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At least 397 records · Page 22
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NASA NTRS2010

Metal Abundances in the Hottest Known DO White Dwarf (KPD 0005+5106)

We performed a new analysis of UV and optical spectra of KPD 0005+5106. We find T(sub eff) = 200000 +/- 20000 K, log(g) = 6.7+/-0.3, M =.64 Stellar mass and logL/L = 3.7. The mass fractions of the metals are in the range 0.7 - 4.3 times solar. This abundance pattern is probably unaffected by gravitational settling and radiative levitation, hence, its origin lies in previous evolutionary stages. We speculate about a link of KPD 0005+5106 to the RCrB stars and its possible outcome of a double-degenerate merger event. Keywords: White dwarfs, atmospheres, abundances

Wassermann, Daniel↗
NASA NTRS1992

Possible observational consequences of primordial binaries in globular clusters

The paper investigates the observational consequences of the evolution of a primordial binary population in a globular cluster. For those binary systems which begin mass transfer at orbital periods of less than about 1 d, the system is expected to evolve to a contact binary stage and to eventually merge to form a single star. At longer orbital periods, the binary system will enter into a common envelope stage. For periods less than about 10 d, the system is likely to merge, and the outcome of the evolution is a single red giant star. For systems characterized by orbital periods greater than about 15 d, a detached red dwarf-white dwarf binary system is expected to change. As a consequence of the mergers, a range in masses for the red giant stars is possible, leading to a color spread on the RGB of the color magnitude diagram. The luminosity function of stars at high luminosities is expected to steepen as a result of tidal interaction in the binary. It is suggested that the formation of cataclysmic variables via primordial binaries is expected only in loosely bound clusters of low-velocity dispersion.

Taam, Ronald E.↗
NASA NTRS1986

KPD 2146+4117 - The coolest DBA white dwarf

When Wickramasinghe and Whelan (1977) discovered the first DBA white dwarf LDS 785A, the possibility was considered that this object represented an exotic oddity. The fact that Shipman et al. (1985) have discovered five additional objects of this type suggests that the DBA stars are in fact a real class of degenerate object. This paper is concerned with the DBA white dwarf KPD 2146+4117, which was observed spectroscopically in October 1982 on a 1.5-m telescope with an image-tube scanner. The spectrum, obtained with 13 A resolution, covered the 3900-6800 A range. It reveals prominent absorption lines of both hydrogen and He I. Methods of analysis are discussed.

Downes, R. A.↗
NASA NTRS1980

IUE observations of circumstellar emission from the late type variable R Aquarii /M7 + pec/

IUE observations of R Aquarii (M7 + pec) have been obtained in low dispersion in order to study its circumstellar emission. Strong permitted, semiforbidden, and forbidden emission lines are identified that are superposed on a bright ultraviolet continuum. From the analysis it is deduced that the strong emission-line spectrum that involves semiforbidden C III, C IV, semiforbidden Si III, forbidden O II, and forbidden O III probably arises from a dense compact nebula the size of which is comparable to the binary system of which R Aqr is the primary star. Low-excitation emission lines of Fe II, Mg II, O I, and Si II suggest the presence of a warm chromosphere (T less than about 10,000 K) in the primary M7 late type giant. The secondary is identified as a white dwarf, comparable to or somewhat brighter than the sun, since such a star can produce enough ionizing photons to excite the continuum and emission-line spectrum and yet be sufficiently faint to escape detection by direct observation. The UV continuum observed is attributed to Balmer recombination and not to blackbody emission from the hot companion. The general spectral properties of R Aqr between 1200 A and 3200 A are discussed in the context of the model for the circumstellar nebula, the companion, and the mass-loss rate of the primary star.

Michalitsianos, A. G.↗
NASA NTRS1996

Physics of systems containing neutron stars

This grant dealt with several topics related to the dynamics of systems containing a compact object. Most of the research dealt with systems containing Neutron Stars (NS's), but a Black Hole (BH) or a White Dwarf (WD) in situations relevant to NS systems were also addressed. Among the systems were isolated regular pulsars, Millisecond Pulsars (MSP's) that are either Single (SMP's) or in a binary (BMP's), Low Mass X-Ray Binaries (LMXB's) and Cataclysmic Variables (CV's). Also dealt with was one aspect of NS structure, namely NS superfluidity. A large fraction of the research dealt with irradiation-driven winds from companions which turned out to be of importance in the evolution of LMXB's and MSP's, be they SMP's or BMP's. While their role during LMXB evolution (i.e. during the accretion phase) is not yet clear, they may play an important role in turning BMP's into SMP's and also in bringing about the formation of planets around MSP's. Work was concentrated on the following four problems: The Windy Pulsar B197+20 and its Evolution; Wind 'Echoes' in Tight Binaries; Post Nova X-ray Emission in CV's; and Dynamics of Pinned Superfluids in Neutron Stars.

