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NASA NTRS1993

Far-ultraviolet radiation from disk globular clusters

IUE spectra obtained in a survey of the metal-rich disk system of globular clusters are presented. Significant FUV fluxes were detected in the 1200-2000-A short-wavelength (SWP) range of the IUE Observatory in several disk globular clusters. These clusters are the most metal-rich known to have an FUV flux. Three clusters show spectral energy distrbutions (SEDs) clearly rising at shorter wavelengths, not unlike the upturns observed in the bulges of metal-rich elliptical galaxies. Several others with weak SWP detections appear to have flat or uncertain spectral energy distributions. Blue stragglers provide a possible explanation for flux redder than 2000 A in clusters showing weaker flux in the SWP region, and with flat or declining SEDs.

Rich, R. M.↗
NASA NTRS1991

CVs and millisecond pulsar progenitors in globular clusters

The recent discovery of a large population of millisecond pulsars in globular clusters, together with earlier studies of both low luminosity X-ray sources and LMXBs in globulars, suggest there should be significant numbers of CVs in globulars. Although they have been searched for without success in selected cluster X-ray source fields, systematic surveys are lacking and would constrain binary production and both stellar and dynamical evolution in globular clusters. We describe the beginnings of such a search, using narrow band H-alpha imaging, and the sensitivities it might achieve.

Grindlay, J. E.↗
NASA NTRS1993

Stellar black holes in globular clusters

The recent discovery of large populations of millisec pulsars associated with neutron stars in globular clusters indicates that several hundred stellar black holes of about 10 solar masses each can form within a typical cluster. While, in clusters of high central density, the rapid dynamical evolution of the black-hole population leads to an ejection of nearly all holes on a short timescale, systems of intermediate density may involve a normal star's capture by one of the surviving holes to form a low-mass X-ray binary. One or more such binaries may be found in the globular clusters surrounding our galaxy.

Kulkarni, S. R.↗
NASA NTRS1984

Determination of the mass of globular cluster X-ray sources

The precise positions of the luminous X-ray sources in eight globular clusters have been measured with the Einstein X-Ray Observatory. When combined with similarly precise measurements of the dynamical centers and core radii of the globular clusters, the distribution of the X-ray source mass is determined to be in the range 0.9-1.9 solar mass. The X-ray source positions and the detailed optical studies indicate that (1) the sources are probably all of similar mass, (2) the gravitational potentials in these high-central density clusters are relatively smooth and isothermal, and (3) the X-ray sources are compact binaries and are probably formed by tidal capture.

Grindlay, J. E.↗
NASA NTRS1988

Intrinsic integrated UBVRI colors of Galactic globular clusters

Published observational data on 50 Galactic globular clusters, including spectral classifications, homogenized colors, and color excesses, are compiled in extensive tables, graphs, and diagrams and analyzed to determine the intrinsic-color/integrated-spectral-type relationship in the UBVRI system. These relationships are found to exhibit significant slopes, although the RI colors do not contribute substantially to the intrinsic-color determination. The values of a(B-V) for the northern and southern Galactic hemispheres are found to be 0.068 + or - 0.006 and 0.039 + or - 0.003 mag, respectively.

Reed, B. Cameron↗
NASA NTRS1992

Magnetic fields and radiative shocks in protogalaxies and the origin of globular clusters

The paper examines the hypothesis that globular clusters formed from gravitational instability in dense sheets of gas produced behind radiative shocks inside protogalaxies, such as those produced by the collision of subgalactic mass fragments partaking of the virial motions within the protogalaxy, in order to determine the differences which result if a magnetic field is present in the preshock medium. The MHD conservation equations are solved along with rate equations for nonequilibrium ionization, recombination, molecular formation and dissociation, and the equations of radiative transfer for steady-state shocks of velocity 300 km/s in a gas of preshock densities of 0.1-1 cu cm, and magnetic field strengths of 0.1-6 micro-G. The magnetic field is found to limit the degree of postshock compression and, thereby, to reduce the level of external radiation flux required to suppress H2 formation and cooling.

