Also, the energy dependence of the yields and ratio are compared to the results of other heavy-ion experiments.

Strange particle spectra from collisions in STAR at RHIC

Production of strange hadrons in high energy p+p collisions provides baseline information to which results from A+A collisions may be compared in the search for possible signatures of a deconfined state. However, p+p collisions also demonstrate some surprising effects of their own and should be investigated for the insight they may provide. We present mid-rapidity transverse mass spectra and ⟨pT⟩ for K0S, Λ and Ξ produced in p+p collisions measured by the STAR collaboration at RHIC. Comparisons will be made with available model calculations as well as lower energy results and Au+Au results from the same detector.

Modelling collision products of triple-star mergers

In dense stellar clusters, binary–single and binary–binary encounters can ultimately lead to collisions involving two or more stars during a resonant interaction. A comprehensive survey of multistar collisions would need to explore an enormous amount of parameter space, but here we focus on a number of representative cases involving low-mass (0.4-, 0.6- and 0.8-M⊙) main-sequence stars. Using both smoothed particle hydrodynamics (SPH) calculations and a much faster fluid sorting software package (mmas), we study scenarios in which a newly formed product from an initial collision collides with a third parent star. By varying the order in which the parent stars collide, as well as the orbital parameters of the collision trajectories, we investigate how factors such as shock heating affect the chemical composition and structure profiles of the collision product. Our simulations and models indicate that the distribution of most chemical elements within the final product is not significantly affected by the order in which the stars collide, the direction of approach of the third parent star, or the periastron separations of the collisions. Although the exact surface abundances of beryllium and lithium in the product do depend on the details of the dynamics, these elements are always severely depleted due to mass loss during the collisions. We find that the sizes of the products, and hence their collisional cross-sections for subsequent encounters, can be sensitive to the order and geometry of the collisions. For the cases that we consider, the radius of the product formed in the first (single–single star) collision ranges anywhere from roughly 2–30 times the sum of the radii of its parent stars. The size of the final product formed in our triple-star collisions is more difficult to determine, but it can easily be as large or larger than a typical red giant. Although the vast majority of the volume in such a product contains diffuse gas that could be readily stripped in subsequent interactions, we nevertheless expect the collisional cross-section of a newly formed product to be greatly enhanced over that of a thermally relaxed star of the same mass. Our results also help establish that the algorithms of mmas can quickly reproduce the important features of our SPH models for these collisions, even when one of the parent stars is itself a former product.

The formation of millisecond radio pulsars in close binary systems

An analysis of the basic parameters of a sample of radio and X-ray pulsars that are members of close binary systems is used to separate them into several families according to the nature of the pulsar companions and the previous evolution of the systems. To quantitatively describe the main parameters of close binaries containing neutron stars, we have performed numerical modeling of their evolution. The main driving forces of the evolution of these systems are the nuclear evolution of the donor, the magnetically coupled and radiation-induced stellar winds of the donor, and gravitational-wave radiation. We have considered donors that are low-mass stars in various stages of their evolution, nondegenerate helium stars, and degenerate stars. The systems studied are either the products of the normal evolution of close binaries with large initial component-mass ratios or result from inelastic collisions of old neutron stars with single and binary low-mass, main-sequence stars in the dense cores of globular clusters. The formation of single millisecond pulsars requires either the dynamical disruption of a low-mass (≲0.1M⊙) donor or its complete evaporation under the action of the X-ray radiation of the millisecond pulsar. The observed properties of binary radio pulsars with eccentric orbits combined with the bimodal spatial-velocity distribution of single radio pulsars suggest that it may be possible to explain the observed rotational and spatial motions of all radio pulsars as a result of their formation in close binaries. In this case, neutron stars formed from massive single stars or the components of massive wide binaries probably cannot acquire the high spatial velocities or rapid rotation rates that are required for the birth of a radio pulsar.

Head-on/near head-on collisions of neutron stars with a realistic equation of state

It has been conjectured that in head-on collisions of neutron stars (NSs), the merged object would not collapse promptly even if the total mass is higher than the maximum stable mass of a cold NS. In this paper, we show that the reverse is true: even if the total mass is less than the maximum stable mass, the merged object can collapse promptly. We demonstrate this for the case of NSs with a realistic equation of state (EOS) (the Lattimer-Swesty EOS) in head-on and near head-on collisions. We propose a ``prompt collapse conjecture'' for a generic NS EOS for head-on and near head-on collisions.

Self-similar spherical shock solution with sustained energy injection

We present the generalization of the Sedov-Taylor self-similar strong spherical shock solution for the case of a central energy source varying in time, $E=A t^k$, where $A$ and $k$ are constants. The known Sedov-Taylor solution corresponds to a particular adiabatic case of $k=0$ or \emph{instant shock} with an instant energy source of the shock, $E=A$. The self-similar hydrodynamic flow in the nonadiabatic $k\neq0$ case exists only under the appropriate local entropy (energy) input which must be supported by some radiative mechanism from the central engine. The specific case of $k=1$ corresponds to a permanent energy injection into the shock, or injection shock with a central source of constant luminosity, $L=A$, $E=A t$. The generalized self-similar shock solution may be applied to astrophysical objects in which the duration of central source activity is longer than the shock expansion time, e.g. the early phase of SN explosions, strong wind from stars and young pulsars, non-steady spherical outflow from black holes and collapsing dense stellar clusters with numerous neutron star collisions.