Age and initial helium abundance of stars in the globular cluster M15.
Age and initial helium abundance of stars in M15 globular clusters estimated from stellar evolution lifetime ratio
Iben, I., Jr.↗
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The evolution of the system of globular clusters
The physical processes affecting the evolution of the system of globular clusters are reviewed. Aspects of internal structure evolution discussed include evaporation, core collapse, expansion, tidal shocks, and interaction with other massive components of the Galaxy such as black holes, molecular clouds, and other clusters. The evolution of orbital parameters is examined, including dynamical friction, scattering by massive subcomponents, and interactions with nearby systems. In addition, an overview of the system evolution is presented.
Ostriker, Jeremiah P.↗
A new age diagnostic applied to the globular clusters NGC 288 and NGC 362
A new age diagnostic for globular clusters is described which uses the difference between the turnoff and the base of the giant branch as the age indicator. As a first application, it is shown that there is a difference in age of 3.1 + or - 0.9 Gyr between the classic 'second parameter' pair NGC 288 and NGC 362. The existence of this age difference is independent of metal abundance differences between the two clusters of up to 0.5 dex. This age difference is corrected for various combinations of relative oxygen enhancement, and it is concluded that the difference in age remains significant for all plausible scenarios.
Sarajedini, Ata↗
Far-ultraviolet photometry of the globular cluster omega Cen
We present far-ultraviolet images of the globular cluster omega Centauri obtained with the Ultraviolet Imaging Telescope (UIT) during the 1990 December Astro-1 mission. A total of 1957 sources are detected at 1620 A to a limiting ultraviolet (UV) magnitude of 16.4 in the central 24 min diameter region of the field and a limit of 15.6 over the remainder of the 40 min diameter field. Over 1400 of these sources are matched with stars on a Stroemgren u band charge coupled devices (CCD) frame obtained with the Cerro Tololo Inter-American Observatory (CTIO) 0.9 m telescope to produce a (far-UV, u) color-magnitude diagram (CMD). Completeness of the sample and error estimates are determined by photometry of artificial stars added to the images. The horizontal branch (HB) of the CMD is heavily populated hotter than 9000 K. A large number of 'extreme HB' stars are found hotter than a conspicuous break in the HB at T(sub e) approximately 16000 K. There is also a significant population of stars above the HB, the brightest of which is 4 mag brighter than the HB. Most of the hotter of these appear to be 'AGB-manque' or 'Post-Early Asymptotic Giant Branch' stars. We compare the observations to recent theoretical evolutionary tracks for the zero-age HB and subsequent phases. The tracks match the data well, with the exception of the hotter HB stars, many of which fall below the zero-age horizontal branch. It is unclear as yet whether these are a special population or an artifact of errors in the models or photometry. We identify 33 stars with T(sub e) greater than or approximately = 50000 K, which are hotter than zero-age HB stars with envelope masses of 0.003 solar mass.
Whitney, Jonathan H.↗
WFPC2 observations of the globular cluster M30
We describe images of the center of the dense globular cluster M30 (NGC 7099) obtained with the Hubble Space Telescope Wide Field/Planetary Camera 2 (HST WFPC2). Data taken in the F336W, F439W, and F555W filters (approximately U, B, and V) yield a color-magnitude diagram with photometric errors of 1 sigma = 0.05 mag for stars with V less than 17.5, and 1 sigma approximately 0.1 mag at V = 20, which is 1.5 mag fainter than the main-sequence turnoff. Simulations show that the star identifications are essentially complete for stars with V less than 19.5, even in the densest regions of the cluster. The projected radial distribution of stars can be represented by a power law distribution: N(r) approximately r(exp alpha), with slope alpha = -0.4 +/- 0.15, into radii less than 0.4 sec (0.015 pc). The radial profile is also consistent with a flat core of any radius smaller than approximately 1.5 sec (0.05 pc), but inconsistent with any core radius larger than 2.5 sec (0.09 pc). A total of 30 blue straggler candidates within 20 sec of the cluster center have been identified from the three-color data. Their radial distribution is strongly centrally concentrated when compared to the horizontal branch, red giant branch, or main-sequence stars in the cluster. The abundance of blue stragglers (relative to red giants and subgiants) within r approximately less than 20 sec of the center of M30 is f(sub BS) = 0.19 =/- 0.04, about twice as high as in other dense clusters.
Yanny, Brian↗
The structure of eight globular clusters
Star counts on short-exposure plates are presented for eight globular clusters. The data are given to several limiting magnitudes for each cluster. Projected radial distributions are determined from the star counts; no significant segregation is observed among the distributions to various limiting magnitudes. The structural parameters of the clusters (core radii and concentration factors) are determined from best-fitting King curves to the observed distributions. The central densities, total masses, central escape velocities, and relaxation times are also calculated. NGC 6093 (M80) and NGC 7099 (M30) have dense compact stellar cores, similar to the established globular X-ray sources. Upper limits are given for the masses of possible central black holes in the eight clusters.
