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At least 1,495 records · Page 83
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NASA NTRS1988

CCD time-resolved photometry of faint cataclysmic variables. I

CCD time-resolved V and B differential light curves are presented for the dwarf novae AR And, FS Aur, TT Boo, UZ Boo, AF Cam, AL Com, AW Gem, X Leo, RZ Leo, CW Mon, SW UMa, and TW Vir. The time-series observations ranged from 2 to 6 hours and have accuracies of 0.025 mag or better for the majority of the runs. Except for AR And, X Leo, CW Mon, and TW Vir, the periods are below the cataclysmic-variable period gap (about 2 hours), and the systems are potential SU UMa stars. Photometric periods for five of the stars are the first such determinations, while those for the other seven generally confirm previous spectroscopic or photometric observations. In several cases, the photometric modulations are large amplitude (up to 0.5 mag). The results on AL Com and SW UMa indicate they may be magnetic variables.

Howell, Steve↗
NASA NTRS1988

Exosat and IUE observations of SW UMa during superoutburst

IUE and Exosat observations obtained during the March 1986 superoutburst of the SU UMa type dwarf nova SW UMa are presented. The observations show the presence of a hot, extensive, optically thick disk. The UV fluxes show an increase of 1000 over the quiescent values, implying a large increase in mass accretion rate. The Exosat obserations show an increase in the soft X-ray count rate by a factor of 44 over quiescence, while the medium-energy flux remains undetected. The most surprising result is that the softness ratio in superoutburst is similar to the quiescent value of 0.3. No variation at the orbital or spin periods or at the superhump period are evident in the superoutburst X-ray data. A transient variation is observed at 5.1 minutes similar to that observed in the optical a few days after the X-ray observations.

Szkody, Paula↗
NASA NTRS1988

Mass mortality and extraterrestrial impacts

The discovery of iridium enrichment at the Cretaceous/Tertiary boundary resulted in formulation of hypothesis of a cometary or asteroid impact as the cause of the biological extinctions at this boundary. Subsequent discoveries of geochemical anomalies at major stratigraphic boundaries like the Precambrian/Cambrian, Permian/Triassic, Middle/Late Jurassic, resulted in the application of similar extraterrestrial impact theories to explain biological changes at these boundaries. Until recently the major physical evidence, as is the location of the impact crater site, to test the impact induced biological extinction was lacking. The diameter of such a crater would be in the range of 60 to 100 km. The recent discovery of the first impact crater in the ocean provide the first opportunity to test the above theory. The crater, named Montagnais and located on the outer shelf off Nova Scotia, Canada, has a minimum diameter of 42 km, with some evidence to a diameter of more than 60 km. At the Montagnais impact site, micropaleontological analysis of the uppermost 80 m of the fall-back breccia represented by a mixture of pre-impact sediments and basement rocks which fills the crater and of the basal 50 m of post-impact marine sediments which overly the impact deposits, revealed presence of diversified foraminiferal and nannoplankton assemblages. The sediments which are intercalated within the uppermost part of the fall-back breccia, had to be deposited before the meteorite impact. The post-impact deposits were laid down almost immediately after the impact as also supported by the micropaleontological data. In conclusion, micropaleontological studies of sediments from the first submarine impact crater site identified in the ocean did not reveal any mass extinction or significant biological changes at the impact site or in the proximal deep ocean basin.

Jansa, L. F.↗
NASA NTRS1988

Analysis of IUE data on V426 Ophiuchi - Outburst and orbital variability

Time-resolved IUE spectra throughout two orbits of the dwarf nova V426 Oph during quiescence are presented. It is found that the visual and C IV fluxes remain relatively constant during both orbits, whereas the UV coninua and Mg II lines vary by 20-30 percent. The time scales of this variability are discussed. Analysis of other spectra at brighter optical magnitudes shows that the UV lags the optical during a probable rise to outburst, similar to the behavior of SS Cyg. The reddening of V426 Oph is determined to be E(B-V) = 0.1. The contribution of the white dwarf to the UV flux at quiescence is estimated to be 25 percent, based on the system parameters of Hessman (1988) and the period-temperature relation of Sion (1987).