Shapiro, Paul R.↗
NASA NTRS1989

Photographic, near-infrared, and CCD photometry of the distant globular cluster AM-1

New photographic, CCD, and near-infrared photometry of stars in the distant globular cluster AM-1 are presented. Based on the photographic and CCD photometry down to the horizontal branch, a distance modulus (m-M)v = 20.4 mag is derived, with E(B-V) = 0.0 mag, corresponding to a distance of 120 kpc from the sun. Using the (V-K) colors of two giants in AM-1, a metallicity of Fe/H = -2.0 + or - 0.2 is deduced, in good agreement with earlier determinations. Using complete photometry and star counts, a core radius of 10 arcsec (6 pc) and a tidal radius of 100 arcsec (58 pc) are derived. Despite the low intrinsic luminosity (Mv = -5.0 mag) and the large galactocentric distance of this globular cluster, the apparent magnitude distribution of the giant branch stars of AM-1 is not found to be unusual.

Madore, Barry F.↗
NASA NTRS1993

Optical, X-ray and gamma-ray observations of compact objects in globular clusters

In the past three years, a new era of study of globular clusters has begun with multiwavelength observations from the current generation of astronomical telescopes in space. We review the recent results obtained from our studies of compact binaries and x-ray sources in globulars with ROSAT and Hubble Space Telescope (HST) as well as our balloon-borne hard x-ray telescope EXITE (Energetic X-ray Imaging Telescope Experiment) and ground-based observations (CTIO). With ROSAT, we have obtained the most sensitive high resolution soft x-ray images of clusters which show multiple low luminosity sources in cluster cores that are likely indicative of the long-sought population of cataclysmic variables (CVs). We have obtained deep H-alpha images of two clusters with HST and found CV candiates for 3 of the ROSAT sources in the core of NGC 6397. New CTIO imaging and spectroscopy of two 'dim source' fields in omega-Cen are also described. With EXITE we carried out the first hard x-ray imaging observations of the cluster 47 Tuc; such studies can ultimately limit the populations of millisecond pulsars and pulsar emission mechanisms. A long ROSAT exposure on 47 Tuc also shows probable cluster diffuse emission, possibly due to hot gas from ablating millisecond pulsars. Multiwavelength studies of globular clusters may provide new constraints on problems as diverse as the origin of CVs and low mass X-ray binaries (LMXBs) and the origin of hot gas in globulars.

Grindlay, J. E.↗
NASA NTRS1996

An Ultraviolet-Excess Optical Candidate for the Luminous Globular Cluster X-Ray Source in NGC 1851

The intense, bursting X-ray source in the globular cluster NGC 1851 was one of the first cluster sources discovered, but has remained optically unidentified for 25 years. We report here on results from Hubble Space Telescope WFPC2 multicolor images in NGC 1851. Our high spatial resolution images resolve approximately 200 objects in the 3 minute radius Einstein X-ray error circle, 40 times as many as in previous ground-based work. A color-magnitude diagram of the cluster clearly reveals a markedly UV-excess object with B approximately 21, (U - B) approximately -0.9, only 2 minutes from the X-ray position. The UV-excess candidate is 0.12 minutes distant from a second, unremarkable star that is 0.5 mag brighter in B; thus ground-based studies of this field are probably impractical. Three other UV-excess objects are also present among the approximately 16,000 objects in the surveyed region of the cluster, leaving an approximately 5% probability that a UV-excess object has fallen in the X-ray error circle by chance. No variability of the candidate is seen in these data, although a more complete study is required. If this object is in fact the counterpart of the X-ray source, previous inferences that some globular cluster X-ray sources are optically subluminous with respect to low-mass X-ray binaries in the field are now strengthened.

