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At least 667 records · Page 37
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NASA NTRS1993

The spectrum of the central star of the planetary nebula in M22

New optical observations of the central star of the planetary nebula IRAS 18333-2357 in the globular cluster M22 show lines of H I, He II, C IV, N IV, and N V. The spectrum closely resembles the sdO star KS 292, which has surface abundances enhanced by products of hydrogen shell burning and helium burning, and an effective temperature of 75,000 K. The lines of C in IRAS 18333-2357 seem somewhat stronger than in KS 292, and the lines of N are considerably stronger. The presence of substantial hydrogen is surprising in view of the hydrogen-poor nature of the nebular ejecta. If IRAS 18333-2357 is as hot as its analog, its luminosity is about 14,000 lunar luminosity. This value is higher than that theoretically expected for single-star evolution of M22 cluster stars.

Harrington, J. P.↗
NASA NTRS1993

Mass loss in M67 giants - Evidence from isochrone fitting

A comparison between the color-magnitude diagram of M67 and a new set of theoretical evolutionary models which include all phases from the unevolved main-sequence through core-helium burning and onto the AGB is presented. The present 5-Gyr solar abundance isochrone is found to yield an excellent fit to the whole of the M67 color-magnitude diagram. A differential technique that compares the gap in color between clump giants and normal red giants, on one hand, with the temperature gap between core He-burning tracks and first-ascent RGB tracks, on the other, strongly indicates that the clump giants in M67 have masses of 0.70 solar mass or less. The extremely large amount of mass loss that is deduced is well in excess of that found for globular cluster stars. Possible resolutions of this problem are that degree of mass loss increases with total stellar mass, or with metallicity.

Tripicco, Michael J.↗
NASA NTRS1993

Detection of an ultraviolet and visible counterpart of the NGC 6624 X-ray burster

We have detected, in images taken with the HST FOC, the UV and optical counterpart of the X-ray source 4U 1820-30 in the globular cluster NGC 6624. Astrometric measurements place this object 2 sigma from the X-ray position of 4U 1820-30. The source dominates a far-UV FOC image and has the same flux at 1400 A as was seen through the large IUE aperture by Rich et al. (1993). It has a B magnitude of 18.7 but is not detected in V. It is 0.66 arcsec from the center of NGC 6624, a fact that may change the interpretation of the P-average of the 11 minute binary orbit. The flux drops between 1400 and 4300 A at a rate that is nearly as steep as that of a Rayleigh-Jeans curve. The flux is far too large to come from the neutron star directly but could accord with radiation from a heated accretion disk and/or the heated side of the companion star.

King, I. R.↗
NASA NTRS1993

A reprocessing model for the ultraviolet and optical light from 4U 1820-30

We show that the recently discovered optical and ultraviolet light from the X-ray burst source 4U 1820-30 in the globular cluster NGC 6624 is due to reprocessing of the X-rays in the outer regions of an optically thick, geometrically thin accretion disk. We suggest that observation of orbital modulation of the reprocessed light, due to the variable contribution made as the heated face of the companion turns toward and away from the observer, would provide constraints on the inclination of the binary orbit, and we suggest that detection in the reprocessed flux of the 'red noise' already observed in the X-rays would provide useful constraints on the geometry and physics of the accretion disk.

Arons, Jonathan↗
NASA NTRS1993

Genesis of a planet in Messier 4

The anomalous spin period second derivative of the binary millisecond pulsar PSR 1620-26 in the globular cluster M4 is best explained by a sub-Jovian mass planet in a moderately eccentric about 7 AU orbit about the pulsar binary. We consider formation scenarios for PSR 1620-26. A planet scavenged from a single main-sequence star during an exchange encounter naturally produces systems such as PSR 1620-26. The position of the pulsar just outside the core of M4 is shown to fit naturally with the preferred formation scenario and permit a planet to have survived in the inferred orbit about the binary. It is possible that the orbital eccentricity of the binary was induced by the planet. A confirmation of a planet in eccentric orbit about PSR 1620-26 would strongly suggest that planets form ubiquitously around low-mass main-sequence stars, even stars of low metallicity.

Sigurdsson, Steinn↗
NASA NTRS1993

The Galactic bulge and halo

The relationship between the Galactic halo and bulge is discussed, with reference to early results from a survey for K giants in a field 2 kpc from the Galactic center and Zinn's (1992) data on globular-cluster kinematics. The data are currently too meager to decide whether the inner halo shades gradually into the bulge, although there are some interesting kinematical similarities between bulge stars and Zinn's Old Halo clusters in the inner Galaxy. Useful observations that would help decide this matter are also discussed.

