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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 325 records · Page 18
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NASA NTRS1983

KQ Mon and the nature of the UX Ursa Majoris Nova-like variables

The UX Ursa Majoris stars form a group of nova-like variables with common photometric and spectroscopic properties. These objects appear to be related to the cataclysmic variables. However, there is no information that they have undergone major outbursts. The present investigation is concerned with a new object, KQ Mon, which has been included in a program of study conducted with the IUE satellite. A description of observations of KQ Mon is presented, and the relationship of this star to other UX UMa stars is examined. Attention is given to the nature of the UX Ursa Majoris stars. It is argued that the accretion rates of the UX UMa stars are higher than, for example, the dwarf novae during quiescence and that the higher accretion rates of the UX UMa stars are responsible for their lack of major outbursts.

Sion, E. M.↗
NASA NTRS1985

Strong earthquakes, novae and cosmic ray environment

Observations about the relationship between seismic activity and astronomical phenomena are discussed. First, after investigating the seismic data (magnitude 7.0 and over) with the method of superposed epochs it is found that world seismicity evidently increased after the occurring of novae with apparent magnitude brighter than 2.2. Second, a great many earthquakes of magnitude 7.0 and over occurred in the 13th month after two of the largest ground level solar cosmic ray events (GLEs). The causes of three high level phenomena of global seismic activity in 1918-1965 can be related to these, and it is suggested that according to the information of large GLE or bright nova predictions of the times of global intense seismic activity can be made.

Yu, Z. D.↗
NASA NTRS1985

CNO abundances resulting from diffusion in accreting nova progenitors

The configurations of potential nova progenitors at the onset of a thermonuclear runaway (TNR) are obtained by evolving a series of white dwarf (WD) models through the quasi-static accretion phase preceding the TNR. Special attention is paid to the effect of diffusion on composition profiles. The initial parameters of the accretion phase are the WD mass, the WD luminosity, and the accretion rate. They lead to a wide range of combinations of accreted mass and heavy element content of the envelope Z(env). It is found that the WD mass is the most important parameter in determining the accreted mass. A lower limit exists for the mass of a WD capable of producing a nova-like outburst which decreases with decreasing WD mass. There is a strong correlation between the accretion rate and Z(env), higher Z(env) values resulting from lower accretion rates. Changes in WD luminosity have no dramatic effect on the outcome of the accretion phase.

Kovetz, A.↗
NASA NTRS1985

Multifrequency observations of dwarf novae outbursts

Studies using EXOSAT and IUE data combined with ground-based optical and IR photometry, spectroscopy and polarimetry of the transition timescales and physical properties of the high accretion rate states of the disks in cataclysmic variables are discussed. Implications for dwarf novae stem from comparison of the timing and amplitude of the optical and IR rise with that of the UV and X-ray; the development of the optical and UV line profile changes as the outburst develops; and the similarity of the outburst flux distributions in different systems and different types of outbursts (standstills and supermaxima). These features of dwarf novae outbursts present interesting comparisons with the outbursts of systems like TV Col and GK Per.

Szkody, P.↗
NASA NTRS1986

X-ray, UV, and optical observations of the dwarf nova BV Puppis

Exosat, IUE, optical spectroscopic, and broad-band photometric data on BV Pup are presented and analyzed in order to study the system characteristics and their relation to the high excitation of the emission lines. It is observed that: (1) at quiescence and outburst, the He II 4686 A/H-beta ratio for the star is larger than for other observed dwarf novae; (2) the X-ray flux is high for a system at outbursts and the X-ray flux correlates with an increased He II flux over the quiescent values; (3) the flat slope of the flux distribution at quiescence implies relatively low accretion rate; and (4) the abnormal UV line spectrum suggests a different geometry/opacity for the line emission zone compared to normal dwarf novae.

Szkody, Paula↗
NASA NTRS1987

Multiwavelength monitoring of the dwarf nova VW Hydri. IV - Voyager observations

Results from Voyager far-ultraviolet (500-1200 A) observations of the dwarf nova VW Hyi are presented as part of a coordinated, multiwavelength program. Data from one normal outburst and one superoutburst are discussed in detail. Far-ultraviolet (1050 A) light curves are produced showing a significant delay (0.5 day) in the rise to maximum at 1050 A with respect to optical wavelength, followed by a simultaneous decline. The superoutburst data show a distinct double-peaked light curve with the first rise and decline closely resembling that of a normal outburst. These data suggest that the rise to supermaximum in the far-ultraviolet is also delayed with respect to optical wavelengths. The spectral distribution of VW Hyi shows the steeply falling spectrum shortward of 1200 A, characteristic of dwarf novae in outburst and absorption features at 985 (N III, C III and Ly gamma) and 1030 (Ly beta and O VI). No flux shortward of 912 A was detected in VW Hyi.