Deutsch, Eric W.↗
NASA NTRS1986

Head-on binary collisions in globular clusters

Collision probabilities between identical binaries of three different binding energies are computed for binaries in the cores of globular clusters. The more tightly bound a binary is, the smaller the chance is that it suffers a collision. It is found that a given binary in a globular cluster core probably does not suffer more than a very few collisions. If it is not broken up into single stars, it will become so tightly bound that collisions involving it are very rare. Since it is found that head-on collisions are so rare, the collisions which are probably more important in affecting the evolution of the cluster core are the long-range (large impact parameter) encounters.

Hoffer, James B.↗
NASA NTRS1994

RR Lyrae stars and color-magnitude diagram of the globular cluster NGC 6388

We present new V, B-V, and V, V-R color-magnitude diagrams for the bulge globular cluster NGC 6388. These diagrams indicate that NGC 6388 is a metal-rich globular cluster with color-magnitude morphology similar to that of 47 Tucanae. We have conducted a search for new variable stars close to NGC 6388, finding three new RR Lyrae stars. The membership of these and previously discovered RR Lyrae stars near NGC 6388 is discussed. There is reason for believing that some of these variables are nonmembers. Others, however, may belong to the cluster and may be similar to the RR Lyrae star V9 in 47 Tuc.

Silbermann, N. A.↗
NASA NTRS1988

Globular cluster luminosity functions

The use of luminosity functions in determining the helium content, age, and initial mass function of globular clusters, and in testing the stellar structure theory is reviewed. The main sequence turnoff and subgiant phases are examined. The Thomas (1967) peak in the luminosity function and the gap near the base of the giant branch of globular clusters found in the CM are discussed as probes of stellar structure. Research in the area of faint star photometry, dealing with the lower main sequence luminosity function and the interplanetary magnetic field is presented.

Demarque, Pierre↗
NASA NTRS1992

CCD photometry of the globular cluster NGC 5897 - Morphology of the color-magnitude diagram

The paper presents CCD photometry in the B and V bands of the Galactic globular cluster NGC 5897. The color-magnitude diagram (CMD) obtained for this cluster is used to examine the properties of the cluster and to compare the NGC 5897 to the well-known globular cluster M3. It was found that the metallicity of the NGC 5897 is in the range of the metallicity of M3 and that the age of NGC 5897 is about 2 Gyr greater than that of M3. The CMD for NGC 5897 also reveals a significant population of blue straggler stars (BSS) more massive than the cluster subgiant branch stars. A pseudomain sequence is constructed for NGC 5897 and the previously studied (Sarajedini and Da Costa, 1991) global cluster 6101, which includes the BSS and extends to the faintest regions of the unevolved main sequence.

Sarajedini, Ata↗
NASA NTRS1981

Horizontal branch stars, and galactic and magellanic cloud globular clusters

Seven blue horizontal branch stars in the field were observed and a few HB stars were isolated in globular clusters. Energy distributions are compared to assess possible differences and also used in comparison with model atmospheres. Observed energy distributions of HB stars in NGC 6397 are used to estimate the total number of HB stars which produced the integrated fluxes as observed by ANS. Preliminary results are given for colors of globular clusters observed in the Magellanic Clouds and for their extent, based on the Washburn IUE extraction.

Deboer, K. S.↗
NASA NTRS1989

The detection of an IR excess toward the globular cluster RV Tauri variable M28 V17

The paper presents the results of a search for IR emission excesses toward three globular cluster stars. Two of the stars are RV Tau variables (M28 V17 and M5 V85) and one is believed to be a W Vir variable (M5 V42) which is historically linked to the RV Tau class. Consideration is given to the implications of the observed detections, in particular, the establishment of M28 V17 as a star in a globular cluster that is indistinguishable from the Preston type A subclass.

Nook, Mark A.↗
NASA NTRS1988

Ultra-compact and X-ray binaries in globular clusters

A summary of ongoing work on the properties, origin, and evolution of low-mass X-ray binaries in globular clusters and in the Galactic Bulge is presented. This discussion centers on the insights into these problems that can be derived from study of the ultracompact binary systems, such as the recently discovered 11-minute binary in the globular cluster NGC 6624.

Grindlay, Jonathan E.↗