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At least 307 records · Page 17
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Are there two kinds of blue stragglers in globular clusters?

Blue stragglers made from collisions may have excess He abundance outside the core compared to those made from binary mergers. This effect may be observable, particularly in ultraviolet colors. Most blue stragglers have been discovered in clusters sufficiently sparse that collisions are improbable, and there is direct evidence for binarity in several cases. However, the newly discovered blue straggler sequences in the cores of NGC 6397 and 47 Tuc show characteristics which may be more easily explained if they are formed from collisional mergers.

Bailyn, Charles D.↗
NASA NTRS1992

Oxygen-enhanced models for globular cluster stars. III - Horizontal-branch sequences

A large grid of horizontal-branch (HB) evolutionary sequences which have been calculated with core expansion and semiconvection and with enhanced oxygen composition are presented and described. Tracks for 10 different metallicities are computed; they range from (Fe/H) = -0.47 to -2.26 and comprise a total of 115 sequences. The evolution is traced from the zero-age horizontal-branch (ZAHB) to the lower AGB at a point where log L/solar luminosity = 2.25. All of the sequences are illustrated on both the theoretical H-R diagram and on the B, V color-magnitude diagram. A complete set of tables for the ZAHB models and a representative sample of tabulations of the track parameters are provided. The phenomena which control HB evolution morphology, and existing certainties in theoretical HB models are discussed.

Dorman, Ben↗
NASA NTRS1993

Globular cluster formation in merging galaxies?

The properties of a population of luminous blue clusters in the merging galaxy NGC 3597 are discussed, and the observability of the signature of such a population in later evolutionary stages of a merger is assessed. NGC 3597 is an advanced merger which has already developed into an elliptical-like object with a de Vaucouleurs brightness distribution, but intense star formation is continuing in the central 4 kpc. A population of unresolved blue objects that are concentrated towards the central region is identified as clusters belonging to NGC 3597.

Lutz, D.↗
NASA NTRS1992

An Ultraviolet Imaging Telescope study of the globular cluster M79 (NGC 1904)

The Ultraviolet Imaging Telescope detects about 100 blue and extremely blue horizontal-branch (HB) stars in M79. Comparison of the ultraviolet color-magnitude diagram of M79 with stellar interior models shows that the distribution of total masses of the detected stars along with the zero-age HB includes the range of 0.495-0.62 solar mass, corresponding to envelope masses 0.01-0.13 solar mass. The total number of HB stars in M79 is estimated at 220 +/- 10. Also detected are two hot stars brighter than the horizontal branch by at least 1.5 mag at 1520 A. One of these stars (UIT 1) appears to be descending in luminosity along a pre-white dwarf track. The other (UIT 2) is most likely a post-HB star evolving toward the asymptotic giant branch.

Hill, Robert S.↗
NASA NTRS1992

Stellar encounters involving neutron stars in globular cluster cores

Encounters between a 1.4 solar mass neutron star and a 0.8 solar mass red giant (RG) and between a 1.4 solar mass neutron star (NS) and an 0.8 solar mass main-sequence (MS) star have been successfully simulated. In the case of encounters involving an RG, bound systems are produced when the separation at periastron passage R(MIN) is less than about 2.5 R(RG). At least 70 percent of these bound systems are composed of the RG core and NS forming a binary engulfed in a common envelope of what remains of the former RG envelope. Once the envelope is ejected, a tight white dwarf-NS binary remains. For MS stars, encounters with NSs will produce bound systems when R(MIN) is less than about 3.5 R(MS). Some 50 percent of these systems will be single objects with the NS engulfed in a thick disk of gas almost as massive as the original MS star. The ultimate fate of such systems is unclear.

Davies, M. B.↗
NASA NTRS1995

Gamma-ray emission from globular clusters. Shock high energy emission from the Be-Star/Pulsar System PSR 1259-63. Echoes in x-ray novae

This grant covers work on the Compton phase 3 investigation, 'Shock High Energy Emission from the Be- Star/Pulsar System PSR 1259-63' and cycle 4 investigations 'Diffuse Gamma-Ray Emission at High Latitudes' and 'Echoes in X-Ray Novae'. Work under the investigation 'Diffuse Gamma-Ray Emission at High Latitudes' has lead to the publication of a paper (attached), describing gamma-ray emissivity variations in the northern galactic hemisphere. Using archival EGRET data, we have found a large irregular region of enhanced gamma-ray emissivity at energies greater 100 MeV. This is the first observation of local structure in the gamma-ray emissivity. Work under the investigation 'Echoes in X-Ray Novae' is proceeding with analysis of data from OSSE from the transient source GRO J1655-40. The outburst of this source last fall triggered this Target of Opportunity investigation. Preliminary spectral analysis shows emission out to 600 keV and a pure power low spectrum with no evidence of an exponential cutoff. Work is complete on the analysis of BATSE data from the Be-Star/Pulsar Sustem PSR 1259-63.

