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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 451 records · Page 25
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NASA NTRS1978

Extreme-ultraviolet observations of dwarf novae from Apollo-Soyuz

The paper reports on EUV observations of four dwarf novae (SS Cyg, Z Cha, VW Hyi, and AE Aqr) in July 1975 during the Apollo-Soyuz Test Project, which were performed by using pointed grazing-incidence optics and a channel-electron multiplier as a photon detector. An EUV flux in the range from 55 to 150 A was detected from SS Cyg during an optical outburst at an intensity level considerably below that of Rappaport et al. (1974). Upper limits are placed on the EUV fluxes from Z Cha, VW Hyi, and AE Aqr. Multicolor optical photometry of SS Cyg and AE Aqr is also examined which was obtained simultaneously with the EUV observations. It is concluded that intrinsic EUV flux variability is probably a characteristic of SS Cyg and that the same system exhibits an extremely large range of soft X-ray luminosities while remaining at about the same visible light level. Several models for the SS Cyg system are compared with the EUV data.

Margon, B.↗
NASA NTRS1978

Origin of the coronal line emission from novae

It is proposed that the coronal line emission from V1500 Cyg and other fast novae originates in gas shocked to about 10 to the 6th power K as a result of supersonic random motions within the principal ejecta. This model simultaneously explains the similar widths of the coronal and nebular lines, the temperature and luminosity of the coronal line region, the gradual decrease in ionization, the variability of the coronal line profiles, and the progressive sharpening of velocity structure in the nebular line profiles. The ejecta are characterized by approximately 10 to the 3rd power clouds with a relative velocity dispersion of about 10 to the 2.2 power km/s, compared with an expansion velocity about 10 to the 3.2 power km/s. Alternative models for the coronal lines involving photoionization or interaction of the principal shell with circumstellar gas or with a fast wind from the central object are considered and rejected.

Shields, G. A.↗
NASA NTRS1979

Soft X-ray emission from the vicinity of the dwarf nova AY Lyrae

The detection of a low-intensity soft X-ray source in the region of the dwarf nova AY Lyrae is reported. The source, designated H1839 plus 37, appeared in superpositions of HEAO A2 X-ray scans taken on October 9, 1977; it was observed only in the energy interval 0.18 to 0.43 keV. The peak intensity was 0.014 plus or minus 0.0027 counts/sq cm/sec. If a thermal bremsstrahlung spectrum is assumed, the estimated upper temperature limit of the source is 5 million K; an upper limit of 1.4 million K is implied if a black-body fit is assumed.

Cordova, F. A.↗
NASA NTRS1980

The evolution of rapid oscillations in an outburst of a dwarf nova

High-speed photometric observations of the dwarf nova AH Her on nine consecutive days during an outburst have been made, and rapid coherent oscillations on every day except two near maximum light have been detected. The period, amplitude, and luminosity for each day is presented and the progression of the periods is discussed.

Hildebrand, R. H.↗
NASA NTRS1980

Detection of periodic light variations in the old nova V 603 Aquilae /1918/

Periodic visible light variations of the old nova V 603 Aql (1918) were detected with the Fine Error Sensor Instrument (FES) aboard the IUE satellite. Continuous observations were carried out during more than two complete cycles. The light curve shows a broad maximum and a pronounced minimum which was covered three times, and is tentatively interpreted as an eclipse of the accretion disk around the white dwarf by the late main sequence component.

Rahe, J.↗
NASA NTRS1981

On the nature of the nova-like variable CD-42 deg 14462

Low dispersion long and short wavelength IUE spectra of the nova like system CD-42 deg 14462 were obtained on August 24 U.T. The short wave spectrum exhibits absorption features due to C III (lambda 1175), Lalpha 1216), NV (lambda1240), HeII (lambda 1640), SiIV (lambda1394), NIV (lambda1875) with CIV (lambda1550) as a P Cygni feature with blue shifted absorption suggesting the presence of material leaving the system. Possible interpretations of this object are discussed.

Guinan, E. F.↗
NASA NTRS1980

A wave model for dwarf novae

The rapid coherent oscillation during a dwarf nova outburst is attributed to an accretion-driven wave going around the white dwarf component of the binary system. The increase and decrease in the period of this oscillation is due to the change in the velocity of the wave as it is first being driven and then damped. Qualitatively, a large number of observations can be explained with such a model. The beginnings of a mathematical representation of this model are developed.