Szkody, Paula↗
NASA NTRS1987

UV flux distributions of novalike variables

Previous estimates of distance, reddening, orbital inclination, and white dwarf mass for a sample of nine nova-like cataclysmic variables are presented, along with their representative IUE fluxes. Blackbody and Kurucz (1979) model atmosphere fluxes are used to construct sequences of model accretion disk spectra. It is found that the Kurucz-type accretion disk models do not reproduce simultaneously the UV flux level and color.

Wade, Richard A.↗
NASA NTRS1988

Quasiperiodicity in cataclysmic variable stars caused by solar-type magnetic cycles

Cyclical variations of orbital periods, quiescent magnitudes and outburst intervals in the activity of cataclysmic variable binary stars are inter-related and are ascribed to variations in radii of the secondaries, caused by solar-type (sunspot) magnetic cycles. In the nova remnant DQ Herculis the observed variations in orbital period and quiescent magnitude are consistent with this mechanism. But accretion onto the white dwarf, from an accretion disk acquired from its companion, cannot explain the observed variation of the 71-second oscillations.

Warner, Brian↗
NASA NTRS1989

On the stream-accretion disk interaction - Response to increased mass transfer rate

The time-dependent interaction between the stream of mass from the inner Lagrangian point and the accretion disk, resulting from an increasing mass transfer rate is calculated. The calculation is fully three-dimensional, using a pseudoparticle description of the hydrodynamics. It is demonstrated that the results of such calculations, when combined with specific observations, have the potential of both determining essential parameters, such as the viscosity parameter alpha, and can distinguish between different models of dwarf nova eruptions.

Dgani, Ruth↗
NASA NTRS1988

Static stability variations during a winter marine cyclone development

The relationship between lower tropospheric static stability and a case of explosive marine cyclone development is examined. The case studied is a cyclone which developed between January 18-21, 1979, about 500 km south of Nova Scotia. Diagnoses of the static stability parameter for this case are presented. Also, the static stability parameters and sea level pressures during marine cyclone development and continental development are compared.

Tsou, Chih-Hua↗
NASA NTRS1989

Thermal light variations of active galactic nuclei

Thermal limit cycles in accretion disks are a promising explanation of the quasi-periodic outbursts of dwarf novae. A similar mechanism may lead to observable light variations in active galactic nuclei (AGNs) at wavelengths 1-5 microns. The variations should obey a characteristic period-luminosity relation. For low-luminosity AGNs, these variations could have a period of less than about 10 yr.

Clarke, C. J.↗
NASA NTRS1988

Cataclysmic variables

Recent observations of cataclysmic variables (CVs) at different wavelengths are reviewed, with a focus on their implications for theoretical models. Consideration is given to disk CVs (the flux distribution of the disk and changes during dwarf-nova outbursts), magnetic CVs (flux distributions and components), and the underlying stars. Typical data are presented in graphs, tables, and sample spectra, and it is concluded that more detailed multiwavelength observations are needed to improve models of radiative transfer and viscosity effects in accretion disks.

Szkody, Paula↗
NASA NTRS1989

Discovery of a cool expanding shell at -1200 kilometers per second around V471 Tauri

High-resolution IUE spectra of V471 Tauri reveal the presence of a very-high-velocity cool expanding gas in the line of sight to the binary system with an expansion velocity of -1200 km/s. The summed strength of the coadded absorption is 125 mA + or - 25 mA, with FWHM = 30 km/s. It is suggested that the observed absorption may be related to the narrow coadded absorption at -590 km/s noted by Bruhweiler and Sion (1966). The large expansion velocity suggests a possible association with an ancient nova outburst.

Sion, Edward M.↗
NASA NTRS1989

Meteoritic silicon carbide and its stellar sources - Implications for galactic chemical evolution

Interstellar silicon carbide grains in meteorites provide a novel means for studying the carbon-star population of about 5 x 10 to the 9th years ago. Their C-12/C-13 ratios differ greatly from the solar value but resemble those of present-day csrbon stars, implying little change in the galactic C-13 inventory. Isotope data on nitrogen and silicon suggest that the silicon carbide grains come mainly from red giants, with small contributions from novae.

Ming, Tang↗
NASA NTRS1989

Dust clouds around red giant stars - Evidence of sublimating comet disks?

