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At least 595 records · Page 33

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NASA NTRS1985

High-speed photometry of the eclipsing dwarf nova OY Carinae

High-speed photometry of the eclipsing dwarf nova OY Car in the quiescent state is presented. OY Car becomes highly reddened during eclipse, with minimum flux colours inconsistent with optically thick emission in the U and B bandpasses. Mass ratios in the range 6.5 to 12 are required to reconcile the eclipse structure with theoretical gas stream trajectories. Primary eclipse timings reveal a significant decrease in the orbital period and the duration of primary eclipse indicates the presence of a luminous ring about the white dwarf. The hotspot eclipse reveals a hotspot which is elongated along the rim of the accretion disc, with optical emission being non-uniformly distributed along the rim. The location of the hotspot in the accretion disc implies a disc radius larger than that of an inviscid disc, with variation in the position of the hotspot being consistent with a fixed stream trajectory.

Cook, M. C.↗

NASA NTRS1985

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 NTRS1986

IUE observations of the dwarf novae OY Car in superoutburst

The May 1985 superoutburst of OY Car, an eclipsing member of the SU UMa subclass of dwarf novae, was observed extensively with IUE to obtain phase-resolved spectra throughout several orbital cycles, when the superhump was well developed and lay close in phase to the eclipse. The UV lines and continuum vary with orbital phase and show a strong UV eclipse coincident with the optical eclipse. This implies that the superhump source at eclipse is not a strong contributor in the ultraviolet. The UV emission line spectrum and lack of X-ray eclipse suggest the presence of an extended coronal region. The flux evolution observed is unusual in that gross changes occur during the decline from superoutburst. The later spectra actually appear hotter than spectra earlier in the decline, and suggest that, early on, the inner disk is obscured from view.

Hassall, B. J. M.↗

NASA NTRS1987

Numerical modelling of the classical nova outburst

A mechanism is described that promises to explain how nova outbursts take place on white dwarf of 1 solar mass or less and for accretion rates of 4 x 10 to the -10 solar mass/yr or greater.

Kutter, G. S.↗

NASA NTRS1987

Reconnection-driven oscillations in dwarf nova disks

A class of oscillations observed during eruption of dwarf novae has been interpreted as oscillations of the accretion disks in these systems.These oscillations are quasi-periodic with coherence times typically between three and 15 cycles. It is shown that magnetic field reconnection at high magnetic Reynolds number can drive disk oscillations. The expected stochastic geometry of disk magnetic fields could naturally produce the observed phase incoherency.

Tajima, T.↗

NASA NTRS1987

IUE observations of the dwarf nova HL Canis Majoris and the winds of cataclysmic variables

An observational and theoretical study is conducted of the P Cygni profiles of cataclysmic variables, with attention to the profiles of the dwarf nova HL CMa, in light of the ionization structure of the wind for a given geometry. It is found that a spherically symmetric wind is capable of generating the observed profile shapes when the accretion disk is limb-darkened, provided that the acceleration of the wind is very low and that the wind represents a mass-loss rate of about 10 to the -11th/C IV ionization fraction of the solar mass per year; a wind of this magnitude, however, cannot be driven by radiation pressure.

Mauche, Christopher W.↗

NASA NTRS1988

The 1985 May superoutburst of the dwarf nova OY Carinae. II - IUE and Exosat observations

Ultraviolet (IUE) and X-ray (Exosat) observations of the eclipsing dwarf nova OY Carinae during the superoutburst of May 1985 are presented. From the lack of X-ray eclipse and UV behavior, it is deduced that the X-ray flux originates in an optically thin corona comparable in size to the Roche lobe, and not directly from the white dwarf or boundary layer. The asymmetric UV line emission originates partly in the accretion disk and partly in a wind. There is a strong modulation of the UV continuum flux that is thought to be caused by extended vertical disk structure shadowing the inner regions.

Naylor, T.↗

NASA NTRS1987

Hydrodynamic studies of oxygen, neon, and magnesium novae

Results are presented from recent theoretical studies that have examined the properties of nova outbursts on ONeMg white dwarfs. These outbursts are much more violent and occur much more frequently than outbursts on CO white dwarfs. Hydrodynamic simulations of both kinds of outbursts are in excellent agreement with the observations.

Starrfield, Sumner↗

NASA NTRS1988

Photometry and spectroscopy of a newly discovered polar - Nova Cygni 1975 (V1500 CYG)

The paper reports photometric and spectroscopic observations which led to the conclusion that Nova Cygni 1975 (V1500 Cyg) is a polar (of AM Her-type).The CCD photometry confirms the constancy of the photometric period which is again interpreted as an orbital cycle. The time-resolved MMT spectra make it possible to reconstruct, under several assumptions, the basic system parameters: M1=0.9M solar mass and M2=0.31M solar mass.

Kaluzny, Janusz↗

NASA NTRS1988

Optical and ultraviolet observations of nova Vul 1987

The outburst for a nova discovered in Nov. 1987 and followed since then is summarized. Although it was possible to observe it with the IUE at maximum, its ultraviolet energy faded rapidly, and after the first 2 weeks it was impossible to observe it at IUE wavelengths. It is observed to form a thick dust shell and is in the nebular stage.

