Showing papers on "White dwarf published in 1982"
TL;DR: In this article, the evolution of carbon-oxygen white dwarfs accreting helium in binary systems was investigated from the onset of accretion up to the point at which a thermonuclear explosion occurs.
Abstract: As a plausible explosion model for a Type I supernova, the evolution of carbon-oxygen white dwarfs accreting helium in binary systems was investigated from the onset of accretion up to the point at which a thermonuclear explosion occurs. The relationship between the conditions in the binary system and the triggering mechanism for the supernova explosion is discussed, especially for the cases with relatively slow accretion rate. It is found that the growth of a helium zone on the carbon-oxygen core leads to a supernova explosion which is triggered either by the off-center helium detonation for slow and intermediate accretion rates or by the carbon deflagration for slow and rapid accretion rates. Both helium detonation and carbon deflagration are possible for the case of slow accretion, since in this case the initial mass of the white dwarf is an important parameter for determining the mode of ignition. Finally, various modes of building up the helium zone on the white dwarf, namely, direct transfer of helium from the companion star and the various types and strength of the hydrogen shell flashes are discussed in some detail.
804 citations
TL;DR: In this paper, a supernova model based on accreting carbon-oxygen white dwarfs is presented and the explosion is triggered by the strong helium shell flash which occurs when the accretion forms a helium layer of substantial mass.
Abstract: Supernova models based on accreting carbon-oxygen white dwarfs are presented. In these models, the explosion is triggered by the strong helium shell flash which occurs when the accretion forms a helium layer of substantial mass. The hydrodynamical behavior of the helium shell flash is computed through the explosion for three cases with different accretion rates.
375 citations
TL;DR: In this article, the authors investigated the properties of white dwarfs which are accreting hydrogen-rich matter at rates in the range 1.5 × 10/sup -9/ to 2.5 x 10/Sup -7/ M/sub sun/ yr/sup 1/1/
Abstract: Properties of white dwarfs which are accreting hydrogen-rich matter at rates in the range 1.5 x 10/sup -9/ to 2.5 x 10/sup -7/ M/sub sun/ yr/sup -1/ are investigated in several approximations. Steady-burning models, in which matter is processed through nuclear-burning shells as rapidly as it is accreted, provide a framework for understanding the properties of models in which thermal pulses induced by hydrogen burning and helium burning are allowed to occur. In these latter models, the underlying carbon-oxygen core is chosen to be in a cycle-averaged steady state with regard to compressional heating and neutrino losses. Several of these models are evolved in the quasi-static approximation. Combining results obtained in the steady-burning approximation with those obtained in the quasi-static approximation, expressions are obtained for estimating, as functions of accretion rate and white dwarf mass, the thermal pulse recurrence period and the duration of hydrogen-burning phases. The time spent by an accreting model burning hydrogen as a large star of giant dimensions versus time spent burning hydrogen as a hot dwarf is also estimated as a function of model mass and accretion rate. Finally, suggestions for detecting observational counterparts of the theoretical models and suggestions for further theoretical investigations aremore » offered. Subject headings: stars: accretion: stars: interiors: stars: novae: stars: symbiotic: stars: white dwarfs« less
215 citations
TL;DR: In this paper, the steady flux from 4U1916-05 which undergoes periodic absorption dips every 50 minutes was demonstrated and it was suggested that variations in the depth and duration of these events are caused by a bulge in the edge of the accretion disk, at the point where the gas stream impacts the disk.
Abstract: The steady flux from 4U1916-05 which undergoes periodic absorption dips every 50 minutes was demonstrated This period represents the underlying orbital period of the system It is suggested that variations in the depth and duration of these events are caused by a bulge in 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 The spectrum of the dips indicates that the metallicity of the absorbing material is at least a factor 17 below solar values
198 citations
TL;DR: In this article, the thermal properties of hydrogen shell burning on accreting white dwarfs are studied and a general picture for their progress is presented which is described by two parameters, the accretion rate and the mass of the white dwarf.