Morrison, Heather L.↗
NASA NTRS1993

Modelling time variable and total eclipses of the millisecond pulsar PSR 1744-24A

The 11 ms pulsar PSR 1744-24A in the globular clusters Terzan 5 is the first and only known example of a binary eclipsing pulsar showing time-variable eclipses with irregular and nonsymmetric eclipse patterns. Occasionally, the pulsar eclipse becomes total, and no radio emission is detected for an extended period of time. We present here numerical results of a hydrodynamical model aimed at explaining the remarkable eclipse properties of PSR 1744-24A. We show that the hydrodynamic time scale for the partial or total pulsar enshrouding is of order of the orbital period for a relatively large mass loss rate, in agreement with observations. The time-variable eclipse egress positions imply that the mass loss rate from the companion varies within one orbital period. Hydrodynamic modelling of the mass outflow of PSR 1744-24A is of crucial importance for the understanding of a class of millisecond pulsars whose radio emission might be occasionally or permanently quenched by mass outflows from their companions. The properties of PSR 1744-24A suggest a new way of studying millisecond pulsars involving radio, X-ray, gamma-ray, and optical observations.

Tavani, Marco↗
NASA NTRS1995

STS-67 Mission Highlights Resource Tape

The Space Shuttle Mission, STS-67, is highlighted in this video. Flight crew (Stephen S. Oswald (Commander), William G. Gregory (Pilot), Tamara E. Jernigan, Wendy B. Lawrence, John M. Grunfeld (Mission Specialists), Samuel T. Durrance, and Ronald A. Parise (Payload Specialists)) prelaunch and launch activities, EVA activities with payload deployment and retrieval (ASTRO-2 and WUPPE (Wisconsin Ultraviolet Photo Polarimeter Experiment)), spaceborne experiments (astronomical observation and data collection, protein crystal growth, and human physiological processes), and pre-reentry activities are shown. There are astronomical telescopic observation from the two telescopes in the payload, the Hopkins Ultraviolet Telescope and the Ultraviolet Imaging Telescope, of Io and of globular clusters, and their emission spectra is collected via a spectrometer. Earth view film and photography is shown, which includes lightning on terrestrial surfaces, cyclone activity, and cloud cover.

Welch, Chuck↗
NASA NTRS1994

Absolute proper motions to B approximately 22.5: Evidence for kimematical substructure in halo field stars

Radial velocities have been obtained for six of nine stars identified on the basis of similar distances and common, extreme transverse velocities in the proper motion survey of Majewski (1992) as a candidate halo moving group at the north Galactic pole. These radial velocities correspond to velocities perpendicular to the Galactic plane which span the range -48 +/- 21 to -128 +/- 9 km/sec (but a smaller range, -48 +/- 21 to -86 +/- 19 km/sec, when only our own measurements are considered), significantly different than the expected distribution, with mean 0 km/sec, for a random sample of either halo or thick disk stars. The probability of picking such a set of radial velocities at random is less than 1%. Thus the radial velocity data support the hypothesis that these stars constitute part of a halo moving group or star stream at a distance of approximately 4-5 kpc above the Galactic plane. If real, this moving group is evidence for halo phase space substructure which may be the fossil remains of a destroyed globular cluster, Galactic satellite, or Searle & Zinn (1978) 'fragment.'

Majewski, Steven R.↗
NASA NTRS1994

Hubble Space Telescope Planetary Camera observations of Arp 220

Planetary Camera images of peculiar galaxy Arp 220 taken with V, R, and I band filters reveal a very luminous object near the position of the western radio continuum source, assumed to be the major nucleus, ans seven lesser objects within 2 sec of this position. The most luminous object is formally coincident with the radio source to within the errors of Hubble Space Telescope (HST) pointing accuracy, but we have found an alternate, more compelling alignment of maps in which the eastern radio source coincides with one of the lesser objects and the OH radio sources reside near the surfaces of other optical objects. The proposed centering places the most luminous object 150 pc (0.4 sec) away from the western radio source. We explore the possibilities that the objects are either holes in the dense dust distribution, dusty clouds reflecting a hidden bright nucleus, or associations of bright young stars. We favor the interpretation that at least the brightest two objects are massive young star associations with luminosities 10(exp 9) to 10(exp 11) solar luminosity, but highly extinguished by intervening dust. These massive associations should fall into the nucleus on a time scale of 10(exp 8) yr. About 10% of the enigmatic far-IR flux arises from the observed objects. In addition, if the diffuse starlight out to a radius of 8 sec is dominated by stars with typical ages of order 10(exp 8) yr (the time since the alleged merger of two galaxies), as indicated by the blue colors at larger radius, then the lower limit to the reradiation of diffuse starlight contributes 3 x 10(exp 11) solar luminosity to the far-infrared flux, or greater than or equal to 25% of the total far-IR flux. Three additional bright objects (M(sub V) approximately equals -13) located about 6 sec from the core are likely young globular clusters, but any of these could be recently exploded supernovae instead. The expected supernovae rate, if the dominant energy source is young stars, is about one per month for the region where the intense far-infrared flux originates. Also, individual giant dust clouds are visible in these images. Their typical size is 300 pc (1 sec).

