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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 235 records · Page 13
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NASA NTRS1995

On the nature of the companion to HD 114762

The results of a recent study of coplanarity tendencies between stellar-equatorial and binary-orbital planes are applied to the low-metal F9 V star HD 114762, for which a possible substellar companion was reported by Latham et al. (1989). High-resolution spectroscopy is performed on HD 114762 in order to extract its projected rotational velocity (v sin i). This is then combined with an expected rotational velocity determined via age-scaling, providing an estimate of the star's equatorial inclination, which then combined with the results of the coplanarity study provides an approximate indication of the companion's orbital inclination. Although the formal uncertainties in this process preclude an accurate estimate of the companion's inclination, the results suggest that the inclination is low, possibly low enough to force the companion's mass above the limit for hydrogen fusion. It is thus possible the companion may be nothing more exotic than a low-mass M star, as opposed to a brown dwarf. These results add support to a similar conclusion recently obtained by Cochran et al. (1991).

Hale, Alan↗
NASA NTRS1981

Ultraviolet spectroscopy of old novae and symbiotic stars

The IUE spectra are presented for two old novae and for two of the symbiotic variables. Prominent emission line spectra are revealed as a continuum whose appearance is effected by the system inclination. These data provide evidence for hot companions in the symbiotic stars, making plausible the binary model for these peculiar stars. Recent IUE spectra of dwarf novae provide additional support for the existence of optically thick accretion disks in active binary systems. The ultraviolet data of the eclipsing dwarf novae EX Hya and BV Cen appear flatter than for the noneclipsing systems, an effect which could be ascribed to the system inclination.

Lambert, D. L.↗
NASA NTRS1997

The Origin of Warped, Precessing Accretion Disks in X-ray Binaries

The radiation-driven warping instability discovered by Pringle holds considerable promise as the mechanism responsible for producing warped, precessing accretion disks in X-ray binaries. This instability is an inherently global mode of the disk, thereby avoiding the difficulties with earlier models for the precession. Here we follow up on earlier work to study the linear behavior of the instability in the specific context of a binary system. We treat the influence of the companion as an orbit-averaged quadrupole torque on the disk. The presence of this external torque allows the existence of solutions in which the direction of precession of the warp is retrograde with respect to disk rotation, in addition to the prograde solutions that exist in the absence of external torques.

Maloney, Philip R.↗
NASA NTRS1992

A magnitude-limited survey of Cepheid companions in the ultraviolet

Results of a magnitude-limited survey of classic Cepheids brighter than 8th mag carried out to search for hot main-sequence companions are presented. Spectra of 76 stars obtained with the IUE satellite in the 2000-3200-A region were compared with the spectra of nonvariable supergiants and also the single Cepheid Delta Cep to search for excess flux at 2500 A from possible companions. Photometric companions were found for 21 percent of the sample. When the Cepheids known to be binary from either orbital motion or spectra in the 1200-2000-A region are included, the percentage of companions rises to 29 percent. If a statistical correction from stars with orbital motion is included, 34 percent have companions. This percentage is compared with that found by Abt et al. (1990) for B2-B5 main-sequence stars. If only systems with periods longer than a year and separations not more than 30 arcsec are considered, only 18 percent of the B stars will become Cepheids with companions.

Evans, Nancy R.↗
NASA NTRS1999

Broad-Band Measurements of Cen X-3 With XTE and CGRO

Centaurus X-3 has played a key role in the development of our understanding of galactic x-ray binary sources. Timing analysis of the UHURU x-ray observations for the luminous Cen X-3 source (L approximately 10(exp 38) erg/s) revealed the first evidence for coherent x-ray pulsations from an object in a binary system (Giaconni 1971; Schreier 1972). It was quickly understood that the luminous pulsed x-ray emission could be generated by the accretion of matter from a companion star onto a rotating neutron star and led to the adoption of binary star models as the fundamental model for galactic x-ray sources (e.g. Pringle and Rees 1972; Lamb 1973). Based on modeling and refined observations since the original measurements, we now believe that Cen X-3 is a high mass x-ray binary system that contains a disk-fed pulsar with a period of 4.84 seconds that is in a 2.087 day orbit around an O-star companion. Since the pulsar discovery, its period has been intermittently monitored and those studies show a long term spin-up of the pulsar punctuated by short intervals of spin-down (e.g. Finger 1994). The implied torques are thought to originate from the interaction of an accretion disk with the magnetic field of a neutron star (Ghosh and Lamb 1979).