Polidan, R. S.↗
NASA NTRS1987

Orbital eccentricity in classical novae

The effect on the orbital parameters of a classical nova of the ejection of mass during the nova explosion is considered. The most easily observable consequence is the generation of a small eccentricity in the orbit which leads to a luminosity modulation at a period just longer than the orbital period. Observation of such an effect would have implications not just for interpreting the dynamics of the explosion but also for measuring the secular effect of tidal interaction after the outburst.

Edwards, D. A.↗
NASA NTRS1987

The spectral evolution of dwarf nova outbursts

The disk instability model for dwarf nova eruptions is investigated by computing the spectral development of the accretion disk through a complete limit cycle. Observed stellar spectra are used to model the radiation emitted by optically thick annuli within the disc. The general findings agree with those of Smak (1984) and Pringle et al. (1986). It is suggested that the 'dwarf nova oscillations' might be a source of information concerning the evolution of the inner disk and that detailed observations of this phenomenon can be used to test various outburst mechanisms.

Cannizzo, John K.↗
NASA NTRS1988

White dwarf heating and the ultraviolet flux in dwarf novae

The heating of the outer layers of the white dwarf which is likely to occur during a dwarf nova outburst is investigated. It is shown that the decline in IUE flux, observed during quiescent intervals in the dwarf novae VW Hydri and WX Hydri, may be due to the outer layers cooling off once the heat source is removed. The calculations here assume uniformity of the heat source over the white dwarf surface. This is unlikely to be realized from disk accretion, and discussion is made of what further calculations are required.

Pringle, J. E.↗
NASA NTRS1988

Polarization of thermal radiation from accretion disks in dwarf novae

Theoretical calculations of linear polarization of thermal radiation from accretion disks in nova-like variables and dwarf novae in eruption are presented. The primary source of polarization is due to electron scattering in the disk's photosphere. It is shown that the degree of polarization can be used as a powerful probe of the structure and evolution of the disk atmosphere. The degree of polarization is considerably higher in the ultraviolet than at optical wavelengths, and the degree of polarization may be higher in an eclipsing system when the central region of the disk is eclipsed. The orbital phase dependence distinguishes intrinsic polarization from the disk to that due to interstellar medium.

Cheng, F. H.↗
NASA NTRS1988

The dwarf nova dippers

Previous papers have shown that the dwarf nova OY Car has an extensive vertical disk structure during its superoutbursts. This structure is analogous to that seen in the low mass X-ray binaries (LMXBs), in that the whole disk flares toward its edge, and there is a bulge or 'dark spot' where the mass transfer stream meets the disk edge. The paper presents evidence suggesting that two more dwarf novae may have such a stream impact bulge, which manifests itself as a dip in the UV light curve, analogous to those seen in the X-ray light curves of the LMXB dippers.

Naylor, T.↗
NASA NTRS1988

V1500 Cygni - Discovery of a magnetic nova

Polarimetric observations of V1500 Cygni, the remnant of Nova Cygni 1975, reveal periodic variations of optical circular polarization with semi-amplitudes of about + or - 1.5 pct. Measured over a six-month baseline, the period of variation, 0.137154 + or - 0.000004 days, is 1.8 percent shorter than the stable photometric period, 0.1396129 days, obtained by Patterson (1979), and refined by Kaluzny and Semeniuk (1987). The circular polarization is interpreted as high-harmonic optical cyclotron emission from an accreting, magnetic white dwarf primary. However, the dominant system light and photometric variability are due to the heated companion star. In this picture, the prenova system was a synchronized AM Herculis-type magnetic variable. During the outburst and subsequent dense-wind phase, coupling between the white dwarf and the expanded envelope and the interaction of the orbiting secondary star were responsible for the asynchronism of the current system and the remarkable photometric variations observed in 1975-1976. Other unusual characteristics of Nova Cygni 1975 are also explained by this model.

