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At least 343 records · Page 19
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NASA NTRS1983

Stellar populations in local group dwarf elliptical galaxies. I - NGC 147

A color-magnitude diagram of NGC 147 to an I magnitude of 23 is presented. The stellar population in the outer parts of this elliptical galaxy resembles that of the globular clusters of the Milky Way. Quantitative comparison of the giant branch with those of globular clusters yields a mean metallicity of -1.2 + or - 0.2, making NGC 147 a part of the general correlation between mass and metallicity seen in ellipticals. The giant branch appears to be broad, which suggests a metallicity dispersion. The absence of asymptotic giant branch stars at luminosities above that of the red giant branch tip sets an upper limit of 10 percent for the fraction of stars in this NGC 147 field that have ages less than 12 Gyr. This result contrasts with the situation in some of the related, but less massive, dwarf spheroidal systems. If the choice is made to assume, rather than determine the stellar content of NGC 147, a distance of 630 + or - 50 kpc is derived, similar to that of M31.

Mould, J. R.↗
NASA NTRS1993

On the effects of cluster density and concentration on the horizontal branch morphology - The origin of the blue tails

Possible relationships between horizontal branch (HB) morphology in Galactic globular clusters and the cluster structure and dynamical evolution are investigated. New HB observables are defined and determined using a theoretical framework deduced from HB models. Data for 53 Galactic globular clusters are used to obtain correlations between the observables. It is found that the net length of the HB and the presence and extent of blue tails in particular are correlated with the cluster density and concentrations, in the sense of more concentrated or denser clusters having bluer and longer HB morphologies. This effect is especially strong for the intermediate metallicity clusters. Thus, the cluster environment can affect the stellar evolution leading to the HB and therefore the HB morphology. This result is interpreted in terms of an enhanced mass removal from the HB progenitors.

Fusi Pecci, F.↗
NASA NTRS1995

Ultraviolet radiation from evolved stellar populations. 2: The ultraviolet upturn phenomenon in elliptical galaxies

We discuss the far-ultraviolet upturn phenomenon (UVX) observed in elliptical galaxies and spiral galaxy bulges. Our premise is the UV radiation from these systems emanates primarily from extreme horizontal branch (EHB) stars and their progeny. We derive the broad-band UV colors 1500-V and 2500-V for globular clusters and elliptical galaxies from the available satellite data and investigate color-color and color-line strength correlation. Clusters can be bluer than any galaxy in 15-V and 25-V, implying larger hot star populations, but galaxies are significantly bluer than clusters in 15-25 at a given 15-V. We attribute this primarily to the effect of metal abundance on the mid-UV (2500 A) light. These redder colors of the galaxies also imply that the UVX in galaxies is not produced by metal-poor subpopulations similar to the clusters. We devlop a simple spectral synthesis formulation for all phases of single star evolution from the zero-age main sequence (ZAMS) to the white dwarf cooling track that requires only one or two parameters for each choice of age and abundance. We provide the ingredients necessary for constructing models with arbitrary horizontal branch (HB) morphologies in the age range 2 less than t less than 20 Gyr and for six metallicities in the range -2.26 less than (Fe/H) less than 0.58; we also consider the efect of enhanced Y in metal-rich models. The maximum lifetime UV output is produced by EHB stars with (M(sub env))(sup 0) approximately 0.02 solar mass and can be up to 30 times higher than for post-asymptotic giant branch (P-AGB) stars. The ultraviolet output of old populations is governed primarily by the distribution of (M(sub env))(sup 0)P(M(sub env))(sup 0), on the ZAHB. The UV output is not very sensitive to (Fe/H) or to Y, but it can change very rapidly with (M(sub env))(sup 0). Thus it is extremely sensitive to the precise nature of giant-branch mass loss. Our models use simple descriptions of P(M(sub env))(sup 0) to bracket the colors produced from any real distribution of stars. Our models accurately predict the range of UV colors observed for the globular clusters, given known constraints on their age, abundances, and HB morphologies. We find that models with (Fe/H) greater than or = 0 that do not contain EHB stars cannot reproduce the colors of most of the galaxies. The models also predict that the fraction of the far-UV light from P-AGB stars, which are spatially resolvable in nearby galaxies, is approximately 70% and approximately 20% for moderate UVX and strong UVX systems, respectively. We find that 25-V, but not 15-V, is sensitive to the age and abundance, though these cannot always be cleanly distinguished. The galaxy colors place limits of (Fe/H) greater than -0.5 and less than 15% on the contribution of globular cluster-type populations to the V light. Galaxy colors are consistent with solar-abundance models with ages in the range 6-14 Gyr. We discuss several implications of the observations and the models, including the question of light metal versus iron peak enhancements in galaxies, whether the UV color-Mg(sub 2) correlation is continuous or discrete, effects of helium abundnace on the UVX, and the key question of whether red giant branch mass loss can be large enough to produce the necessary EHB population in the strong UVX galaxies.

