Introduction and Background
This project is looking for the specific properties and characteristics of globular
clusters that make them more likely to contain X-ray producing binary star systems. This
is of interest because of the unusually high amounts of X-ray binaries in globular clusters
versus any other places in the universe. While globular clusters occupy only a small
fraction of a percent of the galaxy, over a tenth of the total X-ray systems in the galaxy
are found within them.
A globular cluster is about a hundred thousand stars in a roughly spherical area in
space, with the middle being the most dense area and the stars spreading out from there.
Some properties of globular clusters include integrated magnitudes, colors, morphology
parameters, metallicities, radial velocities, and structural parameters
(http://physun.physics.mcmaster.ca/~harris/mwgc.dat).
A binary system star system is two stars in orbit around each other. Only those
systems containing typically a red giant star and either a white dwarf, neutron star, or a
black hole are of interest to the project since only those binary pairs can produce X-rays.
This happens when the giant star and one of the three mentioned stars form a binary pair
where the smaller star takes mass because of it’s greater gravitational force from the
larger star. It is this process of mass redistribution that causes the X-rays to be radiated.
Characteristics being investigated include: Apparent visual distance modulus,
integrated V magnitude of the cluster, absolute visual magnitude (cluster luminosity),
integrated color indices (uncorrected for reddening), metallicity [Fe/H], integrated
spectral type, heliocentric radial velocity (km/s), central concentration [c = log(r_t/r_c)],
core radius, half-mass radius, tidal radius, the logarithm of core relaxation time, the
logarithm of relaxation time at the half-mass radius, central surface brightness, V
magnitudes per square arcsecond, and the logarithm of central luminosity density (Solar
luminosities per cubic parsec). Brief descriptions of these terms can be found in
Appendix A.
Hypothesis
I hypothesize that the properties and/or characteristics of globular clusters that
will be found to effect the formation of X-ray producing binary systems will be within
the structural parameters. Those parameters include: Central concentration, core radius,
half-mass radius, and tidal radius.
Procedure
First, data was gathered on the properties and characteristics of over one hundred
globular clusters in the Milky Way including X-ray radiation amounts. This data will
then be used in a sophisticated statistical program. The tables containing some of this
information can be found in Appendix B with those systems being studied in italics.
Applications
This will help us better understand the universe we live in. More specifically
relevant to the research field of astronomy is further understanding of how stars will
interact with each other where there are dense amounts of stars, such as in clusters. This
may in turn help researchers understand why and how stars will form into X-ray binary
systems in the first place.