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At least 667 records · Page 37
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

A search for spectroscopic binaries among the runaway O type stars

Numerous radial velocity measurements of medium dispersion were made for the 10 brighter stars given in Stone's list of very probable O type runaways. All plates were measured with the KPNO PDS microdensitometer, and a new iterative reductional analysis was used to derive plate velocities, which are estimated to be 1.6 times more accurate internally than those found by using the traditional method. Of thse stars, psi Per, alpha Cam, HD 188209, and 26 Cep are identified as probable velocity variables, while 9 Sge, lambda Cep, and HD 218915 are classed as possible variables. If the source of this variability is Keplerian rather than atmospheric, which cannot be established unequivocally from the observations of this paper, psi Per could be a spectroscopic binary with a black hole companion, and at least 1.2 solar mass. The detection of runaway binary systems from radial velocity measurements is discussed.

Stone, R. C.↗
NASA NTRS2017

Stochastic Template Bank for Gravitational Wave Searches for Precessing Neutron Star-Black Hole Coalescence Events

Gravitational wave searches to date have largely focused on non-precessing systems. Including precession effects greatly increases the number of templates to be searched over. This leads to a corresponding increase in the computational cost and can increase the false alarm rate of a realistic search. On the other hand, there might be astrophysical systems that are entirely missed by non-precessing searches. In this paper we consider the problem of constructing a template bank using stochastic methods for neutron star-black hole binaries allowing for precession, but with the restrictions that the total angular momentum of the binary is pointing toward the detector and that the neutron star spin is negligible relative to that of the black hole. We quantify the number of templates required for the search, and we explicitly construct the template bank. We show that despite the large number of templates, stochastic methods can be adapted to solve the problem. We quantify the parameter space region over which the non-precessing search might miss signals.

gravitational wave↗
NASA NTRS1989

Accretion turnoff and rapid evaporation of very light secondaries in low-mass X-ray binaries

The illumination of companion stars in very low mass X-ray binaries by various kinds of radiation from the neighborhood of the neutron star after accretion has terminated or during accretion is considered. If a neutron star's spun-up period approaches 0.001 s, pulsar kHz radiation can quench accretion by pushing surrounding plasma away from the neutron star, and may leave the companion to be evaporated by the high-energy radiation component expected from an 'isolated' millisecond radiopulsar. Expected accretion-powered MeV gamma-rays and e(+ or -) winds may also be effective in evaporating dwarf companions. Neutron star spin-down energy release may sustain the power in these radiation mechanisms even while accretion falls. Accretion-powered soft X-rays may speed the mass loss of highly evolved dwarf companions, particularly those with a large fraction of carbon and oxygen.

Ruderman, M.↗
NASA NTRS1983

Period-activity relations in close binary systems

The question as to whether the amount of activity observed in stars with the same fundamental stellar parameters depends on rotational velocity is considered in view of close binary systems, making use of their orbital rather than their rotational period. It is noted that the main sequence G star binaries lie systematically lower than the K subgiants for the same rotational periods, so that relations including a variety of spectral classes cannot be used to directly deduce correlations between magnetic fields and activity without suitable correction. The behavior of the present sample suggests that activity increases with later spectral type and with evolution off the main sequence. The chromosphere shows a minor reaction to rotational velocity increase.

Basri, G.↗
NASA NTRS1980

IUE observations of long period eclipsing binaries - A study of accretion onto non-degenerate stars

IUE observations made in 1978-1979 recorded a whole class of interacting long-period binaries similar to beta Lyrae, which includes RX Cas, SX Cas, V 367 Cyg, W Cru, beta Lyr, and W Ser, called the W Serpentis stars. These mass-transferring binaries with relatively high mass transfer rate show two prominent features in the far ultraviolet: a continuum with a color temperature higher than the one observed in the optical region (about 12,000 K), and a strong emission line spectrum with the N V doublet at 1240 A, C IV doublet at 1550 A and lines of Si II, Si III, Si IV, C II, Fe III, AI III, etc. These phenomena are discussed on the assumption that they are due to accretion onto non-degenerate stars.

Plavec, M. J.↗
NASA NTRS2011

Modeling Kepler Transit Light Curves as False Positives: Rejection of Blend Scenarios for Kepler-9, and Validation of Kepler-9 d, a Super-Earth-Size Planet in a Multiple System

Light curves from the Kepler Mission contain valuable information on the nature of the phenomena producing the transit-like signals. To assist in exploring the possibility that they are due to an astrophysical false positive we describe a procedure (BLENDER) to model the photometry in terms of a blend rather than a planet orbiting a star. A blend may consist of a background or foreground eclipsing binary (or star-planet pair) whose eclipses are attenuated by the light of the candidate and possibly other stars within the photometric aperture. We apply BLENDER to the case of Kepler-9 (KIC 3323887), a target harboring two previously confirmed Saturn-size planets (Kepler-9 b and Kepler-9 c) showing transit timing variations, and an additional shallower signal with a 1.59 day period suggesting the presence of a super-Earth-size planet. Using BLENDER together with constraints from other follow-up observations we are able to rule out all blends for the two deeper signals and provide independent validation of their planetary nature. For the shallower signal, we rule out a large fraction of the false positives that might mimic the transits. The false alarm rate for remaining blends depends in part (and inversely) on the unknown frequency of small-size planets. Based on several realistic estimates of this frequency, we conclude with very high confidence that this small signal is due to a super-Earth-size planet (Kepler-9 d) in a multiple system, rather than a false positive. The radius is determined to be 1.64(exp)(sub-14),R, and current spectroscopic observations are as yet insufficient to establish its mass.

