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NASA NTRS1993

Gamma rays from globular clusters

Globular clusters are known to contain a relatively large number of pulsars whose individual and collective emission in the X-ray and gamma-ray energy bands may be detectable by the instruments on board the Compton Gamma-Ray Observatory (CGRO), ROSAT, and possibly SIGMA. We discuss the several types of high-energy emission expected from isolated and interacting binary pulsars in globular clusters. Individual or collective high-energy emission from isolated pulsars is expected to be too low to be detected with current instruments. However, a class of high-luminosity hidden millisecond pulsars enshrouded in the evaporating material from irradiated companion stars can produce unpulsed shock emission detectable by the high-sensitivity instruments of ROSAT and CGRO. Establishing upper limits of high-energy emission from globular clusters will be valuable in constraining models for the formation of cluster millisecond pulsars.

Tavani, Marco↗
NASA NTRS1995

Competition of supermassive black holes and galactic spheroids in the destruction of globular clusters

The globular clusters that we observe in galaxies may be only a fraction of the initial population. Among the evolutionary influences on the population is the destruction of globular clusters by tidal forces as the cluster moves through the field of influence of a disk, a bulge, and/or a putative nuclear component (black hole). We have conducted a series of N-body simulations of globular clusters on bound and marginally bound orbits through poetentials that include black hole and speroidal components. The degree of concentration of the spheroidal component can have a considerable impact on the extent to which a globular cluster is disrupted. If half the mass of a 10(exp 10) solar mass spheroid is concentrated within 800 pc, then only black holes with masses greater than 10(exp 9) solar mass can have a significant tidal influence over that already exerted by the bulge. However, if the matter in the spheroidal component is not so strongly concentrated toward the center of the galaxy, a more modest central black hole (down to 10(exp 8) solar mass) could have a dominant influence on the globular cluster distribution, particularly if many of the clusters were initially on highly radial orbits. Our simulations show that the stars that are stripped from a globular cluster follow orbits with roughly the same eccentricity as the initial cluster orbit, spreading out along the orbit like a 'string of pearls.' Since only clusters on close to radial orbits will suffer substantial disruption, the population of stripped stars will be on orbits of high eccentricity.

Charlton, Jane C.↗
NASA NTRS1982

X-ray illumination of globular cluster puzzles

Globular clusters are thought to be among the oldest objects in the Galaxy, and provide, in this connection, important clues for determining the age and process of formation of the Galaxy. The present investigation is concerned with puzzles relating to the X-ray emission of globular clusters, taking into account questions regarding the location of X-ray emitting clusters (XEGC) unusually near the galactic plane and/or galactic center. An adopted model is discussed for the nature, formation, and lifetime of X-ray sources in globular clusters. An analysis of the available data is conducted in connection with a search for correlations between binary formation time scales, central relaxation times, galactic locations, and X-ray emission. The positive correlation found between distance from galactic center and two-body binary formation time for globular clusters, explanations for this correlation, and the hypothesis that X-ray sources in globular clusters require binary star systems provide a possible explanation of the considered puzzles.

Lightman, A. P.↗
NASA NTRS1976

Radio sources in the field of globular clusters

Eleven globular clusters have been surveyed in the continuum at 2695 and 8085 MHz with the NRAO Green Bank interferometer. The nine clusters discussed probably show unresolved sources with flux densities of the order of 10 mJy. Most of them are not at cluster centers. The dual-frequency data are mostly consistent with nonthermal spectra or with partial resolution, at the higher frequency, of sources with flat spectra. The planetary nebula in M15 has been detected. The empirical statistics of cosmic field sources suggest that they may account for some, but not all, of the sources in the field of globular clusters.

Johnson, H. M.↗
NASA NTRS1993

A supersoft variable low-luminosity X-ray source in the globular cluster M3

The globular cluster M3 (NGC 5272) was observed twice with the ROSAT high-resolution imager in order to study the low-luminosity X-ray source 1E 1339.8 + 2837. In 1992 January 1E 1339.8 + 2837 had an X-ray luminosity of 2 x 10 exp 35 ergs/s over an order of magnitude brighter than it was when observed with the Einstein Observatory. The source was unresolved and very soft; such supersoft outbursts would be difficult to detect in the vast majority of globular clusters which are more heavily absorbed than M3. In 1992 June the source was too faint to be detected. The soft outburst luminosity and the blackbody radius suggest that 1E 1339.8 + 2837 is a cataclysmic variable in which much of the luminosity is generated by steady nuclear burning of accreted material on the surface of the white dwarf primary.