Abstract: By means of analytical solutions of the envelope, thermal properties of hydrogen shell burning on accreting white dwarfs are studied and a general picture for their progress is presented which is described by two parameters, the accretion rate and the mass of the white dwarf. On a white dwarf, the thermal behavior of gas in the burning shell depends on the configuration of the envelope, which gives birth to two distinct types of stable configurations in thermal equilibrium, a high and a low state. In the high state, the nuclear shell burning makes up for the energy loss from the surface. There exists the lower limit to the envelope mass for this state. The nuclear burning rate lies in a narrow range of about a factor of 2.5, irrespective of the mass of the white dwarf, while the range itself varies greatly with the latter. In the low state, the nuclear burning is extinct, and yet the compressional heating by accreted gas balances with the cooling through the diffusion of heat. Therefore, the structure depends on the accretion rate. Thermal instability of nuclear burning sets the upper limit to the envelope mass of this state.
181 citations
01 Nov 1982
TL;DR: In this article, the absolute maximum magnitudes of SNeI and SNeII are discussed, as well as the evidence of SNEI being good standard candles in close binary systems.
Abstract: In the following the frequencies of SNe in external galaxies (Section 1) and in our Galaxy (Section 2) are discussed. The galactic frequencies are compared with SNRs, pulsars, and white dwarfs. In Section 3 the absolute maximum magnitudes of SNeI and SNeII are discussed, as well as the evidence of SNeI being good standard candles. Speculations on the progenitors of SNe in Section 4 are consistent with the assumption that SNeI come from stars with masses > 8 M ⊙, while SNeI could come from accreting white dwarfs in close binary systems.
119 citations
TL;DR: In this article, an analytical solution to the envelope, hydrogen shell flashes on accreting white dwarfs are computed semianlytically from their ignition to their final stage.
Abstract: By applying an analytical solution to the envelope, hydrogen shell flashes on accreting white dwarfs are computed semianlytically from their ignition to their final stage. When the mass of the white dwarf, M, and the mass of the hydrogen-rich envelope, ..delta..M/sub 1/, are specified, their progress is determined uniquely. Structural change and the resultant expansion of the envelope due to the shell flash depend mainly on the weight of overlying layer P(/sub 1/ or the product of the column mass above the burning shell and the surface gravity of the white dwarf. It is proved that this expansion is more rapid and greater for higher P(/sub 1/. When P(/sub 1/ is low (e.g.,< or approx. =4 x 10/sup 18/ dyn cm/sup -2/ for M = 1.0 M/sub sun/), the shell flash is weak, and the envelope settles in thermal equilibrium even before it expands to a solar radius. When P(/sub 1/ is high enough (10/sup 20/ dyn cm/sup -2/), on the other hand, the structural change is so violent that the expansion is accelerated beyond the escape velocity and leads directly to a nova explosion. Because of large P(/sub 1/, such a nova explosion requies a relatively large mass ofmore » the accreted envelope. The necessary mass of the envelope is as large as 10/sup -5/ M/sub sun/ even for white dwarfs close to the Chandrasekhar mass limit, and for less massive white dwarfs, it is still larger because of lower surface gravity. These results indicate that these nova explosions are phenomena which are related to surface gravity. These results indicate that these nova explosion are phenomena which are related to the slow accretion onto massive white dwarfs. Even for the optimal case of the most massive white dwarfs, the accretion rate should be lower than 10/sup -9/ M/sub sun/ yr/sup -1/.« less
115 citations
111 citations
TL;DR: The mass loss rate for cataclysmic variables is of order 10 to the -11th solar mass per yr; this is from 0.01 to 0.001 of the mass accretion rate onto the compact star in the binary as discussed by the authors.
Abstract: An ultraviolet spectrum of the dwarf nova TW Vir during an optical outburst shows shortward-shifted absorption features with edge velocities as high as 4800 km/s, about the escape velocity of a white dwarf. A comparison of this spectrum with the UV spectra of other cataclysmic variables suggests that mass loss is evident only for systems with relatively high luminosities (more than about 10 solar luminosities) and low inclination angles with respect to the observer's line of sight. The mass loss rate for cataclysmic variables is of order 10 to the -11th solar mass per yr; this is from 0.01 to 0.001 of the mass accretion rate onto the compact star in the binary. The mass loss may occur by a mechanism similar to that invoked for early-type stars, i.e., radiation absorbed in the lines accelerates the accreting gas to the high velocities observed.