Kaaret, Philip↗
NASA NTRS1994

The luminosity function for stars with minus 0.6 less than or equal to M (sub nu) less than or equal to 6.5 in the globular cluster M30: Is there a problem with stellar evolution models?

The V band luminosity function (LF) for stars ranging from the tip of the giant branch to approximately 2.5 mag fainter than the main-sequence turnoff (MSTO) in M30 (=NGC 7099, GC 2137-234) is derived from CCD observations. Comparison with the theoretical LFs of Bergbusch & VandenBerg (1992) for (M/H) = -2, 16 Gyr models shows discrepancies which can be described either of two ways: (1) If the observations are normalized to the models over a few magnitudes fainter than the MSTO, then there is an excess (N (sub obs) greater than N(sub predicted)) of subgiant stars within 0.5 mag of the MSTO at the level of 4.8 sigma and an excess of stars (approximately 20%) compared to the models all along the giant branch. (2) If the normalization is made to match predicted and observed numbers of stars on the giant branch, then there is a discrepancy between models and observations in the sense that there is a broad depression in the observed LF fainter than the MSTO that is not present in the models. Consideration of LFs generated from 12 Gyr models or models adjusted to a shorter (m - M)(sub nu) = 14.5) distance modulus diminishes the magnitude of some of these discrepancies but introduces other mismatches between the observations and models. LFs constructed from stellar models that have 10% of the stellar mass in an isothermal core qualitatively match the observations better than the 'standard' models.

Bolte, Michael↗
NASA NTRS1994

Globular cluster photometry with the Hubble Space Telescope. 2: U, V, and I measurements of M15

The projected density distribution of resolved stars near the center of M15 is shown to be consistent with either a power-law cusp N(r) approximately r(exp alpha), with alpha approximately -0.85 +/- 0.2, or with a King model with a core of radius approximately less than 2 sec. The inferred slope is in agreement with the theoretical value, alpha = -0.75, calculated by Bahcall and Wolf for the distribution of equal-mass stars surrounding a massive black hole and is also consistent with the radial profile expected from core collapse without a central black hole. The object AC 214 is a candidate for the central density cusp. Analysis of Monte Carlo simulations of the diffuse light indicates that, using current analysis techniques and available data, the residual light is not a reliable indicator of the true density distribution. This is contrary to earlier work. Photometric measurements in V and I of more than 5 x 10(exp 3) stars (and in U, V, and I of approximately greater than 1500 stars) are used to construct color-magnitude diagrams in the central 1 min of M15. Fourteen blue straggler candidates are identified in the inner 20 sec. The central color gradient noticed by previous researchers is caused by a central depletion of bright red giant stars rather than an excess of blue stragglers or blue horizontal branch stars.

Yanny, Brian↗
NASA NTRS1995

On the luminosity function, lifetimes, and origin of blue stragglers in globular clusters

We compute theoretical evolutionary tracks of blue stragglers created by mergers. Two formation scenarios are considered: mergers of primordial binaries, and stellar collisions. These two scenarios predict strikingly different luminosity functions, which are potentially distinguishable observationally. Tabulated theoretical luminosity functions and lifetimes are presented for blue stragglers formed under a variety of input conditions. We compare our results with observations of the blue straggler sequences in 47 Tucanae and M3. In the case of 47 Tuc, the luminosity function and the formation rate are compatible with the hypothesis that the blue stragglers formed through the collision of single stars. Mergers of primordial binaries are only marginally cosistent with the data, and a significant enhancement of the collision cross section by binary-single-star encounters appears to be ruled out. In the case of M3, we find that the innermost blue stragglers have a luminosity function significantly different from that of the outer stragglers, thus confirming earlier suggestions that there are two distinct populations of blue stragglers in this cluster. The inner stragglers are preferentially brighter and bluer, as would be expected if they were made by collisions, but there are so many of them that the collision rate would need to be enhanced by interactions involving wide binaries. The luminosity function of the outer stragglers is almost identical to the predictions of mergers from primordial binaries and is inconsistent with the collision hypothesis.

Bailyn, Charles D.↗
NASA NTRS2005

XMM-Newton Survey of Globular Cluster Ultracompact Binaries

Our program consisted of an observation of a single source, 4U 0513-40, which we had previously identified as a candidate ultracompact binary (a system with an orbital period below 1 hour). Several other known or suspected ultracompact binaries have shown unusual elemental abundance ratios in their X-ray spectra. In this program, however, our observation found no unusual abundance ratios in the spectrum of 4U 0513-40. This result was included, along with results from a separate Chandra program, in a paper submitted for publication in the Astrophysical Journal.

Chakrabarty, Deepto↗