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At least 127 records · Page 7
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NASA NTRS1984

The H-R diagram of O-type subdwarfs

A spectroscopic survey by Drilling (1983) of nonemission OB+ stars in the Case-Hamburg survey and its extension to b = + or - 30 deg for l = + or - 60 deg has turned up 12 hot, relatively bright O-type subdwarfs (V less than 13). The color temperatures of the subdwarf O stars are estimated at 60,000 to over 100,000 K. The loci or these objects are shown in the H-R diagram together with those of previously analyzed subdwarf O stars. The new hot subdwarfs occupy a previously empty region in the H-R diagram, filling the luminosity range from central stars to white dwarfs. This spread in luminosity is in agreement with their spectral appearance and indicates differences in their origin. The possible origins of these stars are briefly discussed.

Drilling, J. S.↗
NASA NTRS1986

Some topics in the magnetohydrodynamics of accreting magnetic compact objects

Magnetic compact objects (neutron stars or white dwarfs) are currently thought to be present in many accreting systems that are releasing large amounts of energy. The magnetic field of the compact star may interact strongly with the accretion flow and play an essential role in the physics of these systems. Some magnetohydrodynamic (MHD) problems that are likely to be relevant in building up self-consistent models of the interaction between the accreting plasma and the star's magnetosphere are addressed in this series of lectures. The basic principles of MHD are first introduced and some important MHD mechanisms (Rayleigh-Taylor and Kelvin-Helmholtz instabilities; reconnection) are discussed, with particular reference to their role in allowing the infalling matter to penetrate the magnetosphere and mix with the field. The structure of a force-free magnetosphere and the possibility of quasistatic momentum and energy transfer between regions linked by field-aligned currents are then studied in some detail. Finally, the structure of axisymmetric accretion flows onto magnetic compact objects is considered.

Aly, J. J.↗
NASA NTRS1998

Luminosity Dependence and Search Doppler

The research supported by this grant covered two projects: (1) a study of the luminosity dependence of the properties of atoll sources; and (2) a search for Doppler shifts in the pulse arrival times of the anomalous pulsar 4U 0142+61. Following the discovery of kilohertz quasi-periodic oscillations (QPOS) in Sco X-1 studies of the X-ray properties of atoll sources have been dominated by searches for these QPOS, and the study of their dependence on other source properties, such as X-ray luminosity and spectral state. In the project supported by grant NAG5-3269 we have detected kHz QPOs for several atoll sources. The physical interpretation of these QPO is as yet unclear, but simple models (such as the Keplerian beat frequency model) can probably be excluded. The results of this research have been reported. We have studied the X-ray pulsations of the anomalous X-ray pulsar 4U 0142+61 using the Rossi XTE. A detailed search for Doppler shifts did not lead to a positive detection. The upper limits exclude almost all types of possible companion stars, except white dwarfs. However, the latter can be excluded since anomalous X-ray pulsars are very young objects. We therefore conclude that anomalous X-ray pulsars are single neutron stars.

VanParadijs, Johannes A.↗
NASA NTRS1992

X ray observations of late-type stars using the ROSAT all-sky survey

The ROSAT mission made the first x ray survey of the entire sky using an imaging detector. Although ROSAT is a joint NASA/German project and involves direct American participation during its second phase of pointed observations, the all-sky survey remains the sole property of the German investigators. NASA grant represented the first use of ROSAT data analysis funds to support direct American participation in the ROSAT all-sky survey. The project involved a collaborative agreement between the Joint Institute for Laboratory Astrophysics (JILA) and the Max-Planck-Institut fur Extraterrestrische Physik (MPE) where JILA supplied MPE with a post-doctoral research associate with experience in the field of stellar (coronal) x ray emission to work within their ROSAT group. In return, members of the cool star research group at JILA were given the opportunity to collaborate on projects involving ROSAT all-sky survey data. Both sides have benefitted (and still benefit) from this arrangement since MPE suffers from a shortage of researchers who are interested in x ray emission from 'normal' stars and white dwarfs. MPE has also drawn upon experience in optical identification of x ray sources from the Einstein Extended Medium Sensitivity Survey in planning their own identification strategies for the ROSAT all-sky survey. The JILA cool stars group has benefitted since access to all-sky survey data has expanded the scope of their already extensive research programs involving multiwavelength observations of late-type stars. ROSAT was successfully launched on 1 June 1990 and conducted the bulk of the survey from 30 July 1990 to 25 January 1991. Data gaps in the survey have subsequently been made up. At the time of this writing (February 1992), the survey data have been processed once with the Standard Analysis Software System (SASS). A second processing will soon begin with improvements made to the SASS to correct errors and bugs found while carrying out scientific projects with data from the first processing. We outline the major research activities of Dr. Fleming over the past year (detailed accounts of his activities during the first two years of this grant can be found in the first-year and second-year status reports on this grant). Regarding the three specific projects which were proposed in the original proposal, two of them (White Dwarfs and Late M Dwarfs) are near completion. The results are described in two conference proceedings which are appended.