Vestrand, W. Thomas↗
NASA NTRS2015

X-Raying the Coronae of HD 155555

We present an analysis of the high-resolution Chandra observation of the multiple system, HD 155555 (an RS CVn type binary system, HD 155555 AB, and its spatially resolved low-mass companion HD 155555 C). This is an intriguing system which shows properties of both an active pre-main sequence star and a synchronised (main sequence) binary. We obtain the emission measure distribution, temperature structures, plasma densities, and abundances of this system and compare them with the coronal properties of other young/active stars. HD 155555 AB and HD 155555 C produce copious X-ray emission with log L(sub x) of 30.54 and 29.30, respectively, in the 0.3-6.0 kiloelectronvolt energy band. The light curves of individual stars show variability on timescales of few minutes to hours. We analyse the dispersed spectra and reconstruct the emission measure distribution using spectral line analysis. The resulting elemental abundances exhibit inverse first ionisation potential effect in both cases. An analysis of He-like triplets yields a range of coronal electron densities 1010 − 1013 per cubic centimeter. Since HD 155555 AB is classified both as an RS CVn and a PMS star, we compare our results with those of other slightly older active main-sequence stars and T Tauri stars, which indicates that the coronal properties of HD 155555 AB closely resemble that of an older RS CVn binary rather than a younger PMS star. Our results also suggests that the properties of HD 155555 C is very similar to those of other active M dwarfs.

stars: coronae↗
NASA NTRS1988

Nature and evolution of the eclipsing millisecond binary pulsar PSR1957 + 20

A model in which a millisecond pulsar may be able to evaporate a very light companion by a particular component of its energetic radiation is applied to the recently discovered 1.6-ms pulsar PSR1957 + 20. Pulsar turn-on in the very low-mass X-ray binary follows a stage of mass transfer dominated by an evaporative wind from the surface of the companion. The wind is driven by a large MeV gamma-ray flux powered by an accretion dynamo. That source of radiation ceases when it is replaced by that from the millisecond pulsar, which has been spun up by accretion.

Kluzniak, W.↗
NASA NTRS1986

The mass of the classical cepheid SU Cygni

Radial velocity differences between the classical Cepheid SU Cyg and a hot companion were measured from IUE high dispersion spectra. The blue star is itself a close binary. The triple system solution provides the mass ratio of the Cepheid to the masses of the pair of companions. The masses of the companions are inferred from the low dispersion IUE spectra. The mass derived for the Cepheid is 6.3 solar masses, with a range from 7.0 to 5.9 solar masses, which is very close to the evolutionary mass.

Evans, Nancy Remage↗
NASA NTRS1987

The mass of the classical Cepheid SU Cygni

Radial velocity differences between the classical Cepheid SU Cyg and a hot companion were measured from IUE high dispersion spectra. The blue star is itself a close binary. The triple system solution provides the mass ratio of the Cepheid to the masses of the pair of companions. The masses of the companions are inferred from the low dispersion IUE spectra. The mass derived for the Cepheid is 6.3 solar masses, with a range from 7.0 to 5.9 solar masses, which is very close to the evolutionary mass.