PLANETARY SYSTEMS↗
NASA NTRS1994

1ES 1113+432: Luminous, soft X-ray outburst from a nearby cataclysmic variable (AR Ursae Majoris)

A remarkable X-ray transient from the Einstein Slew Survey, 1 ES 1113+432, is identified with a nearby, short-period cataclysmic variable. Wenzel (1993) has confirmed that the optical counterpart is the variable star, AR UMa (cataloged as 'semiregular'), erroneously reported 5.7 min southeast of the true position. One of the Einstein slew observations recorded a flux of 43 IPC counts/s, which is an order of magnitude above the flux observed from the brightest cataclysmic variables in other X-ray surveys. The outburst spectrum is extremely 'soft,' with an implied blackbody temperature of approximately 22 eV. The optical counterpart (V = 16.5) exhibits a strong UV component, TiO bands from an M star, and broadened Balmer emission lines. Optical states as bright as V approx. 13 were found on photographs from the Harvard Plate Library, confirming outburst behavior in the optical counterpart. The historical photographic record suggests that 1ES 1113+432 remains in a low-accretion state most of the time. Both of the soft X-ray spectrum and the transitions between high and low-accretion states are suggestive of the AM Her (magnetic) subclass. Photometric observations in the I band show 0.18 mag modulations at a period of 0.966 hr. These are interpreted as ellipsiodal variations in the secondary star for a binary period of 1.932 hr, which is near the lower boundary of the 'period gap' in the histogram, of orbital periods of accreting white dwarfs. Thus 1ES 1113+432 provides the rare opportunity to study a secondary star in a cataclysmic binary that has evolved through the period gap. The optical spectral features from the secondary imply a spectral type of approximately M6 and a distance of approximately 88 pc. The peak luminosity in the soft X-ray component (unabsorbed) is then estimated to be 3 X 10(exp 33) ergs/s, assuming emission from a blackbody slab with a temperature of 22 eV. While this luminosity is higher than previous measures of the soft X-ray component, it does not exceed the amount of radiation that could be emitted from the accretion-heated surface of a white dwarf.

Remillard, R. A.↗
NASA NTRS1993

The evolution of the lithium abundances of solar-type stars. IV - Praesepe

Echelle observations are presented of lithium in 63 F and G dwarfs of the Praesepe cluster. For stars earlier than about G0V, Praesepe follows the same trends seen in the Hyades, which has approximately the same age and composition. Stars in Praesepe later than about G5V have more Li than their Hyades counterparts, possibly because Praesepe is slightly younger than the Hyades or has slightly lower metallicity. Significant differences in the abundance of Li are seen among stars of the same color, and, as in the Hyades, there is a tendency for the deviant stars to be binaries to the extent that duplicity in Praesepe is known. There are also stars with much less Li than most cluster members yet which appear to be true members of Praesepe. The close binary KW 181 has a normal Li abundance, despite the fact that similar close binaries in the Hyades are Li rich.

Soderblom, David R.↗
NASA NTRS1993

Models for various aspects of dwarf novae and nova-like stars

The first attempts to explain the nature of dwarf novae were based on the assumption of single-star phenomena, in which emission lines were assumed to be caused by circumstellar gas shells. The outburst behavior was tentatively ascribed to the kind of (also not understood) mechanism leading to nova outbursts. The realization that some, and possibly all, dwarf novae and nova-like stars (and novae) are binaries eventually led to models which bore more and more similarities to the modern interpretation on the basis of the Roche model. Not all cataclysmic variables are known binaries. In fact, with respect to the entire number of known objects, the proven binaries are still the minority, but all the brightest variables are in fact known to binaries. Not a single system is known which exhibits the usual characteristics of a cataclysmic variable and at the same time can be declared with certainty to be a single star. Two systems are known, the dwarf nova EY Cyg and the recurrent nova V1017 Sgr, in which, in spite of intensive search, no radial velocity variations have been found; but they still exhibit composite spectra consisting of a bright continuum, an emission spectrum, and a cool absorption spectrum. If the Roche model is correct, it is to be expected that a small percentage of objects is viewed pole-on, so orbital motions do not make themselves felt as Doppler shifts of spectral lines. So even these two systems support the hypothesis that all cataclysmic variables (with the possible exception of symbiotic stars) are binaries. In cataclysmic variables, it seems that the brightness changes observed in dwarf novae and nova-like stars in the optical and the UV are due directly to changes in the accretion disks. The study and understanding of accretion disks in these systems can bear potentially valuable consequences for many other fields in astronomy. The observed spectra of dwarf novae and nova-like stars comprise a fairly large range: pure emission spectra, pure absorption spectra, a mixture of both, asymmetric line profiles, very different slopes of the continuous flux distribution -- and one single system may exhibit all of these features at different times. Agreement and disagreement between computed and observed spectra should show whether or not the Roche model is applicable and where it probably will have to be modified and improved. Except for their outburst behavior and its immediate consequences, novae, dwarf novae, and nova-like stars cannot be physically distinguished from each other.

