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At least 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

Radial velocity studies of cataclysmic binaries. II - The ultrashort period dwarf nova T Leonis

Spectroscopic and photometric observations of T Leo which reveal the object to be an ultrashort period dwarf nova are presented. In particular, the analysis indicates that T Leo has an orbital period of 84.69936 (plus or minus 0.00068) minutes, which is the third shortest orbital period of any known cataclysmic variable and the shortest known for any U Gem type dwarf nova. These observations enable the placement of upper limits of 0.19 and 0.4 solar mass on the mass of the red dwarf and white dwarf, respectively. The overall energy distribution is relatively flat (f-sub-lambda approximately constant) which is consistent with a low mass accretion rate. The models of Williams and Ferguson (1982) suggest that the mass accretion rate is about 10 to the -10th solar mass/yr. A narrow peak component of H-alpha is observed, the phasing of which cannot be explained by the canonical hot spot model. Possible explanations for its origin are presented and discussed. It is predicted that future observations of T Leo may reveal the object to be a SU UMa type dwarf nova.

Shafter, A. W.↗
NASA NTRS1984

IUE observations of DQ Herculis and its nebula, and the nature of the cold nova shells

The nebula ejected in the 1934 outburst of the classical nova DQ Her is remarkable for its unprecedentedly low temperature of Te 500 K as measured by Williams et al. (1978). In this paper, IUE observations are combined with Steward optical spectra. It is confirmed that the gas is quite cold. It is further shown that the gas is ionized by the radiation field of the central object. X-ray, ultraviolet, optical and infrared observations of the underlying binary are combined with the extreme-ultraviolet continuum deduced from the level of ionization of the nebula to obtain a composite energy distribution for the central object. This energy distribution bears no resemblance to that predicted by theoretical models of accretion disks. Photoionization models of the nebula using the deduced continuum, as well as theoretical accretion disk continua, are presented to show that the low electron temperature is the result of the very high metal abundances which characterize nova shells. Infrared fine-structure lines are efficient coolants, and low temperatures are achieved for a wide variety of radiation fields. The implications of these results for nebulae surrounding other old novae are discussed.

Ferland, G. J.↗
NASA NTRS1985

Ultraviolet spectral evolution and heavy element abundances in Nova Austrinae 1981

Nova CrA 1981, a moderately fast galactic nova, was systematically observed over a period of seven months with the IUE satellite. The initial evolution of the UV spectrum was similar to that of previously observed novae; however, several months after maximum, weak emission due to very high ionizating forbidden lines of Na, Mg, Al, and Si appeared, eventually decreasing in strength over the next two months. The spectral development of the lines is consistent with the photoionization of an expanding shell; a high-temperature 'coronal' origin is ruled out. Abundances deduced from the spectra show that most of the observed heavy elements in the ejected material are enhanced with respect to helium, with neon the most abundant of these. In addition, there are substantial deviations of the Na/Mg/Al/Si abundance ratios from solar values. This is interpreted as evidence for the mixing of core material with the accreted envelope on a massive O-Ne-Mg (as opposed to a C-O) white dwarf. Furthermore, the proportion of core material in the ejecta implies that the mass of the white dwarf decreased as a consequence of the outburst.

Williams, R. E.↗
NASA NTRS1985

E2000 + 223 - A newly discovered old nova?

The X-ray source, E2000 + 223, serendipitously discovered by the Einstein Observatory, has been tentatively identified with an old nova shell. Spectroscopic observations of the diffuse optical counterpart show a heavily reddened continuum with bright forbidden S II emission lines at double lambda 6716, 6732. Radio observations at the VLA show a weak (about 1.2 mJy) extended radio source, with a nonthermal spectrum, concident with the optical emission. The observations can be best explained by assuming the object is an old nova which erupted sometime in the eighteenth or nineteenth century, similar to GK Per (Nova 1901) but at a distance of 1.4 kpc.

Takalo, L. O.↗
NASA NTRS1985

Hydrodynamic models for novae with ejecta rich in oxygen, neon and magnesium

The characteristics of a new class of novae are identified and explained. This class consists of those objects that have been observed to eject material rich in oxygen, neon, magnesium, and aluminum at high velocities. We propose that for this class of novae the outburst is occurring not on a carbon-oxygen white dwarf but on an oxygen-neon-magnesium white dwarf which has evolved from a star which had a main sequence mass of approx. 8 solar masses to approx. 12 solar masses. An outburst was simulated by evolving 1.25 solar mass white dwarfs accreting hydrogen rich material at various rates. The effective enrichment of the envelope by ONeMg material from the core is simulated by enhancing oxygen in the accreted layers. The resulting evolutionary sequences can eject the entire accreted envelope plus core material at high velocities. They can also become super-Eddington at maximum bolometric luminosity. The expected frequency of such events (approx. 1/4) is in good agreement with the observed numbers of these novae.

