Showing papers on "Mass segregation published in 1994"

Predictions of a population of cataclysmic variables in globular clusters

Abstract: We have studied the number of cataclysmic variables (CVs) that should be active in globular clusters during the present epoch as a result of binary formation via two-body tidal capture. We predict the orbital period and luminosity distributions of CVs in globular clusters. The results arebased on Monte Carlo simulations combined with evolution calculations appropriate to each system formed during the lifetime of two specific globular clusters, omega Cen and 47 Tuc. From our study of these two clusters, which represent the range of core densities and states of mass segregation that are likely to be interesting, we extrapolate our results to the Galactic globlular cluster system. Although there is at present little direct observational evidence of CVs in globular clusters, we find that there should be a large number of active systems. We predict that there should be more than approximately 100 CVs in both 47 Tuc and omega Cen and several thousand in the Galactic globular cluster system. These numbers are based on two-body processes alone and represent a lower bound on the number of systems that may have been formed as a result of stellar interaction within globular clusters. The relation between these calculations and the paucity of optically detected CVs in globular clusters is discussed. Should future observations fail to find convincing evidence of a substantial population of cluster CVs, then the two-body tidal capture scenario is likely to be seriously constrained. Of the CVs we espect in 47 Tuc and omega Cen, approximately 45 and 20, respectively, should have accretion luminosities above 10(exp 33) ergs/s. If one utilizes a relation for converting accretion luminosity to hard X-ray luminosity that is based on observations of Galactic plane CVs, even these sources will not exhibit X-ray luminosities above 10(exp 33) ergs/s. While we cannot account directly for the most luminous subset of the low-luminosity globular cluster X-ray sources without assuming an evolutionary pattern that is different from that of the majority of CVs in the disk, we are able to account for all of the observed lower luminosity subset of these sources, many of which have been recently discovered through ROSAT observations. In order for our predicted integrated cluster X-ray luminosities to be consistent with observational upper limits, the relation between accretion and X-ray luminosities should be something like that inferred from the Galactic plane population of CVs. Our calculations predict a large number of systems with L(sub acc) is less than 10(exp 32) ergs/s. Although our calculations imply that globular clusters should have an enhancement of CVs relative to the number thought to be present in the Galactic disk, this enhancement is at most roughly an order of magnitude, not comparable to the factor of approximately 100 for low-mass X-ray binaries (LMXBs).

UBV stellar photometry of the 30 Doradus region of the large Magellanic Cloud with the Hubble Space Telescope

Abstract: We report on Planetary Camera observations of the central region of 30 Doradus in the Large Magellanic Cloud (LMC). These images of 30 Doradus are the first `deep' Hubble Space Telescope (HST) exposures that have appropriate photometric calibration. The B band (F439W) image, which shows R136a at the center of the PC6 charge coupled device (CCD) chip, reveals over 200 stars within 3 sec of the center of R13a, and over 800 stars in a 35 sec x 35 sec area. We used Malumuth et al.'s (1991) Point Spread Function (PSF)-fitting method to measure the magnitudes of all stars on the PC6 chip. These new B magnitudes, along with U and V magnitudes from archival PC images, yield a luminosity function, mass density profile, and initial mass function of the 30 Doradus ionizing cluster. The mass distribution is well fit by a King model with a core radius, R(sub c) = 0.96 sec (0.24 pc), a tidal radius, R(sub t) = 110 sec (28 pc), and a total mass, Mass = 16,800 solar mass. Both the luminosity function and initial mass function show evidence for mass segregation, in the sense that the central region has a higher fraction of massive stars than the outer regions. This is the first observational evidence for mass segregation in a very young cluster (age approximately 3 million years). The observations admit the hypothesis that the mass segregation occurred in the process of star formation and/or that the mass segregation is the result of dynamical evolution.