Starrfield, S.↗

NASA NTRS1988

IUE observations of the 1987 superoutburst of the dwarf nova Z Cha

Low resolution IUE observations of the dwarf nova Z Cha during superoutburst are presented. These cover most of the development of the outburst and have sufficient time resolution to probe continuum and line behavior on orbital phase. The observed modulation on this phase is very similar to that observed in the related object OY Car. The results imply the presence of a cool spot on the edge of the edge of the accretion disk, which periodically occults the brighter inner disk. Details of the line behavior suggest that the line originated in an extended wind-emitting region. In contrast to archive spectra obtained in normal outburst, the continuum is fainter and redder, indicating that the entire superoutburst disk may be geometrically thicker than during a normal outburst.

Harlaftis, E.↗

NASA NTRS1989

Analysis of NOVA-1 Doppler data

The intent is to prepare a set of Doppler tracking data for the NOVA-1 Satellite to be included in a data base of satellite tracking data. This data base is to be used in a solution for the gravity field of the Earth. This new gravity field model is needed so that the orbit of the proposed TOPEX (Ocean TOPography EXperiment) satellite can be determined accurately enough for the satellite's missions to be accomplished.

Felsentreger, T. L.↗

NASA NTRS1988

On the outburst recurrence time for the accretion disk limit cycle mechanism in dwarf novae

An attempt is made here to elucidate the dwarf nova outburst time scale in terms of basic physics, and to place limits on the model by using the observed recurrence times. A simple analytical model is developed to describe the limit cycle process, and an expression is obtained for the interval between eruptions. It is shown that the smallness of the amount of mass accreted onto the central white dwarf during eruption can be explained in terms of the propagation of heating and cooling fronts. The dependency of the fraction of the disk mass accreted on the model parameters is derived, and the result is used to construct a semianalytic expression for the recurrence time.

Cannizzo, John K.↗

NASA NTRS1989

Slow expansion of the shell of the recurrent nova T Pyxidis and detection of a faint extended envelope

New H-alpha narrow-based CCD imaging of the recurrent nova T Pyxidis and the detection of a very faint, extended H-alpha halo surrounding the already known shell are reported. A forbidden O III image containing an emitting shell with a morphology different from that of the H-alpha shell is presented, and measurements of the H-alpha shell expansion are reported which rule out the 1966 eruption date for the shell origin, assuming uniform expansion. It is proposed that the observed shell consists of slowly moving, solar abundance ejecta which are photoionized.

Shara, Michael M.↗

NASA NTRS1988

On a polar Nova Cygni 1975 (V1500 Cyg)

Einstein X-ray observations of Nova Cygni 1975 (V1500 Cyg) are reported, including the discovery that this system is a polar. It is suggested that AM Her-type systems (polars) and DQ Her-type systems (intermediate polars) are the extremes of a continuum of magnetic binaries. Such objects as BG CMi, EX Hya and V1500 Cyg and possibly TV Col, AE Aqr, and CP Pup, whose appearance depends on individual properties and geometry, are intermediate between the two classes.

Chlebowski, Tomasz↗

NASA NTRS1989

Evidence for a high-mass white dwarf in nova V1500 Cygni 1975

Time-resolved spectroscopy of V1500 Cyg are presented from 1981, when the nebular lines from the nova ejecta had faded sufficiently to permit reliably radial velocity measurements on emission lines from the binary remnant. The spectra possess a typical AM Herculis signature. The wavelength region 3800-5100 A contains a number of strong lines, in particular the Balmer series and He II 4686 A. All lines show periodic velocity changes with the same period as the photometric variations. On the first night the lines are relatively weak and narrow, are in phase with the photometric cycle, and suffer extreme modulation; two months later they are strong and broad, are roughly 90 deg out of phase with the photometric cycle, and have relatively less variation in their line fluxes. These features support the suggestion that the remnant is an AM Her system. The observed radial velocity of the secondary is 193 + or - 9 km/s. Various constraints suggest that the magnetic white dwarf in this system has a mass over 0.9 solar.

Horne, Keith↗

NASA NTRS1989

The 1989 outburst of V404 Cygni: A very unusual x ray nova

On May 22, 1989, the Japanese Ginga Team discovered a new x ray source that was cataloged as GS 2023+338. This source was subsequently identified as coincident in position with a previously known nova cataloged as V404 Cygni. Its last recorded outburst was in 1938 when it rose to about 12th mag. Spectroscopic data were obtained and confirmed the nature of the outburst. Additional ground based data were obtained by us at CTIO and the VLA. The x ray behavior of this object was reported to be very unusual and it reached a peak of about 17 crab about one week after discovery. Since then it has varied widely in magnitude at all wavelengths at which it has been studied. A brief summary of the observations that were obtained up to the time of the meeting and shortly thereafter is presented.

Wagner, R. M.↗

NASA NTRS1989

Viscous evolution of accretion discs in the quiescence of dwarf novae

Viscous evolution of accretion disks in the quiescence of dwarf novae is investigated semi-analytically. There are two key factors: the inward flow of the initial mass in the disk and the diffusion of the material added into the disk. The results are compared with the brightness temperature distributions obtained by the eclipse mapping of Z Cha and OY Car. The functional form of the viscosity parameter a, its values, the types of outbursts, and the application to soft X-ray transients are discussed.

Mineshige, Shin↗