Ladous, Constanze↗
NASA NTRS1991

Gravitational microlensing by double stars and planetary systems

Almost all stars are in binary systems. When the separation between the two components is comparable to the Einstein ring radius corresponding to the combined mass of the binary acting as a gravitational lens, then an extra pair of images can be created, and the light curve of a lensed source becomes complicated. It is estimated that about 10 percent of all lensing episodes of the Galactic bulge stars will strongly display the binary nature of the lens. The effect is strong even if the companion is a planet. A massive search for microlensing of the Galactic bulge stars may lead to a discovery of the first extrasolar planetary systems.

Mao, Shunde↗
NASA NTRS1972

Stellar evolution.

Aspects of normal stellar evolution are discussed together with evolution near the main sequence, stellar evolution from main sequence to white dwarf or carbon ignition, the structure of massive main-sequence stars, and problems of stellar stability and stellar pulsation. Other subjects considered include variable stars, white dwarfs, close binaries, novae, early supernova luminosity, neutron stars, the photometry of field horizontal-branch stars, and stellar opacity. Transport mechanisms in stars are examined together with thermonuclear reactions and nucleosynthesis, the instability problem in nuclear burning shells, stellar coalescence, and intense magnetic fields in astrophysics. Individual items are announced in this issue.

Chiu, H.-Y.↗
NASA NTRS1982

Ultraviolet and optical spectrum studies of Lambda Andromedae - Evidence for atmospheric inhomogeneities

To pursue the study of solar phenomena in cool stars, chromospheric activity in Lambda Andromedae (HD 222107 has been investigated). This binary, whose primary star is G7-G8 IV-III, shows strong chromospheric emissions and is related to the RS CVn-type systems. The first quantitative measurements of chromospheric and solar-type transition-region emissions as a function of the variable starspot and active-region phenomena in an RS CVn star are presented. The presence of optically darker starspots in Lambda And coincides with the brightening of both Ca II K emission and the ultraviolet transition-region lines. The ultraviolet and optical spectra show attributes of starspots, active regions, and mass flow. Analogies to solar activity are successful in explaining these observations.

Baliunas, S. L.↗
NASA NTRS1987

A search for technetium (Tc II) in barium stars

The authors searched without success for the lines of Tc II at 2647.02, 2610.00 and 2543.24 A in IUE spectra of the barium stars HR 5058, Omicron Vir, and Zeta Cap. The lack of Tc II implies that the observed s-process enhancements were produced more than half a million years ago and supports the suggestion that the spectral peculiarities of barium stars are probably related to the binary nature of the stars.

Little-Marenin, Irene R.↗
NASA NTRS1993

Hydrodynamics of winds from irradiated companion stars in low-mass X-ray binaries

We study the hydrodynamics of evaporative winds driven by X-rays and/or soft gamma-rays irradiating the outer layers of companion stars in low-mass X-ray binaries (LMXBs). We consider several irradiating fluxes and spectra for LMXBs with white dwarf and main-sequence companion stars. The thermal structure of the base of the coronal region, the position of the sonic point, and the value of the mass-loss rate are calculated in the case of spherical geometry. We consider photospheric and coronal heating from both X-ray photoionization and Compton scattering of X-rays and soft gamma-rays with energy about 1 MeV possibly irradiating the companion star in LMXBs. Evaporative winds may play a relevant role for the evolution of a special class of radiation-driven LMXBs, and this study is a step toward a quantitative understanding of the mechanism driving LMXB evolution.

Tavani, Marco↗
NASA NTRS1990

Discovery of five pre-main-sequence binaries in Taurus

The progress of an infrared K band lunar occultation search for binaries among premain-sequence stars is reported. From the fall of 1986 to the fall of 1989, 13 objects have been observed, all in the Taurus star-forming region. Five new binaries have been discovered: DF Tau, DI Tau, FS Tau (Haro 6-5) A, FV Tau (Haro 6-8), and FW Tau, with projected separations ranging from 0.01 to 0.57 arcsec. Including results reported previously, a total of 31 objects, 21 in Taurus and 10 in Ophiuchus, have been surveyed and nine double systems have been found. The detected binary frequency (0.29) seems consistent with that of the solar-type main-sequence field stars.

Chen, W. P.↗