Star Mergers Research Articles

Conformational properties and interaction of polyelectrolyte pH-sensitive stars

Changes in the conformations of two polyelectrolyte stars with amphiphilic segments during their convergence are investigated by the Scheutjens-Fleer method (a numerical method of solving self-consistent field equations). The stars are placed in an aqueous-salt solvent at a pH close to the pK of the segments of polyelectrolyte arms. Individual stars have a two-phase quasi-micellar conformation at a sufficiently strong hydrophobicity of segments in the uncharged state and at a sufficiently low salt concentration. This conformation is formed via separation of star arms into two groups: One consists of uncharged arms and forms a dense core in the star center, while the other consists of arms coming through and forming a corona with charged arms. When stars in this conformation converge, the transition of arms from the corona on the side of the oncoming star to the core occurs inside both stars. When stars touch each other with their cores, the stars merge into a united quasi-micelle. The free energy of interaction of stars is a nonmonotonic function of distance D between their centers. This value grows with a decrease in D until the cores come into contact. Upon the contact and merging of the cores, the free-energy values are characterized by a local minimum corresponding to an ellipsoid-shaped united quasi-micelle. With an increase in the concentration of salt in the solution, individual stars adopt extended conformations. In this case, the interaction of two stars is repulsive at all D values. However, if the salt concentration very slightly exceeds the threshold value of formation of a core in an isolated star, then the initially extended stars during their approach toward each other adopt (at first, each of them separately) a quasi-micellar conformation, while their cores grow as they converge and merge upon contact.

The most magnetic stars

Observations of magnetic A, B and O stars show that the poloidal magnetic flux per unit mass has an upper bound of 10^-6.5 G cm^2/g. A similar upper bound is found for magnetic white dwarfs even though the highest magnetic field strengths at their surfaces are much larger. For magnetic A and B stars there also appears to be a well defined lower bound below which the incidence of magnetism declines rapidly. According to recent hypotheses, both groups of stars may result from merging stars and owe their strong magnetism to fields generated by a dynamo mechanism as they merge. We postulate a simple dynamo that generates magnetic field from differential rotation. The growth of magnetic fields is limited by the requirement that the poloidal field stabilizes the toroidal and vice versa. While magnetic torques dissipate the differential rotation, toroidal field is generated from poloidal by an Omega dynamo. We further suppose that mechanisms that lead to the decay of toroidal field lead to the generation of poloidal. Both poloidal and toroidal fields reach a stable configuration which is independent of the size of small initial seed fields but proportional to the initial differential rotation. We pose the hypothesis that strongly magnetic stars form from the merging of two stellar objects. The highest fields are generated when the merge introduces differential rotation that amounts to critical break up velocity within the condensed object. Calibration of a simplistic dynamo model with the observed maximum flux per unit mass for main-sequence stars and white dwarfs indicates that about 1.5x10^-4 of the decaying toroidal flux must appear as poloidal. The highest fields in single white dwarfs are generated when two degenerate cores merge inside a common envelope or when two white dwarfs merge by gravitational-radiation angular momentum loss.

Lithium production in the merging of white dwarf stars

The origin of R Coronae Borealis stars has been elusive for over 200 years. Currently, two theories for their formation have been presented. These are the final flash scenario, in which a dying asymptotic giant branch star throws off its atmosphere to reveal the hydrogen poor, heavily processed material underneath, and the double degenerate scenario, in which two white dwarfs merge to produce a new star with renewed vigour. Some theories predict that the temperatures reached during the latter scenario would destroy any lithium originally present in the white dwarfs. The observed lithium content of some R Coronae Borealis stars, therefore, is often interpreted as an indication that the final flash scenario best describes their formation. In this paper, we show that lithium production can, indeed, occur in the merging of a helium white dwarf with a carbon-oxygen white dwarf if their chemical composition, particularly that of 3He, is fully considered. The production mechanism is described in detail, and the sensitivity of lithium production to the merger environment is investigated. Nucleosynthesis post-processing calculations of smoothed-particle hydrodynamics (SPH) tracer particles are performed to show that any lithium produced in these environments will be concentrated towards the cloud of material surrounding the R CrB star. Measurements of the lithium content of these clouds would, therefore, provide a valuable insight into the formation mechanism of R CrB stars.

