Globular cluster - Wikipedia
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Unfortunately, the Milky Way lacks large enough numbers of bright i. Some intuition can be developed about which classes of clusters are most likely to contain X-ray sources, but the sample is not large enough to make rigourous tests of whether these parameters have a statistically significant correlation with the probability a cluster will contain an X-ray binary. The use of more distant extragalactic globular clusters, in the large samples of clusters which can be seen in nearby elliptical galaxies, can enhance the sizes of samples of X-ray sources dramatically.
This finding has since been repeated in many other galaxies see e. Another important, but harder-to-address question, is whether the correlation between the stellar interaction rate in a cluster and the probability it will contain an X-ray source is linear. The measurement of stellar interaction rates in extragalactic globular clusters is observationally extremely challenging. None the less, one can fit King models to data, even if the core radius is formally unresolved, but only with high signal-to-noise ratio and good oversampling and understanding of the telescope point spread function e.
Some attempts have been made to test whether the probability a globular cluster will contain an X-ray binary is linearly proportional to the collision rate, based on both King model fits, and other proxies for the collision rate.
These data are deeper than the data obtained in the ACS Virgo Cluster Survey, and the galaxy observed is a factor of almost five closer than M87, lending some confidence that the core radii fitted will be reliable, at least for many of the clusters. They, too find a weaker than linear dependence of probability that an X-ray binary will exist in a cluster on its estimated collision rate.
The Galactic globular clusters have also been studied to determine whether the lower luminosity X-ray sources they contain have numbers which scale with cluster collision rate.
List of globular clusters
Additionally, the lowest collision rate clusters may have their rates of formation of X-ray sources dominated by primoridal systems, rather than by dynamical formation e. The idea that the dependence of the probability a cluster will host an X-ray binary is weaker than linear because of binary destruction in the densest clusters would have major implications on the channels by which X-ray binaries form.
Alternatively, the LMXBs could, potentially, be formed from exchange encounters where the targets are soft binaries, in which case it might be possible for the LMXBs themselves to be hard binaries, while their formation rate depends on the number of soft binaries in the cluster. The hard-soft boundary in most globular clusters occurs for orbital separations of about 1 au — a size scale on which only highly evolved stars can overflow their Roche lobes. It thus seems unlikely that binary destruction plays a major role in regulating the number of X-ray binaries, but given the small number statistics of globular cluster X-ray binaries with known orbital periods, and the importance of the implications for understanding how globular cluster X-ray binaries form, the suggestion merits further investigation.
What is missing from the present analysis in an understanding of how the measurement errors in the samples of cluster parameters made to date, and the systematic offsets between real collision rates and proxy collision rates, affect the conclusions which can be drawn from the data.
In this paper, we look into both issues. Further complications may arise if one wishes to consider the collision rates of neutron stars, as is most appropriate for the bulk of globular cluster X-ray binaries.
One must then, additionally, consider the retention rate of neutron stars in the globular cluster. One should also consider the effects of varying characteristic velocities of interaction in different clusters. Additionally, the lifetimes of many classes of accreting binaries can be long compared to the relaxation time-scale of a globular cluster, and it would be more appropriate to use a weighted integral of the collision rate over time if such a quantity were measurablerather than the actual present collision rate Fregeau Therefore, even if it can be confirmed that the probability a cluster will host an X-ray binary is not linearly proportional to the collision rate, there are a variety of mechanisms apart from binary destruction that may be responsible for such deviations.
Understanding the quantitative details of such deviations may provide a key target for numerical simulations, so it is important to understand the systematic observational biases that may come into attempts to measure the X-ray binary production probability versus collision rate. To do this, we have computed King models for a range of values of cluster concentration, and have computed the collision rates in each of these model clusters.
Following Kingwe define the cluster concentration as the ratio of the cluster tidal radius to its core radius. We normalize the density in units of the central density and the radius in units of the core radius.
The Globular Cluster-Galaxy Connection - fabula-fantasia.info
Figure 1 View large Download slide The integrated collision rates from a single-mass King model plotted versus the value of the central potential W0. Globular cluster formation is prevalent in starburst regions and in interacting galaxies. The mass of the SMBH in such a galaxy is often close to the combined mass of the galaxy's globular clusters.
Very large regions of star formation known as super star clusterssuch as Westerlund 1 in the Milky Waymay be the precursors of globular clusters.