Hertz, P.↗
NASA NTRS1985

Rotation and flattening of globular clusters

Methods for measuring globular cluster ellipticities and the results of such measurements are reviewed, and the processes that determine the shapes of globular clusters and the ways in which they change with time are discussed. The use of the virial tensor theorem to study the connection between the global rotation, velocity anisotropy, and the shape of a self-gravitating system is addressed, and the employment of N-body models to simulate the evolution of globular clusters with initially anisotropic velocity distributions is examined. The application of a simple evaporation model and Fokker-Planck integrations to study the two-body diffusion in globular clusters is reviewed.

Fall, S. M.↗
NASA NTRS1999

A Complete Sample of Hot Post-AGB Stars in Globular Clusters

Ultraviolet images of globular clusters are often dominated by one or two "UV-bright" stars. The most luminous of these are believed to be post-AGB stars, which go through a luminous UV-bright phase as they leave the AGB and move rapidly across the HR diagram toward their final white dwarf state. During the two flights of the ASTRO observatory in 1990 and 1995, the Ultraviolet Imaging Telescope (UIT, Stecher 1997, PASP, 109, 584) was used to obtained ultraviolet (1600 A) images of 14 globular clusters. These images provide a complete census of hot (> 8000 K) post-AGB stars in the observed globular clusters, because the 40' field of view of UIT is large enough to image the entire population of most Galactic globulars, and because the dominant cool star population is suppressed in ultraviolet images, allowing UV-bright stars to be detected into the cluster core. We have begun a program of optical and STIS ultraviolet spectroscopy to determine the fundamental stellar parameters (\log L, T_eff, \log g) of all the hot post-AGB candidates discovered on the UIT images. Among the goals of our program are to test theoretical post-AGB lifetimes across the HR diagram, and to estimate the mass of the currently forming white dwarfs in globular clusters. Two trends are already apparent in our survey. First, the UV-selected sample has removed a bias against the detection of the hottest post-AGB stars, and resulted in the discovery of five cluster post-AGB stars with Teff > 50,000 K. Second, most of the new discoveries have been lower luminosity (2.5 $<$\log L $<$ 3.0) than expected for stars which leave the AGB during the thermally pulsating phase.

Landsman, W.↗
NASA NTRS1982

X-ray diagnostics of globular clusters

The presence of compact X-ray sources in globular clusters allows diagnostic studies of both the X-ray sources themselves and the globular clusters to be carried out. A review of much of this work, primarily based on Einstein X-ray observations and supporting studies of globular clusters at radio through UV wavelengths, is presented. The compact X-ray sources in globular clusters are found to be compact binaries containing neutron stars and - in a separate lower luminosity component of an apparently bimodal luminosity function - possibly white dwarfs. Implications for the formation and evolution of compact binary X-ray sources in globular clusters and in the galactic bulge are discussed. In particular, new evidence is presented that the galactic bulge sources may be compact binaries in the remnants of disrupted globular clusters.

Grindlay, J. E.↗
NASA NTRS1994

Predictions of a population of cataclysmic variables in globular clusters

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).

Di Stefano, R.↗
NASA NTRS1981

X-ray sources in globular clusters

The Einstein X-ray Observatory globular cluster observation program has completed a survey of 30 clusters, using the HRI detector. Preliminary results are reported for a more sensitive survey being conducted with the imaging proportional counter (IPC) of the satellite, which has found four possible new sources in the first 32 clusters observed. Also described are detailed studies of several luminous, globular cluster sources, including the X-ray burst from Terzan 2, and an effort is made to locate the X-ray sources with respect to cluster centers. Sources of systematic error, required optical studies of globular cluster structure, and the implications of the plausible binary systems for cluster core evolution are discussed.