Ruderman, Malvin↗
NASA NTRS1992

Gamma-ray bursts and cosmic rays from accretion-induced collapse

It is shown that the birth of naked or nearly naked neutron stars in accretion-induced collapse or in the bare collapse of white dwarfs can produce cosmological gamma-ray bursts and can provide the required injection rate of cosmic rays into the interstellar space. It is estimated that most of the e(+)e(-) pairs annihilate in flight on a short time scale in the vicinity of the neutron star. It is shown that the gamma-ray bursts, the 0.511 MeV Galactic annihilation radiation, and the cosmic rays exclude the possibility that the large uncertainties in the Galactic pulsar birthrate and the Galactic SN II explosion rate would allow a significant contribution to the pulsar birthrate from naked or nearly naked neutron star formation. The upper bound on the Galactic birthrate of naked or nearly naked neutron stars of less than 1 in 1000 yr makes it very unlikely that a neutrino burst unaccompanied by optical emission from the birth of a naked or nearly naked neutron star will be detected in the near future by underground neutrino telescopes.

Dar, Arnon↗
NASA NTRS1984

VW Hyi - The white dwarf revealed

Nonsimultaneous IUE, optical, and near-IR observations of VW Hyi at quiescence are presented. Using these and UV data from other investigations, a broad feature in the ultraviolet is identified with L-alpha absorption. The presence and width of the line imply that (1) the white dwarf in VW Hyi is directly visible in the UV and (2) the effective temperature of this star is approximately 18,000 + or - 2000 K for log g = 8. The continuum observations, combined with the J and K photometry of Sherrington et al., (1980), can be fit with a combination of this relatively cool white dwarf and a steady-state disk model with an accretion rate of 10 to the -11th solar masses/yr. Additional observations of the hump in the optical light curve can be reasonably fit by a 12,000-K blackbody. Such a source is consistent with the hump being a minor contribution to the system's overall continuum distribution shortward of 2000 A and longward of about 1 micron.

Mateo, M.↗
NASA NTRS2014

Stellar Laboratories: 3. New Ba 5, Ba 6, and Ba 7 Oscillator Strengths and the Barium Abundance in the Hot White Dwarfs G191-B2B and RE 0503-289

Context. For the spectral analysis of high-resolution and high-signal-to-noise (S/N) spectra of hot stars, state-of-the-art non-local thermodynamic equilibrium (NLTE) model atmospheres are mandatory. These are strongly dependent on the reliability of the atomic data that is used for their calculation. Aims. Reliable Ba 5-7 oscillator strengths are used to identify Ba lines in the spectra of the DA-type white dwarf G191−B2B and the DO-type white dwarf RE 0503−289 and to determine their photospheric Ba abundances. Methods. We newly calculated Ba v-vii oscillator strengths to consider their radiative and collisional bound-bound transitions in detail in our NLTE stellar-atmosphere models for the analysis of Ba lines exhibited in high-resolution and high-S/N UV observations of G191−B2B and RE 0503−289. Results. For the first time, we identified highly ionized Ba in the spectra of hot white dwarfs. We detected Ba vi and Ba vii lines in the Far Ultraviolet Spectroscopic Explorer (FUSE) spectrum of RE 0503−289. The Ba vi/Ba vii ionization equilibrium is well reproduced with the previously determined effective temperature of 70 000 K and surface gravity of log g=7.5. The Ba abundance is 3.5 +/- 0.5 × 10(exp−4) (mass fraction, about 23 000 times the solar value). In the FUSE spectrum of G191−B2B, we identified the strongest Ba vii line (at 993.41 Å) only, and determined a Ba abundance of 4.0 +/- 0.5 × 10(exp−6) (about 265 times solar). Conclusions. Reliable measurements and calculations of atomic data are a pre-requisite for stellar-atmosphere modeling. Observed Ba vi-vii line profiles in two white dwarfs' (G191−B2B and RE 0503−289) far-ultraviolet spectra were well reproduced with our newly calculated oscillator strengths. This allowed to determine the photospheric Ba abundance of these two stars precisely.