In astronomical terms, they are described as "metal-poor". The type of stars found in a globular cluster are similar to those in the bulge of a spiral galaxy but confined to a volume of only a few million cubic parsecs.
They are free of gas and dust and it is presumed that all of the gas and dust was long ago turned into stars.
Globular clusters can contain a high density of stars; on average about 0. Planetary orbits are dynamically unstable within the cores of dense clusters because of the perturbations of passing stars.
A planet orbiting at 1 astronomical unit around a star that is within the core of a dense cluster such as 47 Tucanae would only survive on the order of years. Both can be regarded as evidence that supermassive globular clusters are in fact the cores of dwarf galaxies that are consumed by the larger galaxies.
Metallic content[ edit ] Messier 53 contains an unusual number of a type of star called blue stragglers. Astronomers refer to these heavier elements as metals and to the proportions of these elements as the metallicity. These elements are produced by stellar nucleosynthesis and then are recycled into the interstellar mediumwhere they enter the next generation of stars.
Hence the proportion of metals can be an indication of the age of a star, with older stars typically having a lower metallicity. The second group has a slightly longer period of RR Lyrae variable stars. These two populations have been observed in many galaxies, especially massive elliptical galaxies. Both groups are nearly as old as the universe itself and are of similar ages, but differ in their metal abundances.
Many scenarios have been suggested to explain these subpopulations, including violent gas-rich galaxy mergers, the accretion of dwarf galaxies, and multiple phases of star formation in a single galaxy.
In the Milky Waythe metal-poor clusters are associated with the halo and the metal-rich clusters with the bulge. This result argues in favor of the view that type II clusters in the galaxy were captured from a satellite galaxy, rather than being the oldest members of the Milky Way's globular cluster system as had been previously thought.
The difference between the two cluster types would then be explained by a time delay between when the two galaxies formed their cluster systems. Due to these chance encounters, some exotic classes of stars, such as blue stragglersmillisecond pulsars and low-mass X-ray binariesare much more common in globular clusters. A blue straggler is formed from the merger of two stars, possibly as a result of an encounter with a binary system. Astronomers have searched for black holes within globular clusters since the s.
The resolution requirements for this task, however, are exacting, and it is only with the Hubble space telescope that the first confirmed discoveries have been made.
The mass of these intermediate mass black holes is proportional to the mass of the clusters, following a pattern previously discovered between supermassive black holes and their surrounding galaxies.
Claims of intermediate mass black holes have been met with some skepticism. The heaviest objects in globular clusters are expected to migrate to the cluster center due to mass segregation.
As pointed out in two papers by Holger Baumgardt and collaborators, the mass-to-light ratio should rise sharply towards the center of the cluster, even without a black hole, in both M15  and Mayall II.
Large positive values indicate a red star with a cool surface temperaturewhile negative values imply a blue star with a hotter surface. When the stars near the Sun are plotted on an HR diagram, it displays a distribution of stars of various masses, ages, and compositions.
Many of the stars lie relatively close to a sloping curve with increasing absolute magnitude as the stars are hotter, known as main-sequence stars. However the diagram also typically includes stars that are in later stages of their evolution and have wandered away from this main-sequence curve. As all the stars of a globular cluster are at approximately the same distance from us, their absolute magnitudes differ from their visual magnitude by about the same amount.
The main-sequence stars in the globular cluster will fall along a line that is believed to be comparable to similar stars in the solar neighborhood.
The accuracy of this assumption is confirmed by comparable results obtained by comparing the magnitudes of nearby short-period variables, such as RR Lyrae stars and cepheid variableswith those in the cluster.
This in turn provides a distance estimate to the cluster, based on the visual magnitude of the stars. The difference between the relative and absolute magnitude, the distance modulusyields this estimate of the distance.
This differs from the HR diagram of stars near the Sun, which lumps together stars of differing ages and origins. The shape of the curve for a globular cluster is characteristic of a grouping of stars that were formed at approximately the same time and from the same materials, differing only in their initial mass.
As the position of each star in the HR diagram varies with age, the shape of the curve for a globular cluster can be used to measure the overall age of the star population. Certain clusters even display populations that are absent from other globular clusters e. The historical paradigm that all globular clusters consist of stars born at exactly the same time, or sharing exactly the same chemical abundance, has likewise been overturned e.