Grindlay, J. E.↗
NASA NTRS1995

Constraints on the formation history of the elliptical galaxy NGC 3923 from the colors of its globular clusters

We present a study of the colors of globular clusters associated with the elliptical galaxy NGC 3923. Our final sample consists of Wasington system C and T(sub 1) photometry for 143 globular cluster candidates with an expected contamination of no more than 10%. We find that the color distribution of the NGC 3923 globular cluster system (GCS) is broad and appears to have at least two peaks. A mixture modeling analysis of the color distribution indicates that a two-component model is favored over a single-component one at a high level of confidence (greater than 99%). This evidence for more than one population in the GCS of NGC 3923 is similar to that previously noted for the four other elliptical galaxies for which similar data have been published. Furthermore, we find that the NGC 3923 GCS is redder than the GCSs of previously studed elliptical galaxies of similar luminosity. The median metallicity inferred from our (C-(T(sub 1)))(sub 0) colors is (Fe/H)(sub med) = -0.56, with an uncertainty of 0.14 dex arising from all sources of uncertainty in the mean color. This is more metal rich than the median metallicity found for the GCS of M87 using the same method, (Fe/H)(sub med) = -0.94. Since M87 is more luminous than NGC 3923, this result points to significant scatter about any trend of higher GCS metallicity with increasing galaxy luminosity. We also show that there is a color gradient in the NGC 3923 GCS corresponding to about -0.5 dex in Delta(Fe/H)/Delta(log r). We conclude that the shape of the color distribution of individual GCSs and the variation in mean color among the GCSs of ellipticals are difficult to understand if elliptical galaxies are formed in a single protogalactic collapse. Models in which ellipticals and their globular clusters are formed in more than one event, such as a merger scenario, are more successful in accounting for these observations.

Zepf, Stephen E.↗
NASA NTRS1992

Globular cluster formation - The fossil record

Properties of globular clusters which have remained unchanged since their formation are used to infer the internal pressures, cooling times, and dynamical times of the protocluster clouds immediately prior to the onset of star formation. For all globular clusters examined, it is found that the cooling times are much less than the dynamical times, implying that the protoclusters must have been maintained in thermal equilibrium by external heat sources, with fluxes consistent with those found in previous work, and giving the observed rho-T relation. Self-gravitating clouds cannot be stably heated, so that the Jeans mass forms an upper limit to the cluster masses. The observed dependence of protocluster pressure upon galactocentric position implies that the protocluster clouds were in hydrostatic equilibrium after their formation. The pressure dependence is well fitted by that expected for a quasi-statically evolving background hot gas, shock heated to its virial temperature. The observations and inferences are combined with previous theoretical work to construct a picture of globular cluster formation.

Murray, Stephen D.↗
NASA NTRS1985

The optical structure of X-ray globular clusters

CTIO 4-m prime-focus plates of eight globular clusters containing bright X-ray sources have been obtained. Plates in several colors (principally U and B) of each cluster have been digitized. A maximum symmetry method for determining the center of a globular cluster image is described and applied to each cluster in order to determine the cluster center with an accuracy of above 1.0 arc sec. Surface brightness profiles have been derived for seven clusters and fitted with King (1966, 1980) models to yield core radii within about 10 percent. These determinations of the cluster centers and core radii are sufficiently accurate to allow Grindlay et al. (1984) to statistically measure the mass of the globular cluster X-ray sources. Two clusters (NGC 6624 and M15) show significant excesses of light in their cores over the best-fit King model, in agreement with the results of Djorgovski and King (1984). The significance of these excesses, as well as the lack of any dependence of globular cluster structural parameters on color, is discussed.

Hertz, P.↗
NASA NTRS1985

X-raying the dynamics of globular clusters

Major investigations of globular cluster content, internal dynamical structure, and long-term cluster evolution stimulated by X-ray observations are reviewed. The large-scale structure and motions of globulars as revealed by the recently discovered diffuse X-ray emission which may arise from their motion through the interstellar medium of the Galaxy are discussed, and the results of surveys for point sources in globulars are reviewed. The derived luminosity function for these sources out to 10 core radii suggest they are compact binaries containing either degenerate dwarf or neutron star remnants and were formed by tidal capture within the cluster cores. The possibility is considered that the formation and existence of compact binary X-ray sources contributes to the rapid demise of clusters by first heating the cores and causing the clusters to expand, after which they can be more easily disrupted by tidal forces and giant molecular clouds in the Galaxy.

Grindlay, J. E.↗