Stochasticity, a variable stellar upper mass limit, binaries and star formation rate indicators

Using our Binary Population And Spectral Synthesis (BPASS) code we explore the effects on star-formation rate indicators of stochastically sampling the stellar initial mass function, adding a cluster mass dependent stellar upper-mass limit and including binary stars. We create synthetic spectra of young clusters and star-forming galaxies and compare these to observations of H(alpha) emission from isolated clusters and the relation between H(alpha) and FUV emission from nearby galaxies. We find that observations of clusters tend to favour a purely stochastic sampling of the initial mass function for clusters less than 100M(Sun), rather than the maximum stellar mass being dependant on the total cluster mass. It is more difficult to determine whether the same is true for more massive clusters. We also find that binary stars blur some of the observational differences that occur when a cluster-mass dependent stellar upper-mass limit is imposed when filling the IMF. The effect is greatest when modelling the observed H(alpha) and FUV star-formation rate ratios in galaxies. This is because mass transfer and merging of stars owing to binary evolution creates more massive stars and stars that have greater mass than the initial maximum imposed on the stellar population.

The Far, Far Future of Stars

The article reports on projections by astronomers for the future of the universe. It is predicted that as a decrease in interstellar gas slows the formation of stars, heavy elements in the cosmos will give rise to dimmer, low-mass, and longer-lived stars as well as new planets. An overview is given of computer simulations calculating the probability of phenomena such as collisions between planets or galaxies, the merging of stars, and death of stars into red giants, helium white dwarfs, and supernovae. Also discussed are possible outcomes for the end of star formation in trillions of years. INSETS: Enriching the Cosmic Mulch;Enriching the Cosmic Mulch;Enriching the Cosmic Mulch.

Goldilocks Black Holes

Astronomers are investigating middleweight, or intermediate-mass, black holes to understand how black holes found at the center of large galaxies from which nothing, not even light, can escape formed so rapidly. The research suggests that middleweight black holes formed by the collapse of primordial gas clouds in the early universe that bypassed the star stage rather than early stars providing small seeds that merged together. Additional clues about middleweight holes may come from studying bulgeless galaxies which conceal middleweight holes behind dust. INSETS: What Were the Seeds of Supermassive Black Holes?;Early Evidence Favors Gas Collapse over Star Mergers.

Stars acquire youth through duplicity

The origin of unusually hot stars in a sparse cluster has been attributed to their being members of binary systems rather than stellar collisions. This prompts a rethink of how stars merge when they collide. See Letter p.356 Blue straggler stars challenge the standard theory of stellar evolution, as these main sequence stars are brighter and bluer than others in a cluster thought to have formed at about the same time. In theory, they should have already evolved into giants and stellar remnants. Explanations offered to account for these stragglers include stellar collisions, mass transfer from a companion star or mergers in binaries. Aaron Geller and Robert Mathieu have combined precise observations spanning more than a decade with numerical simulations to show that the binary companions of the majority of the blue stragglers in the old open cluster NGC 188 are consistent with a mass transfer origin, and inconsistent with predictions of the other suggested formation channels.