New Ba V↗
NASA NTRS2001

The Origin of Hot Subluminous Horizontal-Branch Stars in (omega) Centauri and NGC 2808

Hot subluminous stars lying up to 0.7 mag below the extreme horizontal branch (EHB) are found in the ultraviolet (UV) color magnitude diagrams of both (omega) Cen and NGC 2808. In order to explore the evolutionary status of these subluminous stars, we have evolved a set of low-mass stars continuously from the main sequence through the helium-core flash to the HB (horizontal branch) for a wide range in the mass loss along the red-giant branch (RGB). Stars with the largest mass loss evolve off the RGB to high effective temperatures before igniting helium in their cores. Our results indicate that the subluminous EHB stars, as well as the gap within the EHB of NGC 2808, can be explained if these stars undergo a late helium-core flash while descending the white-dwarf cooling curve. Under these conditions the convection zone produced by the helium flash will penetrate into the stellar envelope, thereby mixing most, if not all, of the envelope hydrogen into the hot helium-burning interior, where it is rapidly consumed. This phenomenon is analogous to the 'born-again' scenario for producing hydrogen-deficient stars following a very late helium-shell flash. This 'flash mixing' of the stellar envelope greatly enhances the envelope helium and carbon abundances and, as a result, leads to a discontinuous jump in the HB effective temperature. We argue that the EHB gap in NGC 2808 is associated with this theoretically predicted dichotomy in the HB morphology. Using new helium- and carbon-rich stellar atmospheres, we show that these changes in the envelope abundances of the flash-mixed stars will suppress the UV flux by the amount needed to explain the hot subluminous EHB stars in (omega) Cen and NGC 2808. Moreover, we demonstrate that models without flash mixing lie, at most, only approximately 0.1 mag below the EHB, and hence fail to explain the observations. Flash mixing may also provide a new evolutionary channel for producing the high gravity, helium-rich sdO and sdB stars.

Sweigart, Allen V.↗
NASA NTRS2002

A New Formation Mechanism for the Hottest Horizontal-Branch Stars

Hot subluminous stars lying up to 0.7 mag below the extreme horizontal branch (EHB) are found in the ultraviolet color-magnitude diagrams (CMDs) of both omega Cen and NGC 2808. In order to investigate the origin of these subluminous stars, we have constructed a detailed set of evolutionary sequences that follow the evolution of low-mass stars continuously from the zero-age main sequence through the helium-core flash to the HB for a wide range in the mass loss along the red-giant branch (RGB). Stars with the largest mass loss evolve off the RGB to high effective temperatures before igniting helium in their cores. Our results indicate that the subluminous EHB stars, as well as the high temperature gap along the EHB of NGC 2808, can be explained if these stars undergo a late helium-core flash while descending the white-dwarf cooling curve. Under these conditions the convection zone produced by the main helium flash will penetrate into the stellar envelope, thereby mixing most, if not all, of the envelope hydrogen into the hot helium-burning interior, where it is rapidly consumed. This phenomenon is analogous to the 'born-again' scenario for producing hydrogen-deficient stars following a very late helium-shell flash. This 'flash mixing' of the envelope during a late helium-core flash greatly enhances the envelope helium and carbon abundances and, as a result, leads to a discontinuous increase in the HB effective temperature. We argue that the hot HB gap observed in NGC 2808 is associated with this theoretically predicted dichotomy in the RB properties. Using new helium- and carbon-rich stellar atmospheres, we show that the changes in the envelope abundances due to flash mixing will suppress the ultraviolet flux in the spectra of hot EHB stars. We suggest that such changes in the emergent spectral energy distribution are primarily responsible for explaining the hot subluminous EHB stars in omega Cen and NGC 2808. Moreover, we demonstrate that models without flash mixing lie, at most, only approximately 0.1 mag below the EHB, and hence fail to explain the observations.

Sweigart, Allen V.↗
NASA NTRS1995

ROSAT Pointed Observations of Cool Magnetic White Dwarfs

Observational evidence for the existence of a chromosphere on the cool magnetic white dwarf GD 356 has been reported. In addition, there has been theoretical speculations that cool magnetic white dwarfs may be sources of coronal X-ray emission. This emission, if it exists, would be distinct from the two types of X-ray emission (deep photospheric and shocked wind) that have already been observed from hot white dwarfs. We have used the PSPC instrument on ROSAT to observe three of the most prominent DA white dwarf candidates for coronal X-ray emission: GD 356, KUV 2316+123, and GD 90. The data show no significant emission for these stars. The derived upper limits for the X-ray luminosities provide constraints for a revision of current theories of the generation of nonradiative energy in white dwarfs.

Musielak, Z. E.↗
NASA NTRS1981

The interstellar medium and the highly ionized species observed in the spectrum of the nearby white dwarf G191-B2B

High-resolution spectra of the nearby (48 pc) white dwarf G191-B2B, obtained with the International Ultraviolet Explorer, reveal sharp resonance lines of N V, C IV, and Si IV. The origin of these features is most likely linked to the white dwarf, possibly being formed in an expanding halo around the star. Interstellar lines of C II, N I, Mg II, Si II, and Fe II are also seen in the spectrum. Analysis of these features indicates an average neutral hydrogen number density of 0.064 for this line of sight. In combination with the recent EUV and soft X-ray results, this is interpreted to mean that the interstellar medium in the most immediate solar vicinity is of the normal density n approximately equal to 0.1/cu cm of lower ionization, while just beyond it, at least in some directions, is a hot lower density plasma. These results are apparently in conflict with the model of the interstellar medium by McKee and Ostriker (1977) in its present form.