Sparks, W. M.↗
NASA NTRS1981

A model for dwarf novae as progenitors of Type I supernovae

It is suggested that a Type I supernova event can be produced as a result of accretion at low rates (less than 10 to the -10th solar mass/year) onto the white dwarf component of a dwarf-nova close binary. At these low rates, diffusion of the CNO nuclei out of the accreted hydrogen envelope can occur on the accretion time scale, and nuclear burning will occur mainly by the proton-proton chain. Studies of this process show that, when the CNO abundance in the envelope is less than 0.0025 of solar, no thermonuclear flash occurs and no mass is lost. The result of the evolution is a gradually increasing layer of helium beneath the hydrogen-burning shell.

Starrfield, S.↗
NASA NTRS1982

The nova-like variable KQ Mon and the nature of the UX Ursa Majoris stars

The KQ Mon is a UX UMa type nova-like variable discovered by Howard Bond. Optical spectra taken by Bond in 1978 reveal very shallow Balmer absorption lines and He I absorption. Bond also did UBV and high speed photometry in 1978 and early 1981. There has been no evidence of orbital variations but the appearance of the optical spectrum and the presence of low amplitude flickering suggested a strong similarity to CD-42-14462 (=V3885 Sgr) and other members of the UX UMa class. Low dispersion observations of KQ Mon were made with the International Ultraviolet Explorer satellite. Six spectra taken with the shot wavelength prime camera are dominated by strong broad absorption lines due to N V, O I, Si III, Si IV, C IV, He II, N IV, and Al III. There is little evidence of orbital phase modulation over the time baseline of the observations. Unlike UV observations of other UX UMa type objects, KQ Mon exhibits no emission lines or P Cygni type profiles and the velocity displacements appear to be smaller, suggesting the absence of a hot, high velocity wind characterizing other UX UMa stars. The relationship of KQ Mon to other UX UMa disk stars is discussed and a model is suggested to explain their observed properties and the lack of major outbursts.

Sion, E. M.↗
NASA NTRS1982

Ultraviolet spectroscopy of the nova-like variable V3885 Sagittarii /= CD -42 deg 14462/

Low-dispersion long and short wavelength IUE spectra of the nova-like system V3885 Sgr were obtained on 1979 August 24 UT. The UV spectrum exhibits strong, high-temperature absorption lines of C III, C IV, He II, N IV, N V, and Si IV which all have relatively large negative velocities of 600-1000 km/s at their line centers. The C IV (1550 A) features shows a P Cygni-type profile with a blueshifted absorption line; the presence of the P Cygni-type feature strongly suggests a significant mass outflow from the system in the form of a hot stellar wind. The observed continuum fluxes are fitted with theoretical accretion disk model fluxes, and a tentative physical interpretation of the observed spectral properties is presented.

Guinan, E. F.↗
NASA NTRS1982

High-velocity winds from a dwarf nova during outburst

An ultraviolet spectrum of the dwarf nova TW Vir during an optical outburst shows shortward-shifted absorption features with edge velocities as high as 4800 km/s, about the escape velocity of a white dwarf. A comparison of this spectrum with the UV spectra of other cataclysmic variables suggests that mass loss is evident only for systems with relatively high luminosities (more than about 10 solar luminosities) and low inclination angles with respect to the observer's line of sight. The mass loss rate for cataclysmic variables is of order 10 to the -11th solar mass per yr; this is from 0.01 to 0.001 of the mass accretion rate onto the compact star in the binary. The mass loss may occur by a mechanism similar to that invoked for early-type stars, i.e., radiation absorbed in the lines accelerates the accreting gas to the high velocities observed.

Cordova, F. A.↗
NASA NTRS1982

Accretion instability models for Dwarf Novae and X-ray transients

Steady state and time dependent calculations of a model for accretion instability are presented, in which matter accumulates in a cold torus and then undergoes a thermal instability causing the matter to heat and flow down onto the central star. The model is compared to the observations of dwarf novae and certain examples of both hard and soft X-ray transients.

Cannizzo, J. K.↗
NASA NTRS1983

Ultraviolet photometry of dwarf novae in outburst

Broad-band photometry from the Astronomical Netherlands Satellite of five dwarf novae at 1550, 1800, 2200, 2500, and 3300 A is presented. EM Cyg in standstill exhibits a flux excess at 3300 A. The lack of a strong extinction feature at 2200 A is seen as suggesting that the reddening is small toward all five systems. Using an independent estimate of the distances, mass flux rates are derived by fitting steady-state, optically thick accretion disk models to the observed UV luminosity. The rate is found to be 3 x 10 to the 17th g/s for Z Cam and EM Cyg in outburst and a factor of 3 smaller of Z Cam in standstill. The models, however, do not reproduce the slope of the Balmer continuum. Of special interest are the observations of outburst rises of SS Aur, SU UMa, and EM Cyg. SS Aur exhibits an exponential increase at a rate of 3.5 mag in 0.5 d. The outburst in EM Cyg also reveals an exponential rise, but at a rate of only 1.2 mag per day.