Shaya, Edward J.↗
NASA NTRS1994

Evolutionary sequences of very hot, low-mass, accreting white dwarfs with application to symbiotic variables and ultrasoft/supersoft low-luminosity x-ray sources

We present the first detailed model results of quasi-static evolutionary sequences of very hot low-mass white dwarfs accreting hydrogen-rich material at rates between 1 x 10(exp -7) and 1 x 10(exp -9) solar mass/yr. Most of the sequences were generated from starting models whose core thermal structures were not thermally relaxed in the thermal pulse cycle-averaged sense of an asymptotic giant branch stellar core. Hence, the evolution at constant accretion rate was not invariably characterized by series of identical shell flashes. Sequences exhibiting stable steady state nuclear burning at the accretion supply rate as well as sequences exhibiting recurrent thermonuclear shell flashes are presented and discussed. In some cases, the white dwarf accretors remain small (less than 10(exp 11) cm) and very hot even during the shell flash episode. They then experience continued but reduced hydrogen shell burning during the longer quiescent intervals while their surface temperatures increase both because of compressional heating and envelope structure readjustment in response to accretion over thousands of years. Both accretion and continued hydrogen burning power these models with luminosities of a few times 10(exp 37) ergs/s. We suggest that the physical properties of these model sequences are of considerable relevance to the observed outburst and quiescent behavior of those symbiotic variables and symbiotic novae containing low-mass white dwarfs. We also suggest that our models are relevant to the observational characteristics of the growing class of low-luminosity, supersoft/ultrasoft X-ray sources in globular clusters, and the Magellanic Clouds.

Sion, Edward M.↗
NASA NTRS1994

Gradients in giant branch morphology in the core of 47 Tucanae

I describe an algorithm which uses the high spatial resolution of the Hubble Space Telescope to complement the high spatial-to-noise, approximately symmetric point response function, relatively large spatial coverage, and standard filters available from ground based images of crowded fields. Applying this technique to the central regions of the globular cluster 47 Tucanae, I find that the morphology of the giant branch in the core is significantly different from that in more distant regions (r approximately equals 5 to 10 core radii) of the cluster. In particular, there appear to be fewer bright giants in the core, along with an enhanced `asymptotic giant branch' (AGB) sequence. Depletion of giants has been observed in the cores of other dense clusters, and may be due to `stripping' of large stars by stellar encounters and/or mass transfer in binary systems. Central concentrations of true asymptotic giant branch stars are not expected to result from dynamical processes; possibly some of these stars may be evolved blue stragglers.

Bailyn, Charles D.↗
NASA NTRS1994

ROSAT observations of six millisecond pulsars

We present ROSAT observations toward six known millisecond radio pulsars. These observations yielded upper limits to the X-ray flux in the ROSAT band (0.1-2.4 keV) for five pulsars and a possible association of an X-ray source with PSR B1821-24, in the globular cluster M28. At the 99.9% confidence level, the source is pulsed at the expected radio pulsar frequency. We compare our results with predicted X-ray luminosities by Seward & Wang Oegelman. The X-ray luminosities of PSRs B1257+12 and J0437-4715, millisecond pulsars with similar periods and spin-down rates, are found to differ by more than a factor of 25. X-ray emission from radio pulsars has been ascribed to a thermal component arising from a surface hot spot and a power-law magnetospheric component (Halpern & Ruderman). In the context of this model and these observations, we argue that the orientation of the magnetic and rotation axes with respect to the line of sight is very different for PSR J0437-4715 compared to PSR B1257+12. Finally, we suggest that the beaming factor for X-ray emission is independent of the pulsar period, unlike that for radio emission; if so, most millisecond pulsars are visible in the radio but no at X-ray energies.