Adiabatic invariants in stellar dynamics, 3: Application to globular cluster evolution

Abstract: The previous two companion papers demonstrate that slowly varying perturbations may not result in adiabatic cutoffs and provide a formalism for computing the long-term effects of time-dependent perturbations on stellar systems. Here, the theory is implemented in a Fokker-Planck code and a suite of runs illustrating the effects of shock heating on globular cluster evolution are described. Shock heating alone results in considerable mass loss for clusters with R(sub g) less than or approximately 8 kpc: a concentration c = 1.5 cluster with R(sub g) kpc loses up to 95% of its initial mass in 15 Gyr. Only those with concentration c greater than or approximately 1.3 survive disk shocks inside of this radius. Other effects, such as mass loss by stellar evolution, will decrease this survival bound. Loss of the initial halo together with mass segregation leads to mass spectral indices, x, which may be considerably larger than their initial values.

Adiabatic Invariants in Stellar Dynamics: III. Application to Globular Cluster Evolution

Abstract: The previous two companion papers demonstrate that slowly varying perturbations do not result in adiabatic cutoffs and provide a formalism for computing the long-term effects of time-dependent perturbations on stellar systems. Here, the theory is implemented in a Fokker-Planck code and a suite of runs illustrating the effects of shock heating on globular cluster evolution are described. Shock heating alone results in considerable mass loss for clusters with $R_g\lta8\kpc$: a concentration $c=1.5$ cluster with $R_g=8\kpc$ loses up to $95\%$ of its initial mass in $15\Gyr$. Only those with concentration $c\lta1.3$ survive disk shocks inside of this radius. Other effects, such as mass loss by stellar evolution, will increase this survival bound. Loss of the initial halo together with mass segregation leads to mass spectral indices, $x$, which may be considerably larger than their initial values.

Dynamics of Dense Stellar Systems Including the Effects of Stellar Collisions

Abstract: Dynamical evolution of dense stellar systems is followed by integrating Fokker-Planck equation including successive mergers between stars. We have assumed that all the tidal captures lead to mergers. The initial cluster is assumed to be a single component Plummer model with mass of individual stars being 0.7 M⊙. The highest mass that is allowed to form through successive mergers in our model is 32 × 0.7M⊙ =22.4 M⊙. The stellar evolution is simulated by removing stars from the cluster assuming that the stellar material escapes as the star finishes the evolution. We have estimated the number of stars that evolve off per unit time using “evolution function” that depends on the mean age and lifetime. The mean ages are estimated from the rates of formation of stars (through merger). While the formation of massive stars leads to the acceleration of core collapse through mass segregation, the indirect heating effect due to stellar evolution makes the core collapse slow. The net effect depends on the initial conditions. The number of high mass stars depends sensitively on the cluster parameters. Core collapse is usually found to be terminated by the indirect heating effect of evolution of moderate mass stars (mainly 2.8 or 5.6 M⊙ in our models). The maximum number stars in highest mass bin (i.e., M=22.4 M⊙) varies from unity to an order of 101 depending on exact initial conditions for the models appropriate for the Galactic center. These numbers are too small to explain the observed HeI emission line stars which are interpreted as envelope stripped high mass (20~ 40 M⊙) stars. However, if the ejected material from the final stage of stellar evolution can form stars efficiently instead of escaping from the potential well of the stellar system, more high mass stars can be found. The presence of massive black hole in the center would also boost the merger rates.

Dynamics and Evolution of Dense Galactic Nuclei

Abstract: Dynamical processes specific to the dense nuclei of galaxies are reviewed, including mass segregation, core collapse, stellar collisions and mergers, and the disruption of giant star athmospheres by capture of more compact stars. Simple arguments suggest that in the Galactic Centre stars with masses of up to of order 10 M⊙ could be formed through successive mergers, and that the light from old giant stars is significantly depleted within the central 0.4 parsec. In the nucleus of M32 merging of main sequence stars and stripping of giants are marginally important, depending on the central density and the age of the giant population. In both nuclei, the relaxation time is short enough that massive stars or remnants should have segregated to the centre, and perhaps short enough for core collapse.