Linsky, Jeffrey L.↗
NASA NTRS2002

Kronos: A Multiwavelength Observatory for Mapping Accretion-Driven Sources

Kronos is a multiwavelength observatory proposed as a NASA Medium Explorer. Kronos is designed to make use of the natural variability of accreting sources to create microarcsecond-resolution maps of the environments of supermassive black holes in active galaxies and stella-size black holes in binary systems and to characterize accretion processes in Galactic compact binaries. Kronos will obtain broad energy range spectroscopic data with co-aligned X-ray, ultraviolet, and optical spectrometers. The high-Earth orbit of Kronos enables well-sampled, high time-resolution observations, critical for the innovative and sophisticated methods that are used to understand the accretion flows, mass outflows, jets, and other phenomena found in accreting sources. By utilizing reverberation mapping analysis techniques, Kronos produces advanced high-resolution maps of unprecedented resolution of the extreme environment in the inner cores of active galaxies. Similarly, Doppler tomography and eclipse mapping techniques characterize and map Galactic binary systems, revealing the details of the physics of accretion processes in black hole, neutron star, and white dwarf binary systems. The Kronos instrument complement, sensitivity, and orbital environment make it suitable to aggressively address time variable phenomena in a wide range of astronomical objects from nearby flare stars to distant galaxies.

Peterson, Bradley M.↗
NASA NTRS1986

A Study of the Ultraviolet Absorptions in the Spectra of DA White Dwarfs and Ultraviolet Spectra of the Star HR6560

Two projects in conjunction with the International Ultraviolet Explorer Satellite are discussed. These projects were to: (1) study the properties of the H2 and H2+ quasi-molecular absorption features at lambda lambda 1600 and 1400 in the ultraviolet spectra of the hydrogen-rich DA white dwarfs and to search for additional spectroscopic features in the spectra of these stars; and (2) use the ultraviolet portion of the spectrum of the peculiar rare earth-rich late F type star, HR6560 (HD159870), to establish whether or not the element abundance anomalies are produced in conjunction with its having a white dwarf binary companion. The data show that HR6560 is probably not associated with any hot subluminous or degenerate star.

Wegner, Gary A.↗
NASA NTRS1990

Interacting nuclei in distant galaxies

The N-galaxy 3C 390.3 has been monitored spectroscopically since 1974 (Osterbrock, Koski and Phillips 1975; Oke 1988). From various archives and literature, it is found that the Balmer lines change their intensities and profiles in a dramatic manner. The H alpha profile is very broad and peculiar, and the relative intensities of its two humps changes consistently with time, possibly periodically. Before 1980, the blue hump was significantly stronger than the one in the red. From 1980 to 1983 the blue hump became stronger (see Oke 1988). After 1983 the H alpha profile has returned to its early shape and seems to have completed a full circle. Unlike the rapid (on the order of a month or even less) and aperiodic variation in the continuum and integrated line intensities, the change in broad profile seems slow and consistent. Taking the analogy of cataclysmic variables, the double-horn profiles have been observed in cases of interacting stars. For example, the emission lines, both in He II and hydrogen Balmer lines in GD 552 (Stover 1985) show double-horn profiles and periodical changes in their line profiles, including the change in ratios of two humps. It is understood that the D-wave components (Smak 1976) are the signature of an emitting disk and the S-wave component is from the emission at a hot spot which rotates and results in a moving component in the velocity space. The mass flow from the nearby interacting star provides the stream toward the core of a neutron star or white dwarf. Therefore, it is proposed that the variation of broad line profiles observed in 3C 390.3 may be the result of a pair of interacting massive cores. The rotational velocity dominates and produces a variable double-horn profile. However, the line widths observed in broad line radio galaxies are one order larger than that in interacting stars. The Balmer decrements imply a much smaller density (10(exp 10-12) cm(exp-3)) than that in the cataclysmic variables. The much larger velocity and much thinner density make it unlikely that the broad line emission is simply formed in an accretion disk. The authors postulate that a significant rotational motion is involved. If the observed squared profiles are indeed due to the rotational velocity field, one can naturally explain their rare occurrence.