Danner, R.↗
NASA NTRS1994

The helium-to-hydrogen ratio of the UV-bright star Barnard 29 of Messier 13

Barnard 29 is the most luminous UV-bright star of the globular cluster Messier 13. Comparison of its reddening corrected optical region spectrophotometry and IUE low dispersion fluxes and its H-gamma profile with the predictions of metal-poor model atmospheres having the metallicity of M 13 indicated T(sub eff) = 20250 K, log g = 3.15. Observations of He I lambda(4026) taken with echelle spectrograph of the Multi-Mirror telescope using a Reticon detector show that He/H = 0.055 +/- 0.020. Some astrophysical implications of this result are discussed.

Adelman, S. J.↗
NASA NTRS1994

Comparative pulsation calculations with OP and OPAL opacities

Comparative linear nonadiabatic pulsation calculations are presented using the OPAL and Opacity Project opacities. The two sets of opacities include effects due to intermediate coupling and fine structure as well as new abundances. We used two mass luminosity (M-L) relations, one standard (BIT), and one employing substantial convective core overshoot (COV). The two sets of opacities cannot be differentiated on the basis of the stellar pulsation calculations presented here. The BIT relation can model the beat and bump Cepheids with masses between 4 and 7 solar mass, while if the overshoot relation is used, masses between 2 and 6 solar mass are required. In the RR Lyrae regime, we find the inferred masses of globular cluster RRd stars to be little influenced by the choice of OPAL or OP. Finally, the limited modeling we have done is not able to constrain the Cepheid M-L relation based upon period ratios observed in the beat and bump stars.

Kanbur, Shashi M.↗
NASA NTRS1994

A possible optical counterpart for the triple pulsar system PSR B1620-26

We have searched deep optical images of the globular cluster M4 for a possible optical counterpart for the third body in the PSR B1620-26 system. We identify as a possible optical counterpart a star located within 0.3 sec of the nominal pulsar position with V approximately equal to 20. This magnitude is consistent with a main-sequence cluster member with M approximately equal to 0.45 solar mass. However, this star may be blended with fainter stars, and the probability of a chance superposition is nonnegligible.

Bailyn, Charles D.↗
NASA NTRS1995

Very low mass stars and white dwarfs in NGC 6397

Deep Wide Field/Planetary Camera 2 (WFPC2) images in wide bands centered at 606 and 802 nm were taken with the Hubble Space Telescope (HST) 4.6 min from the center of the galactic globular cluster NGC 6397. The images were used to accurately position approximately 2120 stars detected in the field on a color magnitude diagram down to a limiting magnitude m(sub 814) approximately = m(sub I) approximately = 26 determined reliably and solely by counting statistics. A white dwarf sequence and a rich, narrow cluster main sequence are detected for the first time, the latter stretching from m(sub 814) = 18.5 to m(sub 814) = 24.0 where it becomes indistinguishable from the field population. Two changes of slope of the main sequence at m(sub 814) approximately = 20 and m(sub 814) approximately = 22.5 are evident. The corresponding luminosity function increases slowly from M(sub 814) approximately = 6.5 to 8.5 are expected from ground-based observations but then drops sharply from there dwon to the measurement limit. The corresponding mass function obtained bu using the only presently available mass-luminosity function for the cluster's metallicity rises to a plateau between approximately 0.25 and approximately 0.15 solar mass, but drops toward the expected mass limit of the normal hydrogen burning main sequence at approximately 0.1 solar mass. This result is in clear contrast to that obtained from the ground and implies either a substantial modification of the cluster's initial mass function due to dynamical evolution in its lifetime, or that very low mass stars are not produced in any dynamically significant amount by clusters of this type. The white dwarf sequence is in reasonable agreement with a cooling sequence of models of mass 0.5 solar mass at the canonical distance of NGC 6397 with a scatter that is most likely due to photometric errors, but may also reflect real differences in mass or chemical composition. Contamination from unresolved galaxies, which cannot be reliably identified with our filters, makes it difficult to meaningfully compare the observed white dwarf luminosity function with its theoretical counterpart.

Paresce, Francesco↗
NASA NTRS1993

DOPHOT, a CCD photometry program: Description and tests

The design considerations and operational features of DOPHOT, a point-spread function (PSF) fitting photometry program, are described. Some relevant details of the PSF fitting are discussed. The quality of the photometry returned by DOPHOT is assessed via reductions of an 'artificial' globular cluster generated from a list of stars with known magnitudes and colors. Results from comparative tests between DOPHOT and DAOPHOT using this synthetic cluster and real data are also described.

Schechter, Paul L.↗