Dorman, Ben↗
NASA NTRS1991

The rate of neutron star binary mergers in the universe - Minimal predictions for gravity wave detectors

Of the many sources which gravitational wave observatories might see, merging neutron star binaries are the most predictable. Their waveforms at the observable frequencies are easy to calculate. And three systems which will merge in less than a Hubble time have already been observed as binary pulsars: two in the disk of the Galaxy, and one in a globular cluster. From the lifetimes and positions of these, a lower limit to the merger rate in the Galaxy and globular cluster system are inferred with confidence. Taking the merger rate in other galaxies to scale with the star formation rate, the merger rate expected in the local universe is computed. An ultraconservative lower limit to the rate gives three per year within 1 Gpc. The best estimate, still conservative in that it considers only systems like those already observed, gives three per year within 200 Mpc. An upper limit of three mergers per year within 23/h Mpc is set by the rate of Type Ib supernovae. The rates of black hole binary mergers and black hole-neutron star binary mergers are model-dependent, but could be comparable to the given rate of neutron-star binary mergers.

Phinney, E. S.↗
NASA NTRS1993

Protogalactic mergers and cosmochronology

We construct schematic models for chemical evolution and cosmochronology within the expanding and collapsing protogalactic halo followed by formation of the local disk. Star formation is associated with both the rate of protogalactic mergers and the intrinsic gas density of protogalactic clouds and the disk. This leads naturally to a scenario in which star formation in the disk can be delayed by several billion years relative to the formation of the oldest globular clusters. We analyze various cosmochronometers in the context of this model and show that the apparent differences between the maximum globular-cluster ages, the white-dwarf cooling age, and nuclear chronometric ages can be understood. The merger models which satisfy the age constraints imply a relatively late forming peak in luminosity and therefore may be consistent with the observed peak in galaxy number counts at intermediate redshifts. Versions of the model with and without nonbaryonic dark matters can yield significant dark baryonic halos.

Mathews, G. J.↗
NASA NTRS1995

Stellar photometry in the inner bulge of M31 using the Hubble Space Telescope wide field camera

We present photometry of two fields in the M31 bulge imaged with the Hubble Space Telescope (HST) Wide-Field Camara (WFC). The nuclear field (r less than 40 arcsecs = 150 pc) giant branch extends to I = 19.5, M(sub I) = -5 (Cousins system), a full 0.9 mag brighter than the giant-branch tips of metal-poor Galactic globular clusters and M31 halo fields. This is also approximately = 1.5 mag brighter than the giant branches of metal-rich Galactic globular clusters, but is no brighter than Mould's (1986) M31 bulge field 1 kpc from the nucleus. The data also suggest that the brighter stars may be preferentially concentrated to the center. The 648 luminous stars detected in 2 x 10(exp 9) solar luminosity is approximately = 25% that expected from a hypothetical population of evolved asymptotic giant branch (AGB) stars with lifetimes approximately = 10(exp 5) yr, with the cautionary note that we are near the detection limit. The number of bright stars is also consistent with the progeny of blue stragglers, if one uses a lifetime for the thermal-pulsing AGB of 2 x 10(exp 6) yr. We strongly caution that incompleteness becomes severe below I = 19.9 mag and that future surveys are likely to find numbers of bright stars too large to accomodate the blue straggler progeny hypothesis. We have imaged an additional field 2 arcmin = 500 pc south of the nucleus. The brightest stars in this field are also I = 19.5, but bright stars appear less numerous than in the nuclear field. If the population resembles that of the Galactic bulge, then M(sub bol) = -4.5 is a lower limit to the giant-branch tip luminosity; infrared studies should reveal stars 0.5 mag or more brighter. Either high-metallicity or (more likely) age approximately = 10 Gyr may be responsible for the presence of these luminous AGB stars. These observations confirm that previous ground-based infrared studies (e.g., Rich & Mould 1991) very likely detect an extended giant branch and not spurious luminous stars caused by crowding or disk contamination. However, published integrated colors for the M31 bulge/nucleus are extremely red, making it difficult to accomodate a young or intermediate-age population.

