Low-density Globular Clusters Research Articles

The influence of black holes on the binary population of the globular cluster Palomar 5

ABSTRACT The discovery of stellar-mass black holes (BHs) in globular clusters (GCs) raises the possibility of long-term retention of BHs within GCs. These BHs influence various astrophysical processes, including merger-driven gravitational waves and the formation of X-ray binaries. They also impact cluster dynamics by heating and creating low-density cores. Previous N-body models suggested that Palomar 5, a low-density GC with long tidal tails, may contain more than 100 BHs. To test this scenario, we conduct N-body simulations of Palomar 5 with primordial binaries to explore the influence of BHs on binary populations and the stellar mass function. Our results show that primordial binaries have minimal effect on the long-term evolution. In dense clusters with BHs, the fraction of wide binaries with periods >105 d decreases, and the disruption rate is independent of the initial period distribution. Multi-epoch spectroscopic observations of line-of-sight velocity changes can detect most bright binaries with periods below 104 d, significantly improving velocity dispersion measurements. Four BH-MS binaries in the model with BHs suggests their possible detection through the same observation method. Including primordial binaries leads to a flatter inferred mass function because of spatially unresolved binaries, leading to a better match of the observations than models without binaries, particularly in Palomar 5’s inner region. Future observations should focus on the cluster velocity dispersion and binaries with periods of 104–105 d in Palomar 5’s inner and tail regions to constrain BH existence.

Gas Expulsion in MOND: The Possible Origin of Diffuse Globular Clusters and Ultra-faint Dwarf Galaxies

Abstract We study the evolution of star clusters located in the outer regions of a galaxy undergoing a sudden mass loss through gas expulsion in the framework of Milgromian dynamics (MOND) by means of N-body simulations. We find that, to leave a bound star cluster, the star formation efficiency (SFE) of an embedded cluster dominated by deep MOND gravity can be reduced down to . For a given SFE, the star clusters that survive in MOND can bind a larger fraction of mass compared to those of the Newtonian dynamics. Moreover, the more diffuse the embedded cluster is, the less substantial the size expansion of the final star cluster is. The density profiles of a surviving star cluster are more cuspy in the center for more massive embedded clusters, and the central density profiles are flatter for less massive embedded clusters or for lower SFE. This work may help to understand the low concentration and extension of the distant low-density globular clusters and ultra-faint and diffuse satellite galaxies around the Milky Way.

Direct N-body simulations of globular clusters – III. Palomar 4 on an eccentric orbit

Palomar 4 (Pal 4) is a low-density globular cluster (GC) with a current mass ≈30 000 M⊙ in the outer halo of the Milky Way with a two-body relaxation time of the order of a Hubble time. Yet, it is strongly mass segregated and contains a stellar mass function depleted of low-mass stars. Pal 4 was either born this way or it is a result of extraordinary dynamical evolution. Since two-body relaxation cannot explain these signatures alone, enhanced mass-loss through tidal shocking may have had a strong influence on Pal 4. Here, we compute a grid of direct N-body simulations to model Pal 4 on various eccentric orbits within the Milky Way potential to find likely initial conditions that reproduce its observed mass, half-light radius, stellar slope of the mass function and line-of-sight velocity dispersion. We find that Pal 4 is most likely orbiting on an eccentric orbit with an eccentricity of e ≈ 0.9 and pericentric distance of R ≈ 5 kpc. In this scenario, the required 3D half-mass radius at birth is similar to the average sizes of typical GCs (R ≈ 4-5 pc), while its birth mass is about M ≈ 10 M⊙. We also find a high degree of primordial mass segregation among the cluster stars, which seems to be necessary in every scenario we considered. Thus, using the tidal effect to constrain the perigalactic distance of the orbit of Pal 4, we predict that the proper motion of Pal 4 should be in the range −0.52 ≤ μ ≤ −0.38 mas yr and −0.30 ≤ μαcos ≤ − 0.15 mas yr

Chandra X‐Ray ObservatoryObservations of the Globular Cluster M71

We observe the nearby, low-density globular cluster M71 (NGC 6838) with the Chandra X-Ray Observatory to study its faint X-ray populations. Five X-ray sources are found inside the cluster core radius, including the known eclipsing binary millisecond pulsar (MSP) PSR J1953+1846A. The X-ray light curve of the source coincident with this MSP shows marginal evidence for periodicity at the binary period of 4.2 hr. Its hard X-ray spectrum and luminosity resemble those of other eclipsing binary MSPs in 47 Tuc, suggesting a similar shock origin of the X-ray emission. A further 24 X-ray sources are found within the half-mass radius, reaching to a limiting luminosity of -->1.5 × 1030 ergs s−1 (0.3-8 keV). From a radial distribution analysis, we find that -->18 ± 6 of these 29 sources are associated with M71, somewhat more than predicted, and that -->11 ± 6 are background sources, both Galactic and extragalactic. M71 appears to have more X-ray sources in the range -->LX = 1030–1031 ergs s−1 than expected by extrapolating from other studied clusters using either mass or collision frequency. We explore the spectra and variability of these sources and describe the results of ground-based optical counterpart searches.

Old pulsars in the low-density globular clusters MI3 and M53

Millisecond pulsars are conventionally assumed to be spun up through the action of binary companions, although some subsequently lose their companions and appear as isolated pulsars. Such objects should therefore be more numerous in dense stellar systems. We report here the surprising discovery of two pulsars in low-density globular clusters: one is a single 10-ms pulsar (1639+36) in M13 (NGC620S), the other a 33-ms pulsar (1310+ 18) in a 256-day binary in MS3 (NGCS02S). Their ages, inferred from their luminosities and constraints on their period derivatives, seem to be -10^9 years, significantly greater than previously reported ages (≤10^8 years) of cluster pulsars^1. The implied birth rate is inconsistent with the conventional two-body tidal capture model^2,^3, suggesting that an alternative mechanism such as tidal capture between primordial binaries and a reservoir of (hundreds of) primordial neutron stars may dominate the production of tidal binaries in such clusters^1,^4• The period derivative of PSR1639+36 is surprisingly small, and may be corrupted by acceleration due to the mean gravitational potential of the cluster. We discovered the pulsars during a survey of globular^5.

Stellar collisions in globular clusters and the blue straggler problem

The hypothesis that the blue stragglers in globular clusters are produced by physical stellar collisions is critically examined. The most serious objection to the collisional hypothesis is that direct stellar collisions between single stars are not expected to occur frequently enough in low-density globular clusters to account for the observed number of blue stragglers in such clusters. However, if a small fraction of the main-sequence stars in those systems are primordial binaries, then physical stellar collisions can occur at the required rate of one every about 10 to the 8th yr. This is because: (1) mass segregation will result in a high binary frequency in the cluster core; (2) the cross section for a binary-binary collision is about 100 times that for a single-single collision; and (3) physical stellar collisions are likely to occur during strong binary-binary interactions. Since a small fraction of binaries cannot be ruled out in the low-density globular clusters, then neither can the collisional hypothesis for the blue stragglers. Several observations that could be used to test the collisional hypothesis are listed. 52 refs.