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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 613 records · Page 34
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NASA NTRS1990

Orbital periods of recurrent novae

The class of recurrent novae (RN) with thermonuclear runaways contains only three systems (T Pyx, U Sco, and V394 CrA), for which no orbital periods are known. This paper presents a series of photometric observations where the orbital periods for all three systems are discovered. T Pyx is found to have sinusoidal modulation with an amplitude of 0.08 mag and a period of 2.3783 h (with a possible alias of 2.6403 h). U Sco is found to be an eclipsing system with an eclipse amplitude of roughly 1.5 mag and an orbital period of 1.2344 days. V394 CrA is found to have sinusoidal modulation with an amplitude of 0.5 mag and a period of 0.7577 days. Thus two out of three RN with thermonuclear runaways (or five out of six for all RN) have evolved companions.

Schaefer, Bradley E.↗
NASA NTRS1989

Sublimating icy planetesimals as the source of nucleation seeds for grain condensation in classical novae

The problem of grain nucleation during novae outbursts is a major obstacle to our understanding of dust formation in these systems. How nucleation seeds can form in the hostile post-outburst environment remains an unresolved matter. It is suggested that the material for seeding the condensation of ejecta outflow is stored in a primordial disk of icy planetesimals surrounding the system. Evidence is presented that the requisite number of nucleation seeds can be released by sublimation of the planetesimals during outbursts.

Matese, John J.↗
NASA NTRS1990

Model atmospheres for the early stages of novae in outburst

For the photospheres of classical novae during the early stages of their outbursts, continuum and line blanketed models are presented. The expanding envelopes are characterized by small density gradients that lead to very large geometrical extensions and large temperature differences between inner and outer parts. The spectra show large excesses in the IR and small Balmer jumps which may either be in absorption or emission. For the parameters considered most lines are in absorption if powerlaw density distributions and radiative equilibrium are assumed. The effects of modifications in the temperature structure (e.g., heating from shock fronts) and in the density stratifications as well as changes in the abundances are discussed.

Wehrse, R.↗
NASA NTRS1990

Doppler imaging of the dwarf nova U Geminorum

Doppler tomography is applied to time-resolved spectra of the dwarf nova U Geminorum, and it is found that emission from the bright-spot region has neither the velocity of the gas stream nor the velocity of the disk along the stream but lies roughly midway between. It is shown that earlier observations of U Gem were consistent with the present results, and it is concluded that the line emission arises immediately after the shock as the gas from the stream mixes with the disk. The data also show evidence for Balmer emission from the secondary star which, together with the bright spot's emission, produces a complicated S-wave in trailed spectra.

Marsh, Thomas R.↗
NASA NTRS1991

Spectral transformation of the unusual variable star MWC560 to resemble a nova

A dramatic change has occurred in the ultraviolet spectrum of the emission-line star MWC560, so that it now closely resembles the spectrum of a nova shortly after outburst. This event may signal a major mass-ejection episode such as presumably occurred in past centuries in the symbiotic star R Aquarii to produce the well-known bipolar nebula, and it may herald the emergence of a standard symbiotic-star emission-line spectrum in MWC560, corresponding to a change in evolutionary state.

Maran, Stephen P.↗
NASA NTRS1992

TT Crateris - A long-period, double-lined dwarf nova

Time-resolved CCD photometry and spectroscopy of the dwarf nova TT Crt during quiescence are presented. The spectra reveal Balmer emission as well as absorption features of a K5-M0 secondary. The B, V, R light curves are dominated by a periodic variation of full amplitude 0.35 mag in V, which are interpreted as the ellipsoidal variation of the secondary star. The remarkably large amplitude indicates a high inclination (50-65 deg) and a massive white dwarf M greater than 0.8 solar mass. The ratio of the radial velocity semiamplitudes of the primary to the secondary is 0.5-1.4. The best period that can be determined from the observations is 438.17 minutes, with a possible alias at 445 minutes.

Szkody, Paula↗
NASA NTRS1992

The Hubble Space Telescope observations of X-ray nova Muscae 1991 and its spectral evolution

Hubble Space Telescope Faint Object Spectrograph (FOS) Faint Object Camera (FOC) and observations of Nova Muscae 1991 obtained on May 14-15, 1991 are presented and discussed. A nearly featureless continuum with broad 2200 A absorption feature and no Balmer jump is found. The FOS spectrum is modeled, including previous multiepoch IUE, optical, and X-ray data using a simple blackbody accretion disk model. It is found that the mass transfer rate decays exponentially with a characteristic time of about 43 days. The cooling front predicted by the disk instability model should have been observable, but was not seen.

Cheng, F. H.↗
NASA NTRS1993

An interpretation of the narrow positron annihilation feature from X-ray nova Muscae 1991

The physical mechanism responsible for the narrow redshifted positron annihilation gamma-ray line from the X-ray nova Muscae 1991 is studied. The orbital inclination angle of the system is estimated and its black hole mass is constrained under the assumptions that the annihilation line centroid redshift is purely gravitational and that the line width is due to the combined effect of temperature broadening and disk rotation. The large black hole mass lower limit of 8 solar and the high binary mass ratio it implies raise a serious challenge to theoretical models of the formation and evolution of massive binaries.

Chen, Wan↗
NASA NTRS1993

Ultraviolet spectroscopy of Nova Muscae 1991

We report on a series of ultraviolet spectroscopic observations made with the IUE satellite, which sampled critical outburst stages of the soft X-ray transient Nova Muscae 1991. Continuum light curves, and a multi-epoch line analysis are presented. In addition, a single epoch optical-UV spectra obtained with the HST/FOS is discussed. The optical-UV continuum fits approximately the 'canonical' fv varies as V exp 1/3 power low expected for thermal emission from steady state accretion disk. The emission line spectrum contains prominent high ionization lines of C IV and N V. Weaker lines are seen at intermittent epochs, including some of lower ionization. The derivation of galactic reddening which has significant implications for distance estimates is discussed. Comparisons with previous SXT sources are considered.

Shrader, C. R.↗
NASA NTRS1993

On a thermonuclear origin for the 1980-81 deep light minimum of the symbiotic nova PU Vul

The puzzling 1980-81 deep light minimum of the symbiotic nova PU Vul is discussed in terms of a sequence of quasi-static evolutionary models of a hot, 0.5 solar mass white dwarf accreting H-rich matter at a rate 1 x 10 exp -8 solar mass/yr. On the basis of the morphological behavior of the models, it is suggested that the deep light minimum of PU Vul could have been the result of two successive, closely spaced, hydrogen shell flashes on an accreting white dwarf whose core thermal structure and accreted H-rich envelope was not in a long-term thermal 'cycle-averaged' steady state with the rate of accretion.

Sion, Edward M.↗
NASA NTRS1993

BATSE observations of Nova Muscae 1991

The Compton Burst and Transient Source Experiment (BATSE) detected hard x-ray flux from Nova Muscae 1991 (GS/GRS 1124-68) during an interval of approximately 100 days beginning approximately 130 days after the January 1991 main outburst. The light curve during this secondary outburst is roughly symmetric, reaching a maximum around mid-July 1991 at an intensity of approximately 15 percent of the peak intensity during the main outburst. The hard x-ray spectrum displays a soft-to-hard evolution during the rise to maximum; the post-maximum spectral evolution is less well determined. We compare our observations with those of the GRANAT/SIGMA experiment, which covered the initial outburst well but missed most of the secondary outburst.

Paciesas, W. S.↗
NASA NTRS1994

Optical, IUE, and ROSAT observations of the eclipsing nova-like variable V347 Puppis (LB 1800)

Using time-resolved optical spectroscopy and UBVRI and high-speed photometry obtained at Mount Stromlo Observatory, Mount John University Observatory, and the South African Astronomical Observatory; International Ultraviolet Explorer (IUE) ultraviolet spectroscopy; and Roentgen Satellite (ROSAT) survey X-ray fluxes, we present a study of the accretion disk, hot spot, and emission line regions in the bright eclipsing nova-like variable V347 Pup (LB 1800). In the optical and UV, V347 Pup is a strong emission line source with a continuum spectrum which is remarkably red for a high-M cataclysmic variable. Consistent with its high inclination, we interpret the continuum spectrum as the superposition of the spectrum of the cool (T(sub eff) approximately 7000 K) outer edge and the hot (T(sub eff) approximately 100,000 K) inner regions of a self-eclipsed accretion disk. For the assumed parameters, the model matches the level and shape of the observed spectrum for an inclination of approximately 88 and a distance of approximately 300 pc. The prominent hump in the optical and UV light curves just before eclipse manifests the presence of the hot spot where the accretion stream strikes the edge of the disk. The wavelength dependence of the amplitude of the hump is best modeled by a spot having an effective temperature of approximately 25,000 K and an area of approximately 3 x 10(exp 18) sq cm if the spot radiates like a blackbody, or an effective temperatue of approximately 14,000 K and an area of approximately 3 x 10(exp 19) sq cm if it radiates with a stellar spectrum. In either case, the hot spot produces only one-tenth of the predicted luminosity for the assumed mass-transfer rate of 10(exp -8) solar mass/yr. Either the hot spot is 'buried' in the edge of the accretion disk, or a significant fraction of its luminosity is radiated away in lines. The difference in azimuth between the peak of the hump and the dynamically expected location of the hot spot suggests that the spot's emitting surface is rotated forward by approximately 36 deg relative to the edge of the disk.

Mauche, Christopher W.↗
NASA NTRS1994

Quiescent accretion disks in black hole X-ray novae

We present detailed time-resolved spectroscopy of the Balmer emission lines from two black hole binary systems in quiescence, A0620-00 and Nova Muscae 1991. We find extraordinary similarities between the two systems. There are 30-40 km/s velocity variations of the emission lines over the orbital period, the phases of which are not aligned with the expected phase of the motion of the compact primary. Detailed modeling of both systems is complicated by variable hot spot components, regions of optical thickness, and intermittent excess emission in the blue line wings of the H-alpha lines. Both sources also display low velocities at the outer edge of the accretion disk, implying a large primary Roche lobe and extreme mass ratios. These complications suggest that although simple optically thin, Keplerian alpha-disk models provide a useful parameterization of emission lines from these systems, the straightforward physical models they imply should be treated with great caution.

Orosz, Jerome A.↗
NASA NTRS1994

Concurrent X-ray and optical observations of two dwarf novae during eruption

The dwarf novae SU UMa and RU Peg were each observed with the ROSAT Observatory during outburst. SU UMa was also observed twice between outbursts. The spectrum of SU UMa during quiescence is well fitted by a 2.1 keV thermal bremsstrahlung model with a Gaussian line at 0.97 keV absorbed by a column density of 1.4 x 10(exp 20)/sq cm. During the outburst the X-ray count rate dropped by a factor of 3. The outburst spectrum is well fitted to a similar spectrum with a significantly larger absorption. In outburst the mass transfer rate, dot-M, approaches the critical value for the optically thin/thick transition predicted by theoretical models of the boundary layer while the X-ray emission remains optically thin. The best-fit model to the outburst spectrum of RU Peg is a 4.9 keV thermal bremsstrahlung plus a line at 0.89 keV absorbed by a column density of 2.8 x 10(exp 20)/sq cm. The column density is consistent with the upper limit determined for interstellar absorption. The boundary layer at outburst is expected to be optically thick based on the value of dot-M determined from the IUE spectra. There are no ROSAT observations of RU Peg between outbursts, but the flux detected by ROSAT is a factor of 10 lower than that seen with the Einstein Observatory, probably during quiescence. For both CVs the X-ray flux from the boundary layer significantly decreases at the same time that the optical-UV flux greatly increases. We suggest that during outburst most of the boundary layer energy is carried away as kinetic energy of the wind.

Silber, A.↗
NASA NTRS1995

The soft X-ray turnoff of Nova Muscae 1983

Nova GQ Muscae 1983 was detected by ROSAT as a luminous 'supersoft' X-ray source in 1992, nearly a decade after outburst. Further, this is the only classical postnova known to have maintained constant luminosity on a timescale predicted by theoretical models. Follow-up observations were made with the ROSAT position-sensitive proportional counter in 1993 January and September, and complemented with B-band photometry taken in 1993 January. By 1993 January, the X-ray count rate had declined by a factor of 17, while there was neither an appreciable decrease in the optical magnitude nor a change in the amplitude of modulation. In 1993 September the soft X-ray flux was below the ROSAT threshold limit, implying a decrease of a factor greater than or equal to 30 in the count rate. This decline can be interpreted by the turnoff of nuclear processes due to the complete consumption of the residual hydrogen-rich envelope. However, the optical luminosity of the system is not simply coupled to the X-ray luminosity (e.g., through reprocessing).

Shanley, L.↗
NASA NTRS1994

Spectral eclipse mapping of the accretion disk in the nova-like variable UX Ursae Majoris

We analyze narrow-band eclipse light curves of the nova-like cataclysmic variable UX UMa, obtained from low-resolution spectra spanning lambda lambda 3600-9800 A . The light curves for narrow bands in the continuum as well as those for individual spectral lines are treated independently, and are used to construct images of the accretion disk's brightness distribution using the maximum-entropy eclipse-mapping technique. Particular attention is paid to the propagation of statistical uncertainties in the data and to how the analysis may introduce systematic errors in the final result. From the many narrrow band images we have reconstructed the spectra from isolated parts of the accretion disk. These spectra reveal that the inner disk radiates a continuum spectrum which peaks in the near UV and has the hydrogen Balmer lines in absorption (with the exception of H-alpha), whereas the outer disk is much fainter, has a much redder spectrum, and has Balmer emission lines. Our analysis reveals the presence of an uneclipsed component of the total light, whose spectrum is very red and has Balmer lines in emission. This unexpected feature of the eclipse mapping technique offers a new tool for an independent assessment of the secondary star's spectrum in eclipsing cataclysmic variables.

Rutten, R. G. M.↗
NASA NTRS1995

Detection of the hot white dwarf in the magnetic nova V1500 Cygni with the Hubble Space Telescope

A comprehensive investigation of the AM Herculis-like nova system V1500 Cyg is presented, featuring time-resolved spectrophotometry with the Faint Object Spectrograph (FOS) of the Hubble Space Telescope (HST), coeval optical circular spectropolarimetry and spectrophotometry, and new broadband polarimetry. A revised ephemeris for the rotating white dwarf refines our knowledge of the timescale for synchronization with the orbital period -- and for becoming a bona fide AM Herculis system -- to 170 +/- 8 yrs. Three features of the orbital-phase-dependent spectral energy distribution are keys for understanding the relative contributions of several radiation sources in this system: (1) the brightness and shape of the ultraviolet continuum, which rises abruptly at short wavelengths and shows no significant phase dependence for lambda less than 2200 A, (2) the greater than or equal to 1 mag variations in the optical continuum and emission-line fluxes, and (3) a spectral slope in the optical which is reddest when brightest. These data lead us to conclude that the hot, postnova white dwarf is the dominant source of ultraviolet flux and of the unusually large bolometric luminosity of the system, compared with typical active AM Herculis binaries. The flat mid-ultraviolet spectrum requires a radiation component in addition to the white dwarf photosphere. We consider three possible origins: (1) cyclotron emission, (2) relatively broad, optically thick accretion columns with lumps of dense gas, and/or (3) orbiting debris in a disklike geometry. Observational tests are suggested to distinguish among these alternatives.

Schmidt, Gary D.↗
NASA NTRS1995

Hubble Space Telescope FOS spectroscopy of the ultrashort-period dwarf nova WZ Sagittae: The underlying degenerate

Two consecutive Hubble Space Telescope (HST) Faint Object Spectrograph (FOS) spectra of the exposed white dwarf in the ultrashort-period, high-amplitude, dwarf nova WZ Sge, reveal a rich absorption line spectrum of neutron carbon and ionized metals, the Stark-broadened Lyman-alpha absorption wing, the H2 quasi-molecular Lyman-alpha 'satellite' absorption line, and a double-peaked C IV emission line which is variable with orbital phase. A synthetic spectral analysis of the white dwarf yields T(sub eff) = 14,900 K +/- 250 K, log g = 8.0. In order to fit the strongest C I absorption lines and account for the weakness of the silicon absorption lines, the abundance of carbon in the photosphere must be approximately 0.5 solar, silicon abundance is 5 x 10(exp -3) solar, with all other metal species appearing to be 0.1-0.001 times solar. The H2 quasi-molecular absorption is fitted very successfully. The photospheric metals have diffusion timescales of fractions of a year, and thus they must have been accreted long after the 1978 December outburst. The source of the most abundance metal, carbon, is considered. If the time-averaged accretion rate during quiescence is low enough for diffusive equilibrium to prevail, then the equilibrium accretion rate pf neutron carbon is 7 x 10(exp -16) solar mass/yr. A convective dredge-up origin for the concentration of carbon is extremely unlikely, given that the white dwarf atmosphere is H-rich while in single degenerates showing carbon and hydrogen, the C and H are trace elements in a helium background. Additional implications are explored.

Sion, Edward M.↗