Starrfield, S.↗
NASA NTRS1986

A review of the ultraviolet studies of galactic novae

The IUE contribution to studies of galactic novae in quiescence and in outburst are reviewed. A class of novae in which the outburst occurs on an ONeMg white dwarf was identified. Ejected masses, elemental abundances, and ultraviolet light curves for a wide variety of nova outbursts were determined.

Starrfield, Sumner↗
NASA NTRS1988

Gamma-ray limits on Na-22 production in novae

Data accumulated from 1980 to 1987 by the gamma-ray spectrometer aboard the SMM are analyzed to search for annual increases at 1.275 MeV as evidence of Na-22 decay in the Galactic center region and in the ejecta of recent individual novae. No evidence of any 1.275 MeV emission of celestial origin is found. A limit of 3 x 10 to the -6th solar mass is placed on the accumulated Na-22 from many novae occurring near the Galactic center, and a limit of 7 x 10 to the -7th solar mass is placed on the mass of Na-22 ejected by the closest of the recent neon-rich novae.

Leising, Mark D.↗
NASA NTRS1988

The analysis of spectra of novae taken near maximum

A project to analyze ultraviolet spectra of novae obtained at or near maximum optical light is presented. These spectra are characterized by a relatively cool continuum with superimposed permitted emission lines from ions such as Fe II, Mg II, and Si II. Spectra obtained late in the outburst show only emission lines from highly ionized species and in many cases these are forbidden lines. The ultraviolet data will be used with calculations of spherical, expanding, stellar atmospheres for novae to determine elemental abundances by spectral line synthesis. This method is extremely sensitive to the abundances and completely independent of the nebular analyses usually used to obtain novae abundances.

Stryker, L. L.↗
NASA NTRS1988

Observations of classical novae in outburst

The IUE obtained ultraviolet data on novae in outburst. The characteristics of every one of the outbursts are different. Optical and infrared data on many of the same novae were also obtained. Three members of the carbon-oxygen class of novae are presented.

Starrfield, S.↗
NASA NTRS1988

The classical nova outburst

The astrophysics of outbursts in classical and recurrent novae is explored, summarizing and analyzing the results of ground-based and space observations obtained in different regions of the electromagnetic spectrum. The stages of fast and slow outbursts are reviewed; the thermonuclear processes involved are described; and some observations of recent novae are listed and briefly characterized. Particular attention is then given to the specific contributions of optical, UV, IR, radio, and X-ray studies of nova outbursts.

Starrfield, Sumner↗
NASA NTRS1989

Modeling the classical nova outburst. I - Exploring the physics of a new mechanism

Model calculations were performed to describe a mechanism that produces classical nova outbursts on white dwarfs of 1 solar mass or less and for accretion rates of 4 x 10 to the -10th solar mass/yr or greater, i.e., the parameters corresponding to observed data of nova systems. Calculations point to four factors that can induce nuclear runaways of sufficient strength to eject about 0.0001 solar mass at speeds of several hundred to a few thousand km per second, as is observed in classical novae. These are (1) the effects of storage of angular momentum in the star's envelope during the accretion phase; (2) the reduction of centrifugal forces in the star's outer layers during the early nuclear runaway phase, through the inward transport of angular momentum; (3) the inward movement of the zone of peak nuclear burning through the convectively induced shear instability during the runaway phase; and (4) the mixing of original CO stellar matter and H-rich matter, also through the convectively induced shear instability.

Kutter, G. Siegfried↗
NASA NTRS1991

PW Vulpeculae - A nova with nearly solar abundances

Optical spectrophotometry of PW Vulpeculae is combined with ultraviolet data to estimate electron temperatures, densities, and abundances in the ejecta of this slow classical nova. The reddening, the distance, and the evolution of the ultraviolet spectrum are discussed. Abundances are nearly solar, with the exception of nitrogen, which is substantially higher. Although neon has been reported to be enhanced in several novae, it does not seem to be the case for PW Vul. Photoionization model calculations of the ejecta that give a reasonable match of the observed emission spectrum are presented. A strong featureless continuum shows that very hot, presumably shock-heated, gas plays a major role in determining the energetics of this nova. Emission from this hot gas is responsible for the ionization of the nebular gas. A calculation of the masses of both the hot coronal gas and the cooler nebular gas shows that the former may account for most of the mass of the ejecta.