Evans, Nancy Remage↗
NASA NTRS1985

AY Ceti - A flaring, spotted star with a hot companion

AY Ceti is a late-type single-line spectroscopic binary, a bright X-ray source (L/x/ equal to about 1.5 x 10 to the 31st ergs/s), and a spotted star, as evidenced by its prominent photometric wave. In this paper, observations made with the IUE satellite and the VLA radio interferometer are reported. The 1200-2000 A UV spectrum of AY Cet shows a hot stellar continuum and a very broad Ly-alpha absorption line from a previously unobserved white dwarf secondary. The UV spectrum can be matched to the energy distribution of a (T/eff/ = 18,000 K, log g = 8) model atmosphere. Superposed on this hot continuum are high-excitation emission lines typical of chromospheres and transition regions of active late-type stars, e.g., the spotted RS CVn binaries. It is concluded that the bright lines and soft X-ray emission of AY Cet arise from the cool primary star, rather than from mass transfer and accretion onto the secondary as has recently been proposed for the similar system 56 Peg. Two strong radio flares on AY Cet were observed. The second was rapidly variable and left-hand circularly polarized at levels up to pi(c) = 86 + or - 5 percent at 20 cm wavelength. The most likely emission mechanism is an electron-cyclotron maser.

Simon, T.↗
NASA NTRS1988

On black widow evolutionary scenarios for binary neutron stars

The scenario whereby the pulsar 1957 + 20 ablates its companion by soft gamma-ray synchrotron emission (Ruderman et al., 1988) is critically examined, with particular regard to how the outflowing material, beginning at photospheric temperatures, is heated through the cooling barrier to coronal temperatures. Assuming the conductivity to be at most the Spitzer value, this consideration is found to constrain the mass flux more than two orders of magnitude more severely than merely considering cooling near the sonic point. This would imply that the ablation scenario fails by a large margin, even if the emission from the pulsar is beamed along the orbital plane.

Eichler, David↗
NASA NTRS1992

Low-mass X-ray binary evolution and the origin of millisecond pulsars

The evolution of low-mass X-ray binaries (LMXBs) is considered. It is shown that X-ray irradiation of the companion stars causes these systems to undergo episodes of rapid mass transfer followed by detached phases. The systems are visible as bright X-ray binaries only for a short part of each cycle, so that their space density must be considerably larger than previously estimated. This removes the difficulty in regarding LMXBs as the progenitors of low-mass binary pulsars. The low-accretion-rate phase of the cycle with the soft X-ray transients is identified. It is shown that 3 hr is likely to be the minimum orbital period for LMXBs with main-sequence companions and it is suggested that the evolutionary endpoint for many LMXBs may be systems which are the sites of gamma-ray bursts.

Frank, Juhan↗
NASA NTRS2022

Ultraviolet Spectropolarimetry: on the Origin of Rapidly Rotating B Stars

UV spectroscopy and spectropolarimetry hold the key to understanding certain aspects of massive stars that are largely inaccessible (or exceptionally difficult) with optical or longer wavelength observations. As we demonstrate, this is especially true for the rapidly-rotating Be and Bn stars, owing to their high temperatures, geometric asymmetries, binary properties, evolutionary history, as well as mass ejection and disks (in the case of Be stars). UV spectropolarimetric observations are extremely sensitive to the photospheric consequences of rapid rotation (i.e. oblateness, temperature, and surface gravity gradients), far beyond the reach of optical wavelengths. Our polarized radiative-transfer modelling predicts that with low-resolution UV spectropolarimetry covering 120–300 nm, and with a reasonable SNR, the inclination angle of a rapid rotator can be determined to within 5 degrees, and the rotation rate to within 1%. The origin of rapid rotation in Be/n stars can be explained by either single-star or binary evolution, but their relative importance is largely unknown. Some Be stars have hot sub-luminous (sdO) companions, which at an earlier phase transferred their envelope (and with it mass and angular momentum) to the present-day rapid rotator. Although sdO stars are small and relatively faint, their flux peaks in the UV making this the optimal observational wavelength regime. Through spectral modelling of a wide range of simulated Be/n+sdO configurations, we demonstrate that high-resolution high-signal-to-noise ratio UV spectroscopy can detect an sdO star even when ∼1,000 times fainter in the UV than its Be/n star companion. This degree of sensitivity is needed to more fully explore the parameter space of Be/n+sdO binaries, which so far has been limited to about a dozen systems with relatively luminous sdO stars. We suggest that a UV spectropolarimetric survey of Be/n stars is the next step forward in understanding this population. Such a dataset would, when combined with population synthesis models, allow for the determination of the relative importance of the possible evolutionary pathways traversed by these stars, which is also crucial for understanding their future evolution and fate.

