Interactions In Globular Clusters Research Articles

Exploring the Mass Segregation Effect of X-Ray Sources in Globular Clusters. IV. Evidence of Black Hole Burning in ω Centauri

Abstract Using X-ray sources as sensitive probes of stellar dynamical interactions in globular clusters (GCs), we study the mass segregation and binary burning processes in ω Cen. We show that the mass segregation of X-ray sources is quenched in ω Cen, while the X-ray source abundance of ω Cen is much smaller than other GCs, and the binary hardness ratio (defined as , with f b being the binary fraction, and L K being the cumulative X-ray and K-band luminosity of GCs, respectively) of ω Cen is located far below the correlation line of the dynamically normal GCs. This evidence suggests that the binary burning processes are highly suppressed in ω Cen, and other heating mechanisms, very likely a black hole subsystem (BHS), are essential in the dynamical evolution of ω Cen. Through the black hole burning processes (i.e., dynamical hardening of the BH binaries), the BHS is dominating the energy production of ω Cen, which also makes ω Cen a promising factory of gravitational-wave sources in the Galaxy.

A period-dependent spatial scatter of Galactic black hole transients

ABSTRACT There remain significant uncertainties in the origin and evolution of black holes in binary systems, in particular regarding their birth sites and the influence of natal kicks. These are long-standing issues, but their debate has been reinvigorated in the era of gravitational wave detections and the improving precision of astrometric measurements. Using recent and archival characterization of Galactic black hole X-ray binaries (BHXBs), we report here an apparent anticorrelation between Porb (system orbital periods) and scatter in $z$ (elevation above the Galactic plane). The absence of long-period sources at high $z$ is not an obvious observational bias, and two possible explanatory scenarios are qualitatively explored: (1) a disc origin for BHXBs followed by natal kicks producing the scatter in $z$, with only the tightest binaries preferentially surviving strong kicks; and (2) a halo origin, with Porb shortening through dynamical interactions in globular clusters (GCs). For the latter case, we show a correspondence in $z$-scatter between BHXBs and the GCs with most compact core radii of <0.1 pc. However, the known absence of outbursting BHXB transients within Galactic GCs remains puzzling in this case, in contrast to the multitude of known GC neutron star XRBs. These results provide an interesting observational constraint for any black hole binary evolutionary model to satisfy.

Black hole mergers from globular clusters observable by LISA II. Resolved eccentric sources and the gravitational wave background

In paper I of this series we showed that a large percentage of the binary black hole (BBH) mergers that form through dynamical interactions in globular clusters will have significant eccentricity in the ~10^{-3}-10^{-1} Hz LISA band. In this work we quantify the evolution of these highly eccentric binaries through the LISA and LIGO bands, and compute the stochastic gravitational wave background from the merging, eccentric population. We find that the population of BBHs that merge in-between three-body encounters inside their cluster (~50% of all cluster-formed BBH mergers) will have measurable eccentricity for their entire lifetime in the LISA band. The population of BBHs that merge during three-body encounters (~5% of all cluster-formed BBH mergers), will be detectable by LIGO with eccentricities of e~0.1. The gravitational wave background from dynamically assembled BBHs encodes a characteristic bump due to the high initial eccentricities of these systems. The location and amplitude of this bump depends on globular cluster properties.

Evolution of highly eccentric binary neutron stars including tidal effects

This work is the first in a series of studies aimed at understanding the dynamics of highly eccentric binary neutron stars, and constructing an appropriate gravitational-waveform model for detection. Such binaries are possible sources for ground-based gravitational wave detectors, and are expected to form through dynamical scattering and multi-body interactions in globular clusters and galactic nuclei. In contrast to black holes, oscillations of neutron stars are generically excited by tidal effects after close pericenter passage. Depending on the equation of state, this can enhance the loss of orbital energy by up to tens of percent over that radiated away by gravitational waves during an orbit. Under the same interaction mechanism, part of the orbital angular momentum is also transferred to the star. We calculate the impact of the neutron star oscillations on the orbital evolution of such systems, and compare these results to full numerical simulations. Utilizing a Post-Newtonian flux description we propose a preliminary model to predict the timing of different pericenter passages. A refined version of this model (taking into account Post-Newtonian corrections to the tidal coupling and the oscillations of the stars) may serve as a waveform model for such highly eccentric systems.