The dust production by disk comets around intermediate mass stars evolving into red giants is studied, focusing on AGB supergiants. The model of Iben and Renzini (1983) is used to study the observed dust mass loss for AGB stars. An expression is obtained for the comet disk net dust production rate and values of the radius and black body temperature corresponding to peak sublimation are calculated for a range of stellar masses. Also, the fractional amount of dust released from a cometesimal disk during a classical nova outburst is estimated.

Matese, John J.↗
NASA NTRS1989

Computer simulation of dust grain evolution

The latest results are reported from a Monte Carlo code that is being developed at NASA Ames. The goal of this program, is to derive from the observed and presumed properties of the interstellar medium (ISM) the following information: (1) the size spectrum of interstellar dust; (2) the chemical structure of interstellar dust; (3) interstellar abundances; and (4) the lifetime of a dust grain in the ISM. Presently this study is restricted to refractory interstellar material, i.e., the formation and destruction of ices are not included in the program. The program is embedded in an analytic solution for the bulk chemical evolution of a two-phase interstellar medium in which stars are born in molecular clouds, but new nucleosynthesis products and stellar return are entered into a complementary intercloud medium. The well-mixed matter of each interstellar phase is repeatedly cycled stochastically through the complementary phase and back. Refractory dust is created by thermal condensation as stellar matter flows away from sites of nucleosynthesis such as novae and supernovae and/or from the matter returned from evolved intermediate stars. The history of each particle is traced by standard Monte Carlo techniques as it is sputtered and fragmented by supernova shock waves in the intercloud medium. It also accretes an amorphous mantle of gaseous refractory atoms when its local medium joins with the molecular cloud medium. Finally it encounters the possibility of astration (destruction by star formation) within the molecular clouds.

Liffman, K.↗
NASA NTRS1989

Stratospheric Observatory for Infrared Astronomy (SOFIA) science rationale

SOFIA, a proposed 3-meter class telescope in a Boeing 747 aircraft, would have the ability to make astronomical observations over a wavelength range from 0.3 microns to 1.6mm. Relative to the KAO (Kuiper Airborne Observatory) the larger telescope on SOFIA would provide a factor of 10 improvement in sensitivity for compact sources and a factor of 3 improvement in (diffraction-limited) angular resolution at wavelengths beyond 30 microns. In addition, SOFIA will retain the major features of the KAO which have made the airborne astronomy program so successful. Among these are continuous in-flight access to focal plane instruments while flying at or above 41,000 ft altitude; pointing stability of 0.2 arcseconds; and mobility and scheduling flexibility to accommodate targets of opportunity such as comets, eclipses, occultations, and novae.

Davidson, Jacqueline A.↗
NASA NTRS1989

WHT observations of cataclysmic variables in globular clusters. I - V101 in M5 during quiescence and outburst

WHT spectroscopy and photometry are used to investigate the structure of the cataclysmic vaiable V101, in the globular cluster M5. An encouraging consistency is found between the properties derived from the color, magnitude, radial-velocity variations and outburst light curve. All these indicate that V101's orbital period lies above the period gap, is at the distance of M5, and is probably a dwarf nova.

Naylor, T.↗
NASA NTRS1990

The role of synoptic/planetary-scale interactions during the development of a blocking anticyclone

Consideration is given to the development of a blocking anticyclone over the North Atlantic preceded by explosive cyclogenesis about 500 km south of Nova Scotia during January 19-21, 1979. The general behavior of the block is studied using the extended height tendency equation. The relative importance of planetary-scale, synoptic-scale, and scale-interaction forcing of the block is examined.

Tsou, Chih-Hua↗
NASA NTRS1990

The Fe XI-excited fluorescent cascade in Ne IV

A theoretical spectroscopic investigation of the fluorescent cascade to be expected in nitrogen-like Ne IV, when it is resonantly photoexcited by Fe XI in the solar atmosphere, symbiotic stars, or novae, is described. Primary and secondary cascade intensities are obtained as functions of photoexcitation rate, and expected absolute intensities in the solar atmosphere are derived on the basis of observed Fe XI and Ne EUV emission. Comparisons between the spatially resolved solar situation and spatially unresolved stellar cases of cataclysmic variables, in which these ions have been found to coexist, are made. An interesting possibility of periodic time-dependent fluorescence exists for the binary cataclysmics.

Bhatia, A. K.↗