Stockman, H. S.↗
NASA NTRS1988

The 1981 outburst of the old nova GK Persei

Old nova GK Per was observed in 1981 with the IUE, during its rise, maximum, and subsequent return to minimum. In outburst, GK Per is luminous but much redder than dwarf novae or standard model accretion disks. The observed spectrum can be explained qualitatively with the Ghosh and Lamb (1979) model for the interaction of an accretion disk with the magnetic field of the accreting white dwarf. The N V and He2 are enhanced relative to other emission lines during outburst. This can be understood with photoionization by very soft X-rays having a luminosity comparable to that of the hard X-rays.

Wu, C.-C.↗
NASA NTRS1988

Ultraviolet observations of LMC nova 1988

The IUE obtained ultraviolet spectra of a nova in an external galaxy. The spectral features do not seem unusual for a nova at maximum but it is hoped to be able to follow it for a long enough time to be able to study the high ionization lines that appear when the density drops to lower values (the nebular stage). A high dispersion spectrum was also obtained to assist in the line identification and to study the line of sight to the LMC 1 deg of arc away from SN 1987A.

Starrfield, S.↗
NASA NTRS1988

IUE observations of the dwarf nova BV Centauri

The IUE observations of the dwarf nova BV Cen in quiescence are summarized. The reprocessed spectra from January 1979 are analyzed to show that the maximum temperature of the white dwarf primary star, based on its brightness at 1375 A, ranges from 42,000 to about 125,000 K depending upon the adopted values of interstellar reddening, the mass of the white dwarf, and the distance to the system. Physical arguments based on the likely state of the quiescent accretion disk in the BV Cen system suggest that the effective temperature of the white dwarf is significantly lower than this maximum temperature. The disk is more likely to be optically thick than optically thin. The impact of these findings on models for dwarf nova outbursts is discussed, but no firm conclusions can be drawn. The ultraviolet emission-line spectrum from observations obtained in August 1985 is described in detail.

Williger, G.↗
NASA NTRS1990

Ginga observations of the dwarf novae BV Pup and V426 Oph

Ginga X-ray observations of the high excitation dwarf nova BV Pup and the suspected DQ Her dwarf nova V426 Oph are presented. The observation of BV Pup took place during optical quiescence and showed no detectable X-ray flux, implying the X-ray luminosity was below that observed by Exosat during the decline from an outburst. An optical spectrum obtained near the time of the X-ray observations showed no high-excitation He II 4686, so that it is not clear if this was an unusual low-excitation time for BV Pup or if this is a normal phase of the outburst cycle. V426 Oph was observed for four days near maximum outburst brightness with overlapping X-ray coverage on three days. The 2-10 keV flux was a factor of 2 below that observed by Exosat during a quiescent state. There is evidence for a 28 min timescale in the X-ray, and optical data which are not strictly periodic in nature.

Szkody, Paula↗
NASA NTRS1989

PAH emission from Nova Cen 1986

The discovery of broad emission features between 3.2 and 3.6 microns were reported in the spectrum of Nova Cen 1986 (V842 Cen) some 300 days following outburst and remaining prominent for several months. The general characteristics of these features are similar to those attributed to polycyclic hydrocarbon (PAH) molecules in other dusty sources, although the relative strengths are different, and these observations provide the first clear evidence for molecular constituents other than graphite particles in the ejecta of novae.

Hyland, A. R. Harry↗
NASA NTRS1990

PW Vul: A classical nova with nearly solar abundances

Ultraviolet and optical spectrophotometric data are combined in order to determine the physical parameters in the expanding shell of Nova PW Vul 1984. Ultraviolet data obtained with the International Ultraviolet Explorer (IUE) satellite from August 2, 1984 (a few days after discovery) until it became too faint to study with the satellite is presented. The last IUE exposure was obtained in June 1988 but it was continually followed with ground based optical telescopes. A plot showing the variation of electron density and temperature as a function of time is presented. Analysis of the emission line intensities show that the abundances of this very slow nova are solar for helium, carbon, and oxygen while nitrogen is 50 times solar. The CLOUDY code of Ferland is used to model the emission line intensities and find good agreement with the observations. Plots representative of IUE and optical spectra are displayed.

Saizar, P.↗