Wu, C.-C.↗
NASA NTRS1983

A nebula around Nova BT Monocerotis

H-alpha observations of Nova BT Mon obtained on December 15, 1981 using an 800x800-pixel CCD detector on the double spectrograph of the 5-m Hale telescope at Palomar Observatory are reported. The reciprocal dispersion at H-alpha was 6.04 A/pixel and the angular scale along the 1.0-arcsec-wide east-west slit was 0.58 arcsec/pixel; resolution of the combined, processed image is about 12 A. A ring-shaped nebular emission with a center displaced slightly from the stellar image and an expansion distance to BT Mon of about 1800 pc was detected. The velocity diameter is found to be about 1500 km/sec along the ridge line and 2100 km/sec along the 10-unit contour. The mass of the visible nebula is estimated as 0.00003 solar mass, similar to the BT Mon ejection mass determined by Schaefer and Patterson (1983).

Marsh, T. R.↗
NASA NTRS1983

Ultraviolet studies of nova-like variables with the IUE

KQ Mon is a new UX UMa-type nova-like variable. Optical spectra taken in 1978 reveal very shallow Balmer absorption lines and He I (wavelength 4471) absorption. There was no evidence of orbital variations but the appearance of the optical spectrum and the presence of low amplitude flickering suggested a strong similarity to CD-42 degrees 14462 (=V3885 Sgr) and other members of the UX UMa class. KQ Mon was observed at low dispersion with the IUE satellite. Six spectra taken with the short wavelength prime (SWP) camera are dominated by strong broad absorption lines due to N V, O I, Si III, Si IV, C IV, He II, N IV, and A1 III. There is little evidence of orbital phase modulation over the time baseline of the observations. Unlike UV observations of other UX UMa-type objects, KQ Mon exhibits no emission lines or P Cygni-type profiles and the velocity displacements appear to be smaller, suggesting the absence of a hot, high velocity wind characterizing other UX UMa stars. The relationship of KQ Mon to other UX UMa disk stars is discussed and a model is suggested to explain their observed properties and the lack of major outbursts.

Guinan, E. F.↗
NASA NTRS1983

The X-ray emission of the old Nova V603 Aquilae (1918)

X-ray observations of the old Nova V603 Aql have been obtained with the IPC and MPC instruments aboard the Einstein Observatory during 1.7 consecutive orbital cycles. Periodic orbital phase-dependent flux variations are detected with large-amplitude flickering superimposed on the sinusoidal X-ray light curve. The spectrum is hard, characterized by a temperature of 20-30 keV, and the maximum X-ray luminosity observed was about 3 x 10 to the 33rd erg/s. The X-ray measurements are compared with recent optical photometric observations, and origin of the periodic modulation of the optical and X-ray flux is discussed.

Drechsel, H.↗
NASA NTRS1983

Observations of a hard X-ray component in the spectrum of Nova Ophiuchi

The spectrum and time variation of Nova Ophiuchi (H1705-25) in the 10-200 keV range as measured by the UCSD/MIT instruments aboard HEAO 1 during the period 1977 August 25 to September 28 are reported. The composite curve is best fitted by a kT = 2 keV thin thermal bremsstrahlung model below 10 keV and a separate hard X-ray component fitted equally well by a power-law component with photon index 2.19 + or - 0.06 or a kT = 32.1 + or - 2.4 keV thermal bremsstrahlung model. This is the first observation of a hard tail in the spectrum of a transient X-ray source with sufficient statistical significance to allow a detailed study of its spectral and temporal variability. It is found that the intensity variations of the high-energy X-rays are consistent with the variability at lower energies (3-6 keV), but no hard X-ray spectral index variability is found on time scales from 2 days to 2 weeks. The results can be interpreted as due to accretion onto a neutron star (or possibly onto a black hole) that may also be surrounded by an extended corona.

Wilson, C. K.↗
NASA NTRS1984

HEAO 1 high-energy X-ray observations of three bright transient X-ray sources H1705-250 (Nova Ophiuchi), H1743-322, and H1833-077 (Scutum X-1)

The bright X-ray transients H1705-250 (Nova Ophiuchi) and H1743-322 in the energy range 12-180 keV were observed, and the presence of high-energy spectral components was revealed. On the basis of X-ray spectra in the 1-10 keV range, transients are classified as 'soft' (kT = 4 keV) or 'hard' (kT = 17 keV); and both H1705-250 and H1743-322 should be in the 'soft' category. Data have been reexamined for the archetypal 'soft' transient A0600-00 taken by the SAS 3 satellite, but no evidence is found there for a high-energy spectral component. Thus consideration of a wider X-ray energy range makes the 'hard'/'soft' distinction between X-ray transients much less clear.

Cooke, B. A.↗