Ultraviolet astronomy↗
NASA NTRS1990

Acceleration by pulsar winds in binary systems

In the absence of accretion torques, a pulsar in a binary system will spin down due to electromagnetic dipole radiation and the spin-down power will drive a wind of relativistic electron-positron pairs. Winds from pulsars with short periods will prevent any subsequent accretion but may be confined by the companion star atmosphere, wind, or magnetosphere to form a standing shock. The authors investigate the possibility of particle acceleration at such a pulsar wind shock and the production of very high energy (VHE) and ultra high energy (UHE) gamma rays from interactions of accelerated protons in the companion star's wind or atmosphere. They find that in close binaries containing active pulsars, protons will be shock accelerated to a maximum energy dependent on the pulsar spin-down luminosity. If a significant fraction of the spin-down power goes into particle acceleration, these systems should be sources of VHE and possibly UHE gamma rays. The authors discuss the application of the pulsar wind model to binary sources such as Cygnus X-3, as well as the possibility of observing VHE gamma-rays from known binary radio pulsar systems.

Harding, Alice K.↗
NASA NTRS1990

The mass of the classical Cepheid SU Cygni

Velocities of the companion of the classical Cepheid SU Cyg have been measured from 13 IUE high-dispersion spectra. Because the companion is itself a member of a short-period binary system, its short-period orbit and the orbital amplitude of the center of mass in the long-period system are determined from a triple system solution. The velocity amplitude of the short-period center of mass in the long-period orbit is 32.2 + or - 1.6 km/s. This leads to a mass ratio between the mass of the Cepheid and the sum of the masses of the companions of 1.07 + or - 0.05. Combining this mass ratio with the mass function from the Cepheid orbit produces a purely dynamical lower limit to the mass of the Cepheid of 5.9 + or - 0.4 solar masses, in good agreement with the evolutionary mass for the Cepheid without mass loss or convective overshoot near the main sequence.

Evans, Nancy Remage↗
NASA NTRS1993

High-energy emission from the eclipsing millisecond pulsar PSR 1957+20

The properties of the high-energy emission expected from the eclipsing millisecond pulsar system PSR 1957+20 are investigated. Emission is considered by both the relativistic shock produced by the pulsar wind in the nebula surrounding the binary and by the shock constraining the mass outflow from the companion star of PSR 1957+20. On the basis of the results of microscopic plasma physical models of relativistic shocks it is suggested that the high-energy radiation is produced in the range from X-rays to MeV gamma rays in the binary and in the range from 0.01 eV to about 40 keV in the nebula. Doppler boost of the emission in the radiating wind suggests the flux should vary on the orbital time scale, with the largest flux observed roughly coincident with the pulsar's radio eclipse.

Arons, Jonathan↗
NASA NTRS1994

Nonparametric statistical modeling of binary star separations

We develop a comprehensive statistical model for the distribution of observed separations in binary star systems, in terms of distributions of orbital elements, projection effects, and distances to systems. We use this model to derive several diagnostics for estimating the completeness of imaging searches for stellar companions, and the underlying stellar multiplicities. In application to recent imaging searches for low-luminosity companions to nearby M dwarf stars, and for companions to young stars in nearby star-forming regions, our analyses reveal substantial uncertainty in estimates of stellar multiplicity. For binary stars with late-type dwarf companions, semimajor axes appear to be distributed approximately as a(exp -1) for values ranging from about one to several thousand astronomical units. About one-quarter of the companions to field F and G dwarf stars have semimajor axes less than 1 AU, and about 15% lie beyond 1000 AU. The geometric efficiency (fraction of companions imaged onto the detector) of imaging searches is nearly independent of distances to program stars and orbital eccentricities, and varies only slowly with detector spatial limitations.

Heacox, William D.↗