Bruhweiler, F. C.↗
NASA NTRS1982

A first look at the eclipsing cataclysmic variable Lanning 10

Photometry and spectroscopy of a new cataclysmic variable star, Lanning 10 are presented. The star is shown to be an eclipsing binary with a 7h 42m orbital period. Blue flickering is observed to occur on a time scale of 10 minutes. The light curve is analyzed with a numerical simulation of the eclipse of the accretion disk by the lobe-filling star. The disk nearly spans the Roche lobe of the white dwarf, and its surface brightness is a steep function of radius. Spectra show broad H alpha and He I 6678 A emission lines from the accretion disk and a weak absorption feature from the red dwarf. Radial velocity measurements of the H alpha emission line vary systematically with binary phase. The observed velocity amplitude 240 km/s, gives the mass function 0.46 solar masses. A phase shift of the velocity curve relative to the eclipse ephemeris indicates that a brightness asymmetry exists in the inner parts of the disk.

Horne, K.↗
NASA NTRS1982

The discovery of 50 minute periodic absorption events from 4U 1915-05

An analysis of X-ray observations shows that the steady flux from 4U 1915-05 undergoes periodic absorption dips with a period of 50 minutes. This period most probably represents the underlying orbital period of the system, and variations in the depth and duration of these events suggest that they are caused by a bulge on the edge of the accretion disk at the point where the gas stream impacts the disk. The mass-losing star in this system is probably a low-mass white dwarf, and the spectrum of the dips indicates that the metallicity of the absorbing material is at least a factor of 17 below solar values. The discovery of the 50-minute binary period supports earlier suggestions that X-ray burst sources are in binary systems.

White, N. E.↗
NASA NTRS1982

Stellar luminosity functions in the R, I, J, and K bands obtained by transformation from the visual band

The stellar luminosity function that has been measured in the visual band into the R, I, J, and K bands, where it has not yet been obtained directly, is transformed. The transformation is effected by subdividing the total visual function, which includes all stars, into subluminosity functions for each luminosity class (supergiants through white dwarfs), applying the known (V - D) color, D = R, I, J, K for each spectral type, and then summing the resultant transformed subluminosity functions into a total luminosity function for the band D. Simple analytic forms that accurately fit the transformed luminosity functions are given. Consideration is also given to the possibility of a systematic error resulting from the existence of a very red stellar population not accounted for in the visual band luminosity function

Mamon, G. A.↗
NASA NTRS1980

Models for Type I supernovae

Two somewhat disjoint scenarios for Type I supernovae are presented. One scenario is based on mass accretion by a white dwarf in a binary system, while the other involves a star of 8 to 10 solar masses which may or may not be a solitary star. In spite of the apparent dissimilarities of the two models, it may be that each occurs to some extent in nature, for they both share the possibility of producing substantial quantities of Ni-56 and explosions in stars devoid of hydrogen envelopes. It is concluded that these are two properties that must be shared by any viable Type I model.

Woosley, S. E.↗
NASA NTRS1983

The photometric period of 39 AY Ceti

A photometric period of 77.65 days is derived for 39 AY Ceti through a comparison of photoelectric photometry obtained in 1971 and 1972 with that obtained in the periods 1980-1981 and 1981-1982. JD 2444636.0 is an epoch of minimum light, and the full amplitude is found to have been as large as 0.18 mag in V. It is noted that the mean light level has dropped by almost 0.2 mag over the last decade. The present binary star is also noteworthy in light of the recently discovered white dwarf secondary component, and because radial velocity measurements indicate an eccentricity as large as 0.1 and an orbital period which, at 57.1 days, differs considerably from the photometric period presented.

Hopkins, J. L.↗
NASA NTRS1992

Coronal X rays from single, magnetic white dwarfs - A search and probable detection

X-ray emission from a sample of five nearby (6-20 pc), strongly magnetic (10-200 MG), relatively cool (6000-14,000 K), single white dwarfs, two of which may possess coronae was explored. One star (GR 290) at better than 99-percent confidence is detected and upper limits from Einstein Observatory IPC data for four others are given. The detected luminosities and limits are in the range 1.4-12.5 x 10 exp 27 ergs/s.

Arnaud, Keith A.↗