Zheng, Wei↗
NASA NTRS2005

Unveiling the High Energy Obscured Universe: Hunting Collapsed Objects Physics

A large part of energy from space is coming from collapsing stars (SN, Hypernovae) and collapsed stars (black holes, neutron stars and white dwarfs). The peak of their energy release is in the hard-X and gamma-ray wavelengths where photons are insensitive to absorption and can travel from the edge the Universe or the central core of the Galaxy without loosing the primordial information of energy, time signature and polarization. The most efficient process to produce energetic photons is gravitational accretion of matter from a "normal" star onto a collapsed companion (L~GxMcollxdMacc/dtx( 1Rdisc)-dMacc/dt x c2), exceeding by far the nuclear reaction capability to generate high energy quanta. Thus our natural laboratory for "in situ" investigations are collapsed objects in which matter and radiation co-exist in extreme conditions of temperature and density due to gravitationally bent geometry and magnetic fields. This is a unique opportunity to study the physics of accretion flows in stellar mass and super-massive Black Holes (SMBHs), plasmoids generated in relativistic jets in galactic microQSOs and AGNs, ionised plasma interacting at the touching point of weakly magnetized NS surface, GRB/Supernovae connection, and the mysterious origins of "dark" GRB and X-ray flash.

Ubertini, P.↗
NASA NTRS1990

On the nature of the X-ray pulsar near Lynds 1457

The rotation of a magnetized white dwarf or neutron star accidentally located within or behind the Lynds 1457 molecular cloud, is considered as a model for the hard X-ray source H0253 + 193. It is calculated that all models of this type have a low a priori probability because the positions are so remarkably coincident and because the objects involved are not sufficiently common. A neutron star accreting in a close binary is rejected because its likelihood is extremely small and because the observed position, luminosity, and spectrum of the object are all quite atypical for X-ray binaries. Two other possibilities are considered: (1) an isolated white dwarf or neutron star drifting accidentally into the cloud and accreting sufficient gas to power the X-ray source and (2) a white dwarf accreting in a close binary.

Patterson, J.↗
NASA NTRS1995

XTE Science Briefing from KSCNF

The X-ray Timing Explorer (XTE), launched on Dec. 30, 1995, is a Satellite that observes the fast-moving, high-energy worlds of black holes, neutron stars, x-ray pulsars and bursts of X-rays that light up the sky and then disappear forever. This videotape presents a pre-launch science briefing to the press by a few of the scientist and managers associated with the XTE satellite. The moderator for the press briefing is Jim Sahli, from the Public Affairs Office at Goddard Space Flight Center (GSFC). He introduces Alan Bunner, of the High Energy Astrophysics at NASA Headquarters; Fred Lamb, from the University of Illinois; Richard Mashotzky, X Ray Scientist at GSFC; Rick Rothschild, Principal Investigator from the University of California at San Diego; and Dale Schultz, the XTE project manager at GSFC. Dr. Bunner explains the electromagnetic spectrum, the placement of x-rays and the importance of the XTE observations to a better understanding of the Universe. Dr. Lamb, explains the difference between white dwarfs, neutron stars and black holes, and the type of observations that the XTE will give to a further understanding of these phenomena. Dr. Mashotzky expands the viewpoint to beyond the galaxy, and explains the interests of scientists who hope to use XTE to further study Quasars and Active Galactic Nuclei. Dr. Rothschild reviews some of the features of XTE, using a diagram to show the features of interest, such as the X ray Telescopes, and the collecting Proportional Counter Array (PCA.) Mr. Schultz presents a videotape tour of the XTE, in which he shows the scientific instruments and the other features of the satellite. In this tour, the source of each of the instruments is noted. Questions from the members of the press are then fielded. Many of the questions are about the cost of the XTE and any problems that are anticipated in regards to the launch.

Source record↗
NASA NTRS1972

Origin of strong magnetic fields.

A possible mechanism by which extremely strong magnetic fields in neutron stars and white dwarfs could originate involves the existence of thermodynamic equilibrium states (LOFER states) of an electron gas. The essence of the stability theory of a LOFER state is discussed. The extended electronic system considered is in contact with a thermal bath in the presence of an external magnetic field. It is found that it is not necessary to require absolute stability in order to realize a LOFER state in nature.

Canuto, V.↗
NASA NTRS1973

Viscous effects in rapidly rotating stars with application to white-dwarf models. I, II.