Saizar, Pedro↗
NASA NTRS1992

Evidence for a black hole in the X-ray binary Nova Muscae 1991

Optical photometry and spectroscopy of the X-ray Nova Muscae 1991 in quiescence reveal an orbital period of 10.398 +/- 0.014 hr and an absorption-line velocity curve consistent with a sinusoidal modulation at a half-amplitude of 409 +/- 18 km/s. The spectral type of the secondary star is in the range K0 V to K4 V. The value of the mass function, 3.1 +/- 0.4 solar mass, is a conservative lower limit on the mass of the compact primary and suggests that the primary is a black hole. Further considerations of the binary inclination angle and the mass of the secondary strengthen the black hole model. The folded light curves in the I band and the B + V band resemble ellipsoidal variations, with an additional brightening near one of the maxima in the B + V band. The orbital period is 1.4 percent shorter than the photometric period observed during outburst, as expected if the outburst modulations are analogs of 'superhumps' in dwarf novae. In quiescence, the optical properties of the X-ray binary Nova Muscae 1991 bear a striking resemblance to the black hole binary A0620-00, which extends the basis of similarity that was demonstrated during outburst at X-ray and optical wavelengths.

Remillard, Ronald A.↗
NASA NTRS1992

The benefit of amateur observations for research in dwarf novae

Contributions of amateur astronomers to research on dwarf novae, which are based on carefully monitoring the outburst behavior of these objects, are reviewed. These contributions range from scheduling of observations to the observational basis for research on the dwarf nova outburst mechanism. It is suggested, that, with better equipment, observations of orbital light variations in dwarf novae might be performed by amateur astronomers.

La Dous, Constanze↗
NASA NTRS1992

Coordination of multiwavelength groundbased and satellite observations of novae in outburst

Observational data on novae in outburst are reviewed, focusing on the information gained from ultraviolet spectroscopy. Topics discussed include a brief taxonomy of novae, the theoretical framework, observational diagnostics of the outburst, IUE satellite observations, and future trends in novae observations at many wavelengths using space-based observatories to augment the work of ground-based observers.

Shore, Steven N.↗
NASA NTRS1993

Nova dust nucleation - Kinetics and photodissociation

Dust is observed to form in nova ejecta. The grain temperature is determined by the diluted nova radiation field rather than the gas kinetic temperature, making classical nucleation theory inapplicable. We used kinetic equations to calculate the growth of carbon nuclei in these ejecta. For expected values of the parameters too many clusters grew, despite the small sticking probability of atoms to small clusters, and the clusters only reached radii of about 100 A when the carbon vapor was depleted. We then included the effects of cluster photodissociation by ultraviolet radiation from the nova. This suppresses nucleation, but too well, and no grains form at all. Finally we suggest that a few growing carbon nuclei may be protected from photodissociation by a sacrificial surface layer of hydrogen.

Johnson, D. J.↗
NASA NTRS1994

Disk irradiation and light curves of x ray novae

We study the disk instability and the effect of irradiation on outbursts in the black hole X-ray nova system. In both the optical and soft X-rays, the light curves of several X-ray novae, A0620-00, GH 2000+25, Nova Muscae 1991 (GS 1124-68), and GRO J0422+32, show a main peak, a phase of exponential decline, a secondary maximum or reflare, and a final bump in the late decay followed by a rapid decline. Basic disk thermal limit cycle instabilities can account for the rapid rise and overall decline, but not the reflare and final bump. The rise time of the reflare, about 10 days, is too short to represent a viscous time, so this event is unlikely to be due to increased mass flow from the companion star. We explore the possibility that irradiation by X-rays produced in the inner disk can produce these secondary effects by enhancing the mass flow rate within the disk. Two plausible mechanisms of irradiation of the disk are considered: direct irradiation from the inner hot disk and reflected radiation from a corona or other structure above the disk. Both of these processes will be time dependent in the context of the disk instability model and result in more complex time-dependent behavior of the disk structure. We test both disk instability and mass transfer burst models for the secondary flares in the presence of irradiation.

Kim, S.-W.↗
NASA NTRS1994

The early spectral evolution of Nova Cassiopeiae 1993

In this paper we describe the evolution of Nova Cas 1993 over the first two months of its outburst. We present an ultraviolet light curve that covers the period from announcement to just after dust began forming in the ejecta (1994 Feb. 15) and International Ultraviolet Explorer (IUE) spacecraft constraints forced us to halt our observations. We have used spherical, expanding, Non-local Thermodynamic Equilibrium (NLTE) stellar atmospheres to compute synthetic spectra and have compared the results to combined ultraviolet (low-resolution 1200-3400 A and high-resolution 2400-3300 A) spectra. Our fits show that the effective temperature of the ejecta increased from approx. 8000 to about approx. 16 000 K between 1993 Dec. 12 and 1993 Dec. 26. The temperature then increased more slowly to approx. 24 000 on 1994 Jan. 28. A preliminary abundance analysis shows evidence for hydrogen depletion, as we also found for Nova V1974 Cygni; however we find a larger enhancement of carbon, nitrogen, and oxygen. We also show that the principal mechanism for mass ejection in this nova is a radiation pressure driven wind and that mechanical driving is not necessary.

Hauschildt, Peter H.↗