R -PROCESS NUCLEOSYNTHESIS IN DYNAMICALLY EJECTED MATTER OF NEUTRON STAR MERGERS

Although the rapid neutron-capture process, or r-process, is fundamentally important for explaining the origin of approximately half of the stable nuclei with A > 60, the astrophysical site of this process has not been identified yet. Here we study r-process nucleosynthesis in material that is dynamically ejected by tidal and pressure forces during the merging of binary neutron stars (NSs) and within milliseconds afterwards. For the first time we make use of relativistic hydrodynamical simulations of such events, defining consistently the conditions that determine the nucleosynthesis, i.e., neutron enrichment, entropy, early density evolution and thus expansion timescale, and ejecta mass. We find that 10^{-3}-10^{-2} solar masses are ejected, which is enough for mergers to be the main source of heavy (A > 140) galactic r-nuclei for merger rates of some 10^{-5} per year. While asymmetric mergers eject 2-3 times more mass than symmetric ones, the exact amount depends weakly on whether the NSs have radii of ~15 km for a "stiff" nuclear equation of state (EOS) or ~12 km for a "soft" EOS. R-process nucleosynthesis during the decompression becomes largely insensitive to the detailed conditions because of efficient fission recycling, producing a composition that closely follows the solar r-abundance distribution for nuclei with mass numbers A > 140. Estimating the light curve powered by the radioactive decay heating of r-process nuclei with an approximative model, we expect high emission in the B-V-R bands for 1-2 days with potentially observable longer duration in the case of asymmetric mergers because of the larger ejecta mass.

Serendipitous observation of merging stars could explain rare novae

Serendipitous observation of merging stars could explain rare novae

AN EXTENDED GRID OF NOVA MODELS. III. VERY LUMINOUS, RED NOVAE

Extremely luminous, red eruptive variables like RV in M31 are being suggested as exemplars of a new class of astrophysical object. Our greatly extended series of nova simulations shows that classical nova models can produce very red, luminous eruptions. In a poorly studied corner of 3-D nova parameter space (very cold, low-mass white dwarfs, accreting at very low rates) we find bona fide classical novae that are very luminous and red because they eject very slowly moving, massive envelopes. A crucial prediction of these nova models - in contrast to the predictions of merging star ("mergeburst") models - is that a hot remnant, the underlying white dwarf, will emerge after the massive ejected envelope has expanded enough to become optically thin. This blue remnant must fade on a timescale of decades - much faster than a "mergeburst", which must fade on timescales of millennia or longer. Furthermore, the cooling nova white dwarf and its expanding ejecta must become redder in the years after eruption, while a contracting mergeburst must become hotter and bluer. We predict that red novae will always brighten to L~1000 Lsun for about 1 year before rising to maximum luminosity at L~10^6 - 10^7 Lsun. The maximum luminosity attainable by a nova is likely to be L~10^7 Lsun, corresponding to M-12. In an accompanying paper we describe a fading, luminous blue candidate for the remnant of M31-RV; it is observed with HST to be compatible only with the nova model.

Merging in the common envelope and the origin of early R-type stars

Context. Binary systems experiencing one or two common envelope episodes during the red giant branch or the Hertzsprung gap phases can produce a single star, evolving along the Hayashi track, as a final outcome. Even if these objects are expected to be very common in nature, a proper description of their evolution and physical properties is still missing. Moreover, this scenario (red giant merging scenario) has been invoked as the progenitor systems of early-R stars, by assuming that the physical conditions developed as a consequence of the cores merging could produce the mixing into the convective envelope of fresh carbon that was synthesized during the He-flash. Aims. We analyze in detail the red giant merging scenario to verify if the resulting star develops the physical conditions suitable for a dredge-up of C-enriched material from the core to the envelope. Methods. We performed 3D simulations of the merging stars, to check whether He is burnt efficiently during the formation of a self-sustained disk. We therefore did 1D computations of the accretion phase occurring after the merging and of the following evolution up to the settling of quiescent He-burning in the center. We adopted different assumptions on the amount of angular momentum transferred from the disk to the core and on the angular momentum transport. Results. Efficient He-burning does not occur during the merging, because a very high temperature (T > 10 8 K) at the disk/He-core interface develops only for a few minutes. Our computations show that the accretion process is the leading parameter in determining the final properties of the merged object. In particular, the thermal energy delivered by the accreted matter determines the heating of the whole newborn core, thus preventing the developing of highly degenerate physical conditions. This occurrence determines the onset of the He-burning with an He-flash milder and closer to the center, as compared to standard RGB stars. Rotation and different angular momentum transport efficiency plays a secondary role by determining the exact location of the first He-flash. In none of the computed models is material formed in the He-core mixed into the convective envelope, because the H-burning shell, which always active during the He-flashes and later on, acts as a barrier. Conclusions. In the red giant merging scenario, the physical conditions suitable for both a peculiar He-flash and the dredging-up of C-enriched material never occur. Our results speak against the possibility that such an evolutionary scenario could represent the progenitor system of early R-stars.