Rich, R. M.↗
NASA NTRS1995

Derived parameters for NGC 6791 from high-metallicity isochrones

We have computed 8, 10, and 12 Gyr isochrones and physically consistent models of zero-age red horizontal branch stars for stellar masses between 0.55 and 1.3 solar mass, all at (fe/H) = +0.15. Comparison to the NGC 6791 BVI photometry of Kaluzny & Udalski (1992) and Montgomery et al. (1994) yields an age of 10.0 +/- 0.5 Gyr at an apparent distance modulus 13.49 less than (m-M)(sub V) less than 13.70. The color offsets required to fit the isochrones, combined with the spectroscopic results of Friel & Janes (1993), imply that the foreground reddening to NGC 6791 lies in the range 0.24 greater than E(B -V) greater than 0.19 with +0.27 less than (Fe/H) less than + 0.44. These results are derived using a technique by which we predict color shifts and apply these to the isochrones to simulate progressively higher metallicities. The zero-age horizontal branch model suggest that the red horizontal branch stars of NGC 6791 have masses approximately less than 0.7 solar mass. The masses are similar to those found for M67 red horizontal branch stars by Tripicco et al. (1993) and for globular cluster red horizontal branch stars, even though the M67 progenitors are approximately 0.2 solar-mass more massive, while the progenitors of globular cluster horizontal branch stars are similarly less massive. This suggests the presence of a mechanism, not strongly dependent on metallicity, which reduces stellar envelopes on the zero-age horizontal branch to a given mass rather than by a given amount.

Dorman, Ben↗
NASA NTRS1994

Modeling the radial distribution of blue stragglers in M3

Recent surveys of the blue straggler (BS) population in the Galactic globular cluster M3 (NGC 5272) give the first complete characterization of the number density of BSs as a function of radius over an entire globular cluster. The BSs in M3 are overabundant at large radii and significantly underabundant at intermediate radii. Here we present the result of a simulation of the dynamical evolution of a population of BSs in a multimass model of M3. Assuming the BSs were formed in the core through binary interactions (Hut & Verbundt 1983; Leonard 1989; Sigurdsson & Phinney 1993; Hut et al. 1992b; Davies, Benz & Hills 1994), and given some very general assumptions about the recoil that occurs during stellar mergers in interacting binaries, we find an excellent fit to the observed radial distribution of BSs, suggesting strongly that most of the BSs in M3 were formed through binary collisions in the core.

Sigurdsson, Steinn↗
NASA NTRS1977

A new determination of the gas-to-dust ratio in M31

The mean gas-to-dust ratio in M31 is derived from observations of the H I column density and the color excess in the direction of 121 globular clusters. The angular resolution of the H I observations obtained with the Westerbork Synthesis Radio Telescope is comparable to the angular sizes of individual globular clusters. The mean gas-to-dust ratio turns out to be approximately 33 by 10 to the 20th power atoms/sq cm per magnitude; the value depends on radial distance from the center of the galaxy. Some effects that may influence the determination are discussed.