A general approximate numerical technique is proposed for constructing evolutionary sequences of rapidly rotating axisymmetric barytropic equilibrium configurations, with allowance for angular momentum transfer by a nonconstant isotropic viscosity. The principal physical assumption involved is the constancy of the angular momentum per unit mass on cylinders about the axis of rotation. Rapidly rotating nonmagnetic white-dwarf models with a zero-temperature degenerate-electron equation of state are considered as a particular application. The viscosity used in the analysis is that of the degenerate electrons.

Durisen, R. H.↗
NASA NTRS1975

Viscous effects in rapidly rotating stars with application to white-dwarf models. III - Further numerical results

Improved viscous evolutionary sequences of differentially rotating, axisymmetric, nonmagnetic, zero-temperature white-dwarf models are constructed using the relativistically corrected degenerate electron viscosity. The results support the earlier conclusion that angular momentum transport due to viscosity does not lead to overall uniform rotation in many interesting cases. Qualitatively different behaviors are obtained, depending on how the total mass M and angular momentum J compare with the M and J values for which uniformly rotating models exist. Evolutions roughly determine the region in M and J for which models with a particular initial angular momentum distribution can reach carbon-ignition densities in 10 b.y. Such models may represent Type I supernova precursors.

Durisen, R. H.↗
NASA NTRS1978

IUE observations of hot stars - HZ43, BD +75 deg 325, NGC 6826, SS Cygni, Eta Carinae

During the commissioning phase of IUE observations were made of a selection of hot stars: the white dwarf HZ43, the hot subdwarf BD +75 deg 325, the nucleus of the planetary nebula NGC 6826, the dwarf nova SS Cygni, and the peculiar object Eta Carinae. The observations were made in the low dispersion mode and the data have been reduced using a preliminary calibration. The results are presented and discussed for each object.

Heap, S. R.↗
NASA NTRS1979

Helios-2 Vela-Ariel-5 gamma-ray burst source position

The gamma-ray burst of 28 January 1976, one of 18 events thus far detected in interplanetary space with Helios-2, was also observed with the Vela-5A, -6A and the Ariel-5 satellites. A small source field is obtained from the intersection of the region derived from the observed time delays between Helios-2 and Vela-5A and -6A with the source region independently found with the Ariel-5 X-ray detector. This area contains neither any steady X-ray source as scanned by HEAO-A nor any previously catalogued X-ray, radio or infrared sources, X-ray transients, quasars, seyferts, globular clusters, flare stars, pulsars, white dwarfs or high energy gamma-ray sources. The region is however, within the source field of a gamma-ray transient observed in 1974, which exhibited nuclear gamma-ray line structure.

Cline, T. L.↗
NASA NTRS1979

Helios 2-Vela-Ariel 5 gamma-ray burst source position

The gamma-ray burst of January 28, 1976, one of 18 events thus far detected in interplanetary space with Helios 2, was also observed with the Vela 5A and 6A and the Ariel 5 satellites. A small source field is obtained from the intersection of the region derived from the observed time delays between Helios 2 and Vela 5A and 6A, with the source region independently found with the Ariel 5 X-ray detector. This area contains neither any steady X-ray source as scanned by HEAO 1 nor any previously cataloged X-ray, radio, or infrared sources, X-ray transients, quasars, Seyferts, globular clusters, flare stars, pulsars, white dwarfs, or high enery gamma-ray sources. The region is, however, within the source field of a gamma-ray transient observed in 1974 by Jacobson et al. (1978) which exhibited nuclear gamma-ray line structure.

Cline, T. L.↗
NASA NTRS1984

Absorption and Emission of EUV Radiation by the Local ISM

The Berkeley extreme ultraviolet radiation (EUV) telescope flown on the Apollo Soyuz mission in July, 1975 established the existence of a measurable flux of EUV (100 lambda or = or = 1000 A) originating from sources outside the solar system. White dwarfs, flare stars and cataclysmic variables were dicovered to be relatively intense compact sources of EUV photons. Moreover, this and other subsequent experiments have strongly suggested the presence of a truly diffuse component of the FUV radiation field possibly due to thermal emission from hot interstellar gas located in the general vicinity of the Sun. Closer to the H1, 912 A edge, the effect of a few hot O and B stars has been shown to be very important in establishing the interstellar flux density. All these results imply that the local interstellar medium (ISM) is immersed in a non-negligible EUV radiation field which, because of the strong coupling between EUV photons and matter, will play a crucial role in determining its physical structure. The available information on the local ISM derived from the limited EUV observations carried out so far is assembled and analyzed. These include measurements of the spectra of bright EUV sources that reveal clear evidence of H photo absorption at lambda 400 A and of the He ionization edge at 228 A.

Paresce, F.↗