Short Gamma Ray Bursts as electromagnetic counterpart of coalescing binary systems

Coalescing binary systems, consisting of two collapsed objects, are among the most promising sources of high frequency gravitational waves signals detectable, in principle, by ground-based interferometers. Binary systems of Neutron Star or Black Hole/Neutron Star mergers should also give rise to short Gamma Ray Bursts, a subclass of Gamma Ray Bursts. Short-hard-Gamma Ray Bursts might thus provide a powerful way to infer the merger rate of two-collapsed object binaries. Under the hypothesis that most short Gamma Ray Bursts originate from binaries of Neutron Star or Black Hole/Neutron Star mergers, we outline here the possibility to associate short Gamma Ray Bursts as electromagnetic counterpart of coalescing binary systems.

ON THE TRANSITION FROM NUCLEAR-CLUSTER- TO BLACK-HOLE-DOMINATED GALAXY CORES

Giant elliptical galaxies, believed to be built from the merger of lesser galaxies, are known to house a massive black hole at their center rather than a compact star cluster. If low- and intermediate-mass galaxies do indeed partake in the hierarchical merger scenario, then one needs to explain why their dense nuclear star clusters are not preserved in merger events. A valuable clue may the recent revelation that nuclear star clusters and massive black holes frequently co-exist in intermediate mass bulges and elliptical galaxies. In an effort to understand the physical mechanism responsible for the disappearance of nuclear star clusters, we have numerically investigated the evolution of merging star clusters with seed black holes. Using black holes that are 1-5% of their host nuclear cluster mass, we reveal how their binary coalescence during a merger dynamically heats the newly wed star cluster, expanding it, significantly lowering its central stellar density, and thus making it susceptible to tidal destruction during galaxy merging. Moreover, this mechanism provides a pathway to explain the observed reduction in the nucleus-to-galaxy stellar mass ratio as one proceeds from dwarf to giant elliptical galaxies.

On the onset of runaway stellar collisions in dense star clusters - II. Hydrodynamics of three-body interactions

The onset of runaway stellar collisions in young star clusters is more likely to initiate with an encounter between a binary and a third star than between two single stars. Using the initial conditions of such three-star encounters from direct N-body simulations, we model the resulting interaction by means of Smoothed Particle Hydrodynamics (SPH). Our code implements new equations of motion that allow for ecient use of non-equal mass particles and is capable of evolving contact binaries for thousands of orbits. We nd that, in the majority of the cases considered, all three stars merge together. In addition, we compare our SPH calculations against those of the sticky-sphere approximation. If one is not concerned with mass loss, then the sticky sphere approach gives the correct qualitative outcome in approximately 75% of the cases considered. Among those cases in which the sticky-sphere algorithm identies only two particular stars to collide, the hydrodynamic calculations nd the same

The origin of magnetism on the upper main sequence

Abstract We consider the incidence of magnetism in main-sequence stars with mainly radiative envelopes. We propose that the small fraction, which increases with mass, of stars which are magnetic can be explained if towards the end of the formation process, after the stars have developed a substantial radiative envelope, a correspondingly small fraction of stars merge. Such late mergers would produce a brief period of strong differential rotation and give rise to large-scale fields in the radiative envelopes. Such late mergers can also account for the lack of close binaries among these stars.