MOCCA-SURVEY Database. I. Eccentric Black Hole Mergers during Binary–Single Interactions in Globular Clusters

Abstract We estimate the population of eccentric gravitational wave (GW) binary black hole (BBH) mergers forming during binary–single interactions in globular clusters (GCs), using ∼800 GC models that were evolved using the MOCCA code for star cluster simulations as part of the MOCCA-Survey Database I project. By re-simulating BH binary–single interactions extracted from this set of GC models using an N-body code that includes GW emission at the 2.5 post-Newtonian level, we find that ∼10% of all the BBHs assembled in our GC models that merge at present time form during chaotic binary–single interactions, and that about half of this sample have an eccentricity >0.1 at 10 Hz. We explicitly show that this derived rate of eccentric mergers is ∼100 times higher than one would find with a purely Newtonian N-body code. Furthermore, we demonstrate that the eccentric fraction can be accurately estimated using a simple analytical formalism when the interacting BHs are of similar mass, a result that serves as the first successful analytical description of eccentric GW mergers forming during three-body interactions in realistic GCs.

Black hole–neutron star mergers in globular clusters

We model the formation of black hole-neutron star (BH-NS) binaries via dynamical interactions in globular clusters. We find that in dense, massive clusters, 16-61% of the BH-NS binaries formed by interactions with existing BH binaries will undergo mergers driven by the emission of gravitational radiation. If the BHs are retained by the cluster after merging with a NS, the BHs acquire subsequent NS companions and undergo several mergers. Thus, the merger rate depends critically upon whether or not the BH is retained by the cluster after the merger. Results from numerical relativity suggest that kick imparted to a ~7 M_sun BH after it merges with a NS will greatly exceed the cluster's escape velocity. In this case, the models suggest that the majority of BH-NS mergers in globular clusters occur within 4 Gyrs of the cluster's formation and would be unobservable by Advanced LIGO. For more massive BHs, on the other hand, the post merger kick is suppressed and the BH is retained. Models with 35 M_sun BHs predict Advanced LIGO detection rates in the range 0.04 - 0.7 per year. On the pessimistic end of this range, BH-NS mergers resulting from binary-single star interactions in globular clusters could account for an interesting fraction of all BH-NS mergers. On the optimistic end, this channel may dominate the rate of detectable BH-NS mergers.

On the offset of short gamma-ray bursts

Short Gamma-Ray Bursts (SGRBs) are expected to form from the coalescence of compact binaries, either of primordial origin or from dynamical interactions in globular clusters. In this paper, we investigate the possibility that the offset and afterglow brightness of a SGRB can help revealing the origin of its progenitor binary. We find that a SGRB is likely to result from the primordial channel if it is observed within 10 kpc from the center of a massive galaxy and shows a detectable afterglow. The same conclusion holds if it is 100 kpc away from a small, isolated galaxy and shows a weak afterglow. On the other hand, a dynamical origin is suggested for those SGRBs with observable afterglow either at a large separation from a massive, isolated galaxy or with an offset of 10-100 kpc from a small, isolated galaxy. We discuss the possibility that SGRBs from the dynamical channel are hosted in intra-cluster globular clusters and find that GRB 061201 may fall within this scenario.

The gravitational wave emission from white dwarf interactions in globular clusters

In the dense central regions of globular clusters close encounters of two white dwarfs are relatively frequent. The estimated frequency is one or more strong encounters per star in the lifetime of the cluster. Such encounters should be then potential sources of gravitational wave radiation. Thus, it is foreseeable that these collisions could be either individually detected by LISA or they could contribute significantly to the background noise of the detector. We compute the pattern of gravitational wave emission from these encounters for a sufficiently broad range of system parameters, namely the masses, the relative velocities and the distances of the two white dwarfs involved in the encounter.