Bajaja, E.↗
NASA NTRS2009

Flash Mixing of the White Dwarf Cooling Curve Spectroscopic Confirmation in NGC 2808

We present new HST far-UV spectroscopy of two dozen hot evolved stars in NGC 2808, a massive globular cluster with a large population of "blue-hook" stars. The blue-hook stars are found in ultraviolet color-magnitude diagrams of the most massive globular clusters, where they fall at luminosities immediately below the hot end of the horizontal branch (HB), in a region of the HR diagram unexplained by canonical stellar evolution theory. Using new theoretical evolutionary and atmospheric models, we have shown that these subluminous HB stars are very likely the progeny of stars that undergo extensive internal mixing during a late He-core flash on the white dwarf cooling curve. This flash mixing leads to hotter temperatures and an enormous enhancement of the surface He and C abundances; the hotter temperatures and associated decrease in the hydrogen opacity shortward of the Lyman limit makes the stars brighter in the extreme UV but appear sub luminous in the UV and optical. Our far-UV spectroscopy demonstrates that, relative to normal HB stars at the same color, the blue-hook stars of NGC 2808 are hotter and greatly enhanced in He and C, thus providing unambiguous evidence of flash mixing in the subluminous population. Although the C abundance in the blue-hook stars is orders of magnitude larger than that in the normal HB stars, the atmospheric C abundance in both the blue-hook and normal HB stars appears to be affected by gravitational settling. The abundance variations seen in C, Si, and the Fe-peak elements indicate that atmospheric diffusion is at play in our sample, with all of our hot subdwarfs at 25,000 K to 50,000 K exhibiting large enhancements of the iron-peak elements. The hottest subdwarfs in our blue-hook sample may be pulsators, given that they fall in the temperature range of newly-discovered pulsating subdwarfs in omega Cen.

Brown, Thomas M.↗
NASA NTRS1986

IRAS 18333-2357 - An unusual source in M22

The present paper is concerned with a search for IRAS sources positionally associated with globular clusters. This search revealed a strong source with unusual infrared colors, IRAS 18333-2357, near the center of M22. M22 (NGC 6566) is a bright, nearby, metal-poor globular cluster. Attention is given to the observations, the mass and luminosity of IRAS 18333-2357, the mass-loss rate, and the nature of IRAS 18333-2357. It is concluded that IRAS 18333-2357 may possibly be a bipolar nebula similar to OH 0739-14 although element abundance arguments appear to favor a carbon-rich rather than oxygen-rich shell.

Gillett, F. C.↗
NASA NTRS1988

Multiple populations on the horizontal branch. I - Observations in the (log g, log T/eff/)-diagram

Log T(eff) and log g have been determined for blue horizontal branch (HB) stars in the globular clusters M15, M3, M5, M92, and NGC 288 as a first step in determining the physical origin of gaps or bimodal distributions in the blue horizontal branches of a number of globular clusters. The (log g, log T/eff/) diagram for the 'normal' HB cluster M5 matches the standard theory quite well. The stars blueward of the gap in NGC 288 and the blue extension of the M3 HB have significantly lower gravity than predicted by the standard theory. The cooler stars in M3 agree well with the standard theory while those of NGC 2988 lie at slightly lower gravity. In M92, the sequence of stars is broadened toward the low gravity side more than expected either from standard evolution or from observational error. The data for M15 are too scattered to support firm conclusions but seem consistent with NGC 288.

Crocker, Deborah A.↗
NASA NTRS1989

Theoretical colours and isochrones for some Hubble Space Telescope colour systems

Synthetic spectra for effective temperatures of 4000-7250 K, logarithmic surface gravities typical of dwarfs and subgiants, and metallicities from solar values to 0.001 of the solar metallicity were used to derive a grid of synthetic surface brightness magnitudes for 21 of the Hubble Space Telescope Wide Field Camera (WFC) band passes. The absolute magnitudes of these 21 band passes are also obtained for a set of globular cluster isochrones with different helium abundances, metallicities, oxygen abundances, and ages. The usefulness and efficiency of different sets of broad and intermediate bandwidth WFC colors for determining ages and metallicities for globular clusters are evaluated.

Edvardsson, B.↗
NASA NTRS1987

New developments in studies of compact X-ray binaries

Several recent developments, both observational and theoretical, on the study of X-ray binaries and the compact objects they contain are discussed. The recent discovery of the first binary periods for the globular cluster X-ray sources has stimulated a new model for their origin. As a variant of the 'standard' tidal capture origin model, this predicts an enhanced number of neutron stars in globular clusters. Long term timing studies of X-ray binaries may be consistent with many of these systems, primarily X-ray burst sources, being in fact hierarchical triple systems. Finally, the radio studies of Cyg X-3 and other X-ray binaries suggest that nonthermal processes are as important, energetically, as accretion processes in these systems.