A Customized Python Module for CFD Flow Analysis within VisTrails

A relatively simple, customized Python module that plugs smoothly into an otherwise standard workflow within VisTrails facilitates a quantitative analysis of complex fluid flows in simulations of merging binary stars.

TROJAN STARS IN THE GALACTIC CENTER

We performed, for the first time, the simulation of spiral-in of a star cluster formed close to the Galactic center (GC) using a fully self-consistent $N$-body model. In our model, the central super-massive black hole (SMBH) is surrounded by stars and the star cluster. Not only are the orbits of stars and the cluster stars integrated self-consistently, but the stellar evolution, collisions and merging of the cluster stars are also included. We found that an intermediate-mass black hole (IMBH) is formed in the star cluster and stars escaped from the cluster are captured into a 1:1 mean motion resonance with the IMBH. These "Trojan" stars are brought close to the SMBH by the IMBH, which spirals into the GC due to the dynamical friction. Our results show that, once the IMBH is formed, it brings the massive stars to the vicinity of the central SMBH even after the star cluster itself is disrupted. Stars carried by the IMBH form a disk similar to the observed disks and the core of the cluster including the IMBH has properties similar to those of IRS13E, which is a compact assembly of several young stars.

Neutrino alert systems for Gamma Ray Bursts and transient astronomical sources

Gamma Ray Bursts (GRBs) are the most energetic events in the Universe, associated with the death of massive stars (core-collapse supernovae) or the merging of neutron stars or black holes. Discovered in the early 1970s, their cosmological origin was demonstrated only in 1997, when the first distance was measured. Theoretical models predict that the very energetic processes at work in GRBs accelerate charged particles to such energies that they could contribute to the observed high energy neutrinos. These processes will be discussed and the observational consequences, in particular for current and forthcoming neutrino telescopes, presented.

Short γ-ray bursts and gravitational waves from dynamically formed merging binaries

Merging binary systems consisting of two collapsed objects are among the most promising sources of high frequency gravitational wave, GW, signals for ground based interferometers. Double neutron star or black hole/neutron star mergers are also believed to give rise to short hard bursts, SHBs, a subclass of gamma ray bursts. SHBs might thus provide a powerful way to infer the merger rate of two-collapsed object binaries. Under the hypothesis that most SHBs originate from double neutron star or black hole/neutron star mergers, we outline here a method to estimate the incidence of merging events from dynamically formed binaries in globular clusters and infer the corresponding GW event rate that can be detected with Advanced LIGO/Virgo. In particular a sizeable fraction of detectable GW events is expected to be coincident with SHBs. The beaming and redshift distribution of SHBs are reassessed and their luminosity function constrained by using the results from recent SHBs observations. We confirm that a substantial fraction of SHBs goes off at low redshifts, where the merging of systems formed in globular clusters through dynamical interactions is expected.

The Influence of Initial Mass Segregation on the Runaway Merging of Stars

We have investigated the effect of initial mass segregation on the runaway merging of stars. The evolution of multimass, dense star clusters was followed by means of direct N-body simulations of up to 131,072 stars. All clusters started from King models with dimensionless central potentials of 3.0 ≤ W0 ≤ 9.0. Initial mass segregation was realized by varying the minimum mass of a certain fraction of stars whose either (1) distances were closest to the cluster center or (2) total energies were lowest. The second case is more favorable to promote the runaway merging of stars by creating a high-mass core of massive, low-energy stars. Initial mass segregation could decrease the central relaxation time and thus help the formation of a high-mass core. However, we found that initial mass segregation does not help the runaway stellar merger to happen if the overall mass density profile is kept constant. This is due to the fact that the collision rate of stars is not increased due to initial mass segregation. Our simulations show that initial mass segregation is not sufficient to allow runaway merging of stars to occur in clusters with central densities typical for star clusters in the Milky Way.