Eccentric Double White Dwarfs as LISA Sources in Globular Clusters

We consider the formation of double white dwarfs (DWDs) through dynamical interactions in globular clusters. Such interactions can give rise to eccentric DWDs, in contrast to the exclusively circular population expected to form in the Galactic disk. We show that for a 5 yr Laser Interferometer Space Antenna (LISA) mission and distances as far as the Large Magellanic Cloud, multiple harmonics from eccentric DWDs can be detected at a signal-to-noise ratio higher than 8 for at least a handful of eccentric DWDs, given their formation rate and typical lifetimes estimated from current cluster simulations. Consequently, the association of eccentricity with stellar-mass LISA sources does not uniquely involve neutron stars, as is usually assumed. Due to the difficulty of detecting (eccentric) DWDs with present and planned electromagnetic observatories, LISA could provide unique dynamical identifications of these systems in globular clusters.

The Redshift Distribution of Short Gamma-Ray Bursts from Dynamically Formed Neutron Star Binaries

Short-hard gamma-ray bursts (SHBs) may arise from gravitational wave (GW) driven mergers of double neutron star (DNS) systems. DNSs may be "primordial" or can form dynamically by binary exchange interactions in globular clusters during core-collapse. For primordial binaries, the time delay between formation and merger is expected to be short, tau~0.1 Gyr, implying that the redshift distribution of merger events should follow that of star-formation. We point out here that for dynamically formed DNSs, the time delay between star-formation and merger is dominated by the cluster core-collapse time, rather than by the GW inspiral time, yielding delays comparable to the Hubble time. We derive the redshift distribution of merger events of dynamically formed DNSs, and find it to differ significantly from that typically expected for primordial binaries. The observed redshift distribution of SHBs favors dynamical formation, although a primordial origin cannot be ruled out due to possible detection biases. Future red-shift observations of SHBs may allow to determine whether they are dominated by primordial or dynamically formed DNSs.

Field star diffusion in globular clusters

A new interaction of globular clusters with galactic field stars, the deceleration (by dynamical friction) of high-velocity field stars diffusing through individual globular clusters, is investigated. This frictional interaction contributes to cluster heating and, in conjunction with disc shocking and other mechanisms, helps to reflate the evolution of globular clusters. Moreover, penetrating field stars of low relative velocity can even be captured by globular clusters. The calculated rate of capture suggests that there is a modest population of stars having an origin external to the clusters in which they now reside

The formation of hierarchical triple systems by single-star-binary encounters

view Abstract Citations (9) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Formation of Hierarchical Triple Systems by Single-Star--Binary Encounters Bailyn, Charles D. Abstract The possible formation of bound hierarchical triple systems in globular clusters by encounters between single stars and binaries formed by tidal capture is considered. The first step in forming such systems must be the formation of temporarily bound hierarchical resonances. This paper explores the creation of such resonance systems in the context of globular clusters, taking into account both point mass and tidal effects. Numerical simulations show that relatively long-lasting systems can be formed, but only in retrograde or near retrograde orbits. A total cross section for the formation of relatively long-lasting triples on the order of that for the formation of tidal capture binaries is determined. The very low energy of the incoming stars appropriate for interactions in globular clusters is crucial for the formation process. Publication: The Astrophysical Journal Pub Date: June 1989 DOI: 10.1086/167482 Bibcode: 1989ApJ...341..175B Keywords: Binary Stars; Encounters; Globular Clusters; Star Formation; Monte Carlo Method; Simulation; Astrophysics; CLUSTERS: GLOBULAR; STARS: STELLAR DYNAMICS full text sources ADS | data products SIMBAD (2)