Bahcall, N. A.↗
The fragmentation of proto-globular clusters. II - Gravitational instabilities
The remarkable homogeneity in the metal abundances within a typical globular cluster may be a consequence of the fragmentation of the proto-globular clouds. In this paper, the late stages of fragmentation are investigated by examining the growth of gravitational instabilities in the shell. It is found that the short dynamical time scale of the shell leads to the rapid growth of gravitational instabilities, so that the entire process of fragmentation to stellar mass scales can be completed on a time scale much less than the dynamical time scale of the parent cloud.
Murray, Stephen D.↗
Upper limits on the atomic hydrogen abundance in 12 globular clusters.
Description of 21-cm neutral-hydrogen line observations of 12 globular clusters performed with negative results by means of the NRAO 140-ft telescope. Limits on the atomic hydrogen content yielded by these observations over a range of 11 to 85 solar masses are presented.-
Kerr, F. J.↗
Nuclear brightness profiles of the X-ray globular cluster M15 compared to M3, M5, and M13
Densitometer tracings of the globular clusters M3, M5, M13 and M15 indicate that the central peak of the brightness profile for M15 is narrower in the red than in the blue region. It is suggested that the red excess in the nucleus of M15 is due to red giants.
Feibelman, W. A.↗
Ultraviolet fluxes for globular clusters in M31 - A rediscussion
Long-exposure observations of three bright globular clusters in M31 obtained with both the short- and long-wavelength low-resolution cameras of the International Ultraviolet Explorer satellite are discussed. All of the clusters are seen at the longer wavelengths, but only one of the clusters is seen at short wavelengths, and this detection is marginal. The ultraviolet fluxes are in fact known with only poor precision, and previous conclusions concerning the stellar population are weakened accordingly. Discrepancies between the ultraviolet fluxes obtained here and in other published work are described.
Crotts, Arlin P. S.↗
Blue stragglers in the core of the globular cluster 47 Tucanae
High-resolution observations of the core of the globular cluster 47 Tucanae with the Faint Object Camera on the Hubble Space Telescope reveal a high density of 'blue straggler' stars, occupying the upper end of the main sequence from which all stars in the cluster should have long since evolved. Their presence in the dense core supports the hypothesis that they formed by stellar collision and coalescence, and, as the heaviest objects in the cluster, have drifted to the core.
Paresce, F.↗
The formation of primordial binaries in globular clusters by star-disk interactions
The formation of primordial binaries in globular clusters is examined using simple numerical models. Clusters of protostars collapse until their velocity dispersion rises sufficiently to reverse the infall and the cluster reaches equilibrium. During the collapse, interactions between stars and protostellar disks lead to stellar capture. It is found that binary fraction of a few percent typically result. Binary formation is terminated when the velocity dispersion rises to a point at which most encounters result in disk destruction rather than capture. As a result, much gas is returned to the cluster ISM, limiting the star formation efficiency to a value significantly below 100 percent.
Murray, Stephen D.↗
Observations of ultraviolet-bright stars in globular clusters with the Hopkins Ultraviolet Telescope
Two UV-bright stars in globular clusters, UV5 in NGC 1851 and vZ 1128 in M3, were observed with the Hopkins Ultraviolet Telescope (HUT) during the Astro-1 space shuttle mission in 1990 December. The stars' spectra show weak absorption features and no significant emission features other than well known geocoronal lines. Detailed fitting of Kurucz (1991) stellar atmosphere models using a chi(sup 2) minimization technique indicates T(sub eff) = 16 000 K, log g = 2.5, and abundance (-1.0) (logarithm of abundance of elements heavier than helium relative to solar) for UV5, and T(sub eff) = 35 000 K, log g = 4.0, and abundance (-3.5) for vZ 1128. The Kurucz model which best fits vZ 1128 overpredicts the flux in the region below approximately 1000 A, an effect seen in previous models of O-type stars. Our results are robust (to within approximately 1000 K) with respect to uncertainties in interstellar reddening and atomic and molecular hydrogen column densities. We do not see significant molecular hydrogen absorption, which might have indicated material in a circumstellar shell, in either star's spectrum. We estimate the stellar luminosities to be log L/solar luminosity = 3.33 +/- 0.15 for UV5 and log L/solar luminosity = 3.21 +/- 0.12 for vZ 1128. These atmospheric parameters place both stars on the Schoenberner (post-AGB) tracks, though the stellar masses derived from the best-fitting Kurucz models are somewhat less than those predicted by the Schoenberner models. Examination of individual absorption line strengths reveals no significant abundance anomalies in either star.