92 citations
84 citations
TL;DR: In this paper, it was shown that the strength of both sources of neutrons is proportional to the initial abundance of CNO elements and the magnitude and duration of the extended luminosity dip which follows a thermal pulse are such that the well known core-mass--luminosity relationship frequently used in estimating core masses of real asymptotic giant branch stars is not a good approximation.
Abstract: Model asymptotic giant branch stars of composition relevant to the Magellanic Clouds and of mass 0.6 M/sub sun/ and 0.7 M/sub sun/ are evolved far enough to corroborate earlier calculations which show that the dredge up phenomenon does not occur for core masses smaller than or equal to 0.6 M/sub sun/. However, it is found that the /sup 13/C(..cap alpha.., n)/sup 16/O neutron source is weakly active for all small core masses, even if hydrogen does not enter the helium-burning convective shell, and that the /sup 22/Ne(..cap alpha.., n)/sup 25/Mg source operates weakly for core masses as small as 0.6 M/sub sun/. The strengths of both sources of neutrons are proportional to the initial abundance of CNO elements. It is found further, in agreement with earlier work, that the magnitude and duration of the extended luminosity dip which follows a thermal pulse are such that the well known core-mass--luminosity relationship frequently used in estimating core masses of real asymptotic giant branch stars is not a good approximation. The evolution of the 0.6 M/sub sun/ model is followed until it becomes a white dwarf and begins a final thermal pulse. Calculations of the 0.7 M/sub sun/ model are terminated at M/submore » core/approx.0.61 M/sub sun/ when it becomes apparent that several neglected physical processes: carbon recombination and chemical diffusion: may be of importance in influencing both the mixing and the nucleosynthesis properties of additional thermal pulse cycles. Some conjectures as to the possible impact of these processes are offered.« less
TL;DR: The X-ray properties of the main sequence Be stars conclude that they are a widely separated binary system containing an accreting neutron star as mentioned in this paper, which is a common assumption for binary systems.
Abstract: The X-ray properties of the main sequence Be stars conclude that they are a widely separated binary system containing an accreting neutron star.
01 Jan 1982
TL;DR: In this paper, a search engine with the keywords and phrases download Danielle Alloin PDF eBooks in order for you personally to only get PDF formatted books to download that are safer and virus-free.
Abstract: In case you happen to be looking to understand how to acquire Binary and Multiple Stars as Tracers of Stellar Evolution eBooks, you should go thorough investigation on common search engines with the keywords and phrases download Danielle Alloin PDF eBooks in order for you personally to only get PDF formatted books to download that are safer and virus-free you'll discover an array of internet sites catering for your requirements. The majority of these sites have a massive collection of PDF eBooks which you are able to use for your advantage.
TL;DR: In this article, it was shown that even slightly non-spherically symmetric accretion leads to transverse temperature gradients in the dwarfs' accreted envelopes.
Abstract: Practically all studies to date of thermonuclear runaways on degenerate dwarf stars in binary systems have considered only spherically symmetric eruptions. We emphasize that even slightly non-spherically symmetric accretion leads to transverse temperature gradients in the dwarfs' accreted envelopes. Over a rather broad range of parameter space, thermalization time scales in accreted envelopes are much longer than thermonuclear runaway time scales. Thus localized thermonuclear runaways (i.e., runaways much smaller than the host degenerate star) rather than spherically symmetric global eruptions are likely to occur on many degenerate dwarfs. Localized runaways are more likely to occur on more massive and/or hotter dwarfs.
TL;DR: In this article, the authors investigated the role of the mass accretion rate in determining the outcome of the accretion of hydrogen-rich material onto a 1.25 M/sub sun/C/O white dwarf.
Abstract: We investigate the role of the accretion rate in determining the outcome of the accretion of hydrogen-rich material onto a 1.25 M/sub sun/ C/O white dwarf. It is found that there is an upper limit to the mass accretion rate that leads to a nova outburst. The upper limit is between 10/sup -8/ and 10/sup -9/ M/sub sun/ yr/sup -1/ for a 1.25 M/sub sun/ white dwarf. Below this upper limit, the outburst characteristics are very similar for accretion rates down to 10/sup -11/ M/sub sun/ yr/sup -1/.
