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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 343 records · Page 19
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NASA NTRS1982

Ultraviolet comparisons of normal outbursts and a supermaximum in two dwarf novae

IUE spectra of AY Lyr obtained shortly after supermaximum and normal maximum and of RX And after normal maximum are compared. All spectra are compatible with steady state disk models in the ultraviolet while the declining stages show an upturn in flux longward of 2500 A which indicates the presence of a cool component. For the same optical magnitude, the ultraviolet flux of AY Lyr is a factor of two higher after supermaximum than normal maximum, implying a larger accretion rate during supermaximum. The lines of RX And exhibit P Cyg profiles which diminish during the decline. The terminal velocities right after maximum are greater than the escape velocity but then decrease rapidly along with the column density by the third day after maximum light has ended.

Szkody, P.↗
NASA NTRS1983

The nautre of dwarf novae and related systems

The high energy emissions from cataclysmic variables (interacting binary systems containing an accreting white dwarf) and the relationship of the high energy emission of these stars to their optical properties were investigated. The relationship of the cataclysmic binaries to the strong X-ray emitting binaries that contain a neutron star as the accreting star was examined. A soft X-ray survey of cataclysmic variable stars was carried out. Approximately 75% of all the sources observed were detected in short (2000 sec) observations. Relationships between the X-ray and optical luminosity and the dependence of this quantity on the subclass of the variable were investigated and constraints on the nature of these systems were provided.

Lampton, M.↗
NASA NTRS1983

First detection of radio emission from a dwarf nova

The detection of 4.75 GHz radio emissions from a white dwarf star in SU UMa is reported, and the source of the emission is discussed. The emission was discovered during a survey of six dwarf stars with a double horn receiver system. SU UMa was successfully scanned 123 times, with each scan comprising 31 3-sec integrations 30 arcsec apart. Average fluxes for each beam position were calculated, as was the X ray emission of 7.6 x 10 to the 54th/cu cm in the 0.1-4.5 keV band. The small mass outflow projected for the object indicates a source of suprathermal electrons for the radio emissions A cyclotron maser instability is suggested as the mechanism, and future measurements to detect circular polarization as proof of a coherent source are indicated.

Benz, A. O.↗
NASA NTRS1983

An infrared study of the eclipsing dwarf nova U Geminorum

One series of data comprising the present J, H, K broadband IR photometry of the cataclysmic binary system U Geminorum over the course of two orbital cycles was obtained several days before an outburst, while a second was gathered when the system had almost completely returned to quiescence from that outburst. In quiescence, the M4-M5.5 red dwarf star supplies most of the IR luminosity of the system, while the light curves at this time are tidally induced ellipsoidal variations of the red dwarf which exhibit an amplitude such that the red dwarf must fill, or nearly fill, its Roche lobe. This shows directly that Roche lobe overflow is important in cataclysmic variable stars. The range of the disk IR colors after outbursts suggest that cool and optically thick gas, together with optically thin gas, supply most of the IR light of the disk.

Berriman, G.↗
NASA NTRS1985

On the nature of the UX Ursa Majoris-type nova-like variables - CPD-48 deg 1577

A series of low-dispersion spectra and one high-dispersion spectrum have been obtained of the catacylsmic variable CPD-48 deg 1577. Like other UX UMa stars, this variable exhibits the flat continuum of a luminous, thick, steady state accretion disk dominating its light from the far UV to the infrared. A wind mass loss rate of less than about 10 to the -9th solar mass/yr and an accretion rate of roughly 5 x 10 to the -9th solar mass/yr is roughly consistent with the range which characterizes other UX UMa stars. An SWP echelle spectrum reveals detailed line profile information on the N V, Si IV, and C IV resonance lines. He II 1640 A is remarkably weak and even absent in two of the spectra. The known properties and evolutionary status of CPD-48 deg 1577 and the other UX UMa stars are summarized.

Sion, E. M.↗
NASA NTRS1986

Dwarf nova burst asymmetry and the physics of accretion disks

Far-ultraviolet observations of a burst of SS Cyg obtained with the Voyager spacecraft are presented along with optical observations. Representative V band and far-UV light curves are computed based on a thermal instability model, and the dependence of the far-UV delay and the shape of the outburst on the location in the disk where the outburst starts is examined. The results show that burst initiated at large radii produce asymmetric light curves for which the far-UV flux lags the optical flux by about half a day, as observed. For bursts starting at smaller radii, the delay is smaller and the outburst is more rounded and symmetric. For both burst types, the delay with respect to the optical is longer for larger accretion disks and at shorter wavelengths. The physical processes that will promote burst initiation in the outer disk are discussed, emphasizing that this can occur in principle with no modulation of the mass transfer rate from the companion.

Cannizzo, J. K.↗
NASA NTRS1986

Ultraviolet observations of V471 Tauri - Evidence for a nova-like outburst?

A search for accretion of wind-flare material onto the white dwarf in V471 Tauri has unexpected by revealed what appears to be shortward-shifted absorption, centered on two distinct velocities, near heliocentric velocities of -300 and -590 km/s in the line of sight toward V471 Tauri. The possibility that these features are due to instrumental effects is explored. The narrow interstellar-like appearance of the -590 km/s component and lack of variability with orbital phase indicate that the observed high-velocity absorption components are formed at a great distance from the V471 Tauri system and probably owe their origins to a discrete mass ejection event.

Bruhweiler, F. C.↗
NASA NTRS1986

X-ray and optical observations of the ultrashort period dwarf nova SW Ursae Majoris - A likely new DQ Herculis star

Time-resolved X-ray and optical photometric and optical spectroscopic observations of the ultrashort period cataclysmic variable SW UMa are reported. The spectroscopic observations reveal the presence of an s-wave component which is almost in phase with the extreme line wings and presumably the white dwarf. This very unusual phasing in conjunction with the available optical and X-ray data seems to indicate that a region of enhanced emission exists on the opposite side of the disk from the expected location of the hot spot. The photometric observations reveal the presence of a hump in the light curve occurring at an orbital phase which is consistent with the phase at which the region of enhanced line emission is most favorably seen. Changes in the hump amplitude are seen from night to night, and a 15.9 min periodicity is evident in the light curve. The optical and X-ray periodicities suggest that SW UMa is a member of the DQ Her class of cataclysmic variables.

Shafter, A. W.↗
NASA NTRS1989

Ultraviolet spectrophotometry and optical and infrared photometry of the old nova GK Persei

IUE observations in the 1150-3250-A region were obtained of GK Per during the rise, at the maximum, and during the decline of the 2.5-mag optical outburst in 1981. The results support previous predictions for the interaction of an accretion disk with the magnetic field of an accretion star. The luminosity at minimum is found to be 2.3 solar luminosities, with 1/3 of this being attributed to the cool secondary star.

Wu, Chi-Chao↗
NASA NTRS1990

Near-infrared spectroscopy of classical novae in the coronal phase

Spectroscopic observations of V1819 Cyg, V827 Her, and V2214 Oph, obtained at 1-2.5 microns using a 32-channel InSb array detector at the Cassegrain focus of the 2.7-m telescope at McDonald Observatory during 1986-1989, are reported. The data are presented in extensive tables and graphs and characterized in detail, with a focus on the nature of the 'coronal' emission. The ratios of Br-gamma (2.167 microns) and forbidden Si VI (1.960 microns) are found to be consistent with coronal gas temperatures of about 10,000 K and with photoionization as the primary mechanism of origin for the coronal species. Severe constraints on theoretical models involving shock heating and collisional ionization are inferred.

Benjamin, Robert A.↗
NASA NTRS1991

Synchronization of the magnetic Nova V1500 Cygni

The study addresses a program of polarimetric monitoring of V1500 Cyg and a detection of spin-down of the magnetic primary star. Data on the spectral shape of polarized cyclotron flux is presented, and a lower limit to the field strength on the magnetic primary star is derived. The implications which the field strength and observed rate of period change have on locking scenarios are discussed, including the likelihood of identifying other AM Her systems in the process of formation. It is noted that two-color polarimetry of V1500 Cyg indicates a polar magnetic-field strength consistent with those measured in the synchronized AM Her variables; however, the symmetry and long-term repeatability of the polarized flux curves indicate that accretion occurs onto large regions of the white-dwarf surface.

Schmidt, Gary D.↗
NASA NTRS1991

On the X-ray emitting boundary layer of the dwarf nova VW Hydri

The temperature and luminosity of the boundary layer of VW Hyi are constrained to kT(BL) of about 10.5 eV, and L(BL) of about 6 x 10 to the 32nd (d/65 pc)squared ergs/sec. This is based on Voyager far- and extreme-ultraviolet spectrophotometry and a measurement of the column density of neutral hydrogen, combined with Exosat LE filter observations. Results are compared with the accretion-disk luminosity found by Polidan et al. (1990) using concurrent optical, IUE, and Voyager spectrophotometric observations. The value of zeta is found to be about 0.04, although theoretical predictions show comparable luminosities at the boundary layer and the accretion disk - zeta is identical to L(BL)/L(disk), which is about 1 - unless the white dwarf rotates very rapidly. Severe contamination of filter observations due to light from the inner accretion disk is also found. This contamination had previously been understood as a result of the luminous ultrasoft boundary layer.

Mauche, Christopher W.↗
NASA NTRS1992

Fe II fluorescence and anomalous C IV doublet intensities in symbiotic novae

The variation of absolute intensities of Bowen-excited Fe II emission in the symbiotic stars RR Tel, RX Pup, and AG Peg is examined. The C IV doublet intensity ratios in RR Tel were not anomalous between 1979 and 1989, and the ratio had typical values within the optically thin range. The intensity of individual Fe II Bowen-excited lines is correlated with the C IV 1548.2 A flux, suggesting the presence of a foreground Fe II region in which fluorescent-excited material responds to flux variations of C IV 1548.2 A. In RX Pup the combined fluxes of Fe II Bowen-pumped lines can account for an appreciable fraction of the flux deficit in the C IV 1548.2 A line when the C IV doublet ratio is less than the optically thick limit of unity. The Fe II Bowen lines in RX Pup exhibit a velocity range from 0 to 80 km/s, where several strong Fe II emission lines correspond to deep absorption structure in the C IV 1548.2 A line profile. In AG Peg and C IV 1548.2 A flux deficit cannot be explained by Fe II fluorescent absorption alone when the C IV doublet ratio anomaly is at an extreme.

Michalitsianos, A. G.↗
NASA NTRS1992

Spectral evolution of accretion disks of dwarf novae. III - Outburst cycle of SS Cygni

A modified disk atmosphere has been used to produce absorption profiles for H-beta, H-gamma, and He I 4471 A lines to fit those at the maximum light during the October 1981 outbursts of SS Cyg and those during the decline. The coexistence of strong emission cores and absorption wings at the maximum light shows that the mass transfer rate at SS Cyg in the outburst was about 5 x 10 exp 17 g/s. The viscosity parameter in the disk during quiescence is between 0.006 and 0.6. The weak emission and shallow absorption during rise indicate that the outburst is initiated from the inner region of the accretion disk during the outburst. This result is compatible with the disk thermal instability hypothesis for CV outbursts, but incompatible with the mass transfer instability scenario for CV outbursts.

Cheng, F. H.↗
NASA NTRS1992

The photometric period of the recurrent Nova T Pyxidis

The paper presents 1713 photometric measurements of T Pyx from 1966 to 1990. The light curve of T Pyx shows roughly sinusoidal variations with a typical amplitude of 0.09 mag and a time scale of 2 hr. Discrete Fourier transforms of data from individual nights and runs reveal a highly significant periodic modulation. This modulation is proven to be coherent only on time scales shorter than from 6 d to under 1 d. The present period for the modulation is 0.07616 +/- 0.00017 d. Since other stars have variable photometric periods (possibly related to the superhump phenomenon) which are slightly different from the orbital period, an orbital period of about 0.073 d for T Pyx is suggested.

Schaefer, Bradley E.↗
NASA NTRS1992

Eclipse maps of the accretion disk in the classical nova V Persei

The surface brightness distribution in the accretion disk of V Per is reconstructed using a maximum entropy technique. The resulting brightness temperature gradient is much less steep than that from a standard steady state, optically thick disk for all reasonable mass accretion rates. A model including a rim which obscures part of the disk does not resolve this discrepancy. However, if the inner disk is disrupted out to about 0.15-0.25R sub L1 (for example by a magnetic field), the brightness temperature gradient can be very close to T sub b varies as R exp -3/4 (1-b(Rin/R)exp 1/2) law. The value of Rin needed to match this steady state distribution depends on the distance to the system, its size and brightness, and the rotation rate of the white dwarf.

Wood, Janet H.↗