Grindlay, Jonathan E.↗
NASA NTRS2001

Flash Mixing on the White-Dwarf Cooling Curve: Understanding Hot Horizontal Branch Anomalies in NGC 2808

We present an ultraviolet color-magnitude diagram (CMD) spanning the hot horizontal branch (HB), blue straggler, and white dwarf populations of the globular cluster NGC 2808. These data, obtained with the Space Telescope Imaging Spectrograph (STIS), demonstrate that NGC 2808 harbors a significant population of hot subluminous HB stars, an anomaly only previously reported for the globular cluster omega Cen. Our theoretical modeling indicates that the location of these subluminous stars in the CMD, as well as the high temperature gap along the HB of NGC 2808, can be explained if these stars underwent a late helium-core flash while descending the white dwarf cooling curve. We show that the convective zone produced by such a late helium flash will penetrate into the hydrogen envelope, thereby mixing hydrogen into the hot helium-burning interior, where it is rapidly consumed. This phenomenon is analogous to the "born again" scenario for producing hydrogen-deficient stars following a late helium-shell flash. The flash mixing of the envelope greatly enhances the envelope helium and carbon abundances that, in turn, leads to a discontinuous increase in the HB effective temperatures. We argue that the hot HB gap is associated with this theoretically predicted dichotomy in the HB properties. Moreover, the changes in the emergent spectral energy distribution caused by these abundance changes are primarily responsible for explaining the hot subluminous HB stars. Although further evidence is needed to confirm that a late helium-core flash can account for the subluminous HB stars and the hot HB gap, we demonstrate that an understanding of these stars requires the use of appropriate theoretical models for their evolution, atmospheres, and spectra.

Brown, Thomas M.↗
NASA NTRS1976

A search for radio emission from ionized hydrogen in M15

A sensitive search at 3.6 cm for thermal emission from ionized gas in the globular cluster M15 (NGC 7078) reveals no emission greater than the experimental error, which is 0.003 Jy. Based on this flux-density limit, the mass of ionized gas within 3 pc of the cluster center is less than 26 solar masses. This result suggests that the ionized-gas content of M15, which contains an intense X-ray source, is not greatly enhanced compared with similar globular clusters without known X-ray sources. This mass limit is an order of magnitude greater than the amount of gas required to produce the X-ray flux by accretion onto massive black holes.

Klein, M. J.↗
NASA NTRS1984

Infrared observations of galactic bulge X-ray sources

Nine unidentified galactic bulge X-ray sources, the recently identified X-ray burster MXB 1728-34, and two optically identified sources (Sco X-1 and MXB 1735-44) were observed with the NASA 3 m Infrared Telescope Facility. The data constrain both the presence of diffuse infrared sources near the X-ray positions and the flux of possible infrared counterparts. None of the nine unidentified sources lies within obscured globular clusters, although there is marginal evidence for diffuse infrared emission near 4U 1822-00 and 4U 1916-05. This implies that at most two additional luminous galactic bulge X-ray sources lie within undiscovered, obscured globular clusters. No infrared counterparts were detected for unidentified sources; the limits derived are consistent with all of the sources observed being similar to the low mass X-ray binary Sco X-1.

Hertz, P.↗
NASA NTRS1991

A post-core-collapse model for the nucleus of M33

A model for the post-collapse core of M33 is proposed. By analogy with globular clusters, the formation of tight binary systems powers the reexpansion of the core, and it is expected that low-mass X-ray binaries form at a rate about equal to that of all galactic globular clusters combined. About a dozen such binaries should be present, and their combined emission may explain the large and enigmatic unresolved X-ray emission from the nucleus of M33. In addition, tidal formation of cataclysmic binaries may lead to a nova rate of more than about one per century. Numerous blue stragglers have probably formed through mergers caused by stellar collisions, but their density is probably too low to explain the blue color of the nucleus. Young stars are the likely cause for the color, and their presence may complicate the simple dynamical picture presented here.

Hernquist, Lars↗