Van Dyke Dixon, W.↗
Rosat Observations of Nine Globular Clusters
The ROSAT HRI was used to image fields around nine Galactic globular clusters that have central densities in the range of 10(exp 4) - 10(exp 5) solar mass pc(exp -3) and that had not previously been observed with the Einstein Observatory. We detected X-ray sources associated with Pal 2 and NGC 6304 with luminosities of 1.1 x 10(exp 34) ergs/s and 1.2 x 10(exp 33) ergs/s, respectively. No X-ray emission was detected from the source in Ter 6, thus confirming its transient nature. In all, there were 23 serendipitous sources found in the nine fields; none was apparently associated with any of the other seven clusters. The results are discussed in the context of low-luminosity cluster X-ray sources, in general.
Rappaport, S.↗
Predictions of Gamma-ray Emission from Globular Cluster Millisecond Pulsars Above 100 MeV
The recent Fermi detection of the globular cluster (GC) 47 Tucanae highlighted the importance of modeling collective gamma-ray emission of millisecond pulsars (MSPs) in GCs. Steady flux from such populations is also expected in the very high energy (VHE) domain covered by ground-based Cherenkov telescopes. We present pulsed curvature radiation (CR) as well as unpulsed inverse Compton (IC) calculations for an ensemble of MSPs in the GCs 47 Tucanae and Terzan 5. We demonstrate that the CR from these GCs should be easily detectable for Fermi, while constraints on the total number of MSps and the nebular B-field may be derived using the IC flux components.
Venter, C.↗
A 110-ms pulsar, with negative period derivative, in the globular cluster M15
The discovery of a 110-ms pulsar, PSR2127+11, in the globular cluster M15, is reported. The results of nine months of timing measurements place the new pulsar about 2 arcsec from the center of the cluster, and indicate that it is not a member of a close binary system. The measured negative value of the period derivative is probably the result of the pulsar being bodily accelerated in our direction by the gravitational field of the collapsed core of M15. This apparently overwhelms a positive contribution to the period derivative due to magnetic braking. Although the pulsar has an unexpectedly long period, it is argued that it belongs to the class of 'recycled' pulsars, which have been spun up by accretion in a binary system. The subsequent loss of the pulsar's companion is probably due to disruption of the system by close encounters with other stars.
Wolszczan, A.↗
Evolving, dynamical models for collapsed-core globular clusters - M15 and NGC 6624
The stellar populations of the collapsed-core globular clusters M15 and NGC 6624 are investigated by fitting observed surface-brightness and projected velocity-dispersion profiles. The present evolving cluster models were generated by the direct Fokker-Planck method and incorporate realistic stellar mass spectra and energy input from binaries formed by three-body interactions. An evolved power-law mass function with nonluminous remnants of maximum mass 1.0-1.4 solar mass is adopted. M15 is found to be best fitted by postcollapse evolving models with a mass-function slope of x = 0.9 (where x = 1.35 corresponds to the Salpeter mass function) and a maximum remnant mass of 1.3 solar mass. Postcollapse core oscillations, driven by energy input from hard binaries, can produce sufficiently rapid core expansion to explain the resolution of the core of M15 by HST observations reported by Lauer et al. (1991).
Grabhorn, Robert P.↗
Demonstrating the Likely Neutron Star Nature of Five M31 Globular Cluster Sources with Swift-NuSTAR Spectroscopy
We present the results of a joint Swift-NuSTAR spectroscopy campaign on M31. We focus on the five brightest globular cluster X-ray sources in our fields. Two of these had previously been argued to be black hole candidates on the basis of apparent hard-state spectra at luminosities above those for which neutron stars are in hard states. We show that these two sources are likely to be Z-sources (i.e. low magnetic field neutron stars accreting near their Eddington limits), or perhaps bright atoll sources (low magnetic field neutron stars which are just a bit fainter than this level) on the basis of simultaneous Swift and NuSTAR spectra which cover a broader range of energies. These new observations reveal spectral curvature above 6-8 keV that would be hard to detect without the broader energy coverage the NuSTAR data provide relative to Chandra and XMM-Newton. We show that the other three sources are also likely to be bright neutron star X-ray binaries, rather than black hole X-ray binaries. We discuss why it should already have been realized that it was unlikely that these objects were black holes on the basis of their being persistent sources, and we re-examine past work which suggested that tidal capture products would be persistently bright X-ray emitters. We discuss how this problem is likely due to neglecting disc winds in older work that predict which systems will be persistent and which will be transient.
globular clusters: general↗