Journal Article•
TL;DR: In this paper, it was shown that the He I and H I emission lines are associated with the Balmer lines in the outer regions of the accretion disk, indicating that they are either highly evolved cores or they have an outer layer of processed material which was accreted from the white dwarf during earlier evolution of the system.
Abstract: Time-resolved spectroscopy of a sample of eclipsing cataclysmic variables indicates that the He I emission lines are found with the Balmer lines in the outer regions of the accretion disk. Additional observations of lower inclination noneclipsing systems demonstrate that the He I emission is frequently characterized by relatively high singlet to triplet intensity ratios and an inverted line decrement, an unusual situation which can be explained in terms of emission from an optically thick (in the lines) region in local thermodynamic equilibrium. Calculations of steady state accretion disk structure show that the observed characteristics of the He I and H I lines can be reproduced in normal disk models, but only in the helium abundance is assumed to be very high, i.e., He/H> or approx. =100 by number. Enhanced helium abundances are thus implied for some cataclysmic variable secondaries, suggesting that they are either highly evolved cores or that they have an outer layer of processed material which was accreted from the white dwarf during earlier evolution of the system.
01 Jan 1982
TL;DR: The results of recent numerical simulations of supernova explosions are presented and a variety of topics discussed as mentioned in this paper, with particular emphasis on the nucleosynthesis expected from intermediate mass of both Population I and III.
Abstract: The results of recent numerical simulations of supernova explosions are presented and a variety of topics discussed. Particular emphasis is given to i) the nucleosynthesis expected from intermediate mass (10 MƟ 100 MƟ) of both Population I and III. In one dimension, nuclear burning following a ‘failed’ core bounce does not appear likely to lead to a supernova explosion although, in two dimensions, a combination of rotation and nuclear burning may do so. Near solar proportions of elements from neon to calcium and very brilliant optical displays may be created by ‘hypernovae’, the explosions of stars in the mass range 100 MƟ to 300 MƟ. Above ~ 300 MƟ a black hole is created by stellar collapse following carbon ignition. Still more massive stars may be copious producers of 4He and 14N prior to their collapse on the pair instability.
TL;DR: In this paper, numerical calculations of X-ray and UV emission from accreting nonmagnetic degenerate dwarfs are reported, which span the entire range of accretion rates and stellar masses.
Abstract: Numerical calculations of X-ray and UV emission from accreting nonmagnetic degenerate dwarfs are reported, which span the entire range of accretion rates and stellar masses. Calculations include the effects of bremsstrahlung, Compton cooling, radiation pressure, albedo of the stellar surface, Compton degradation and free-free abscription of the X-ray spectrum by the accreting matter. Maximum X-ray luminosity for degenerate dwarfs undergoing spherical accretion is found to be 2.2 x 10 to the 36th ergs/s, which is little changed if accretion occurs radially over only a fraction of the stellar surface, so that the emitted radiation escapes without significant scattering. The temperature characterizing the X-ray spectra produced by degenerate dwarfs strongly depends on the stellar mass and the accretion rate, and it is suggested that the correlation between spectral temperature and luminosity is an important signature of degenerate X-ray sources.
TL;DR: A VLA search for 4.9 GHz radiation from the magnetic cataclysmic variable AM Her, along with the similar EF Eri binary, has led to the discovery of AM Her radio emission having a flux density of 0.67 + or - 0.052 mJy.
Abstract: A VLA search for 4.9 GHz radiation from the magnetic cataclysmic variable AM Her, along with the similar EF Eri binary, has led to the discovery of AM Her radio emission having a flux density of 0.67 + or - 0.052 mJy, where 1 mJy is equal to 10 to the -29th W/sq m per Hz. Neither AM Her circular polarization nor EF Eri were detected. The AM Her data are shown to be consistent with a model in which radiation is due to geosynchrotron emission from electrons of energies of a few hundred keV, which are trapped in the magnetosphere of the white dwarf element of the cataclysmic variable.
TL;DR: The spectrum of the flickering blue variable RW Sex was observed from 10,000 to 1150 A. The star is a cataclysmic variable currently stabilized at maximum and the spectrum is dominated by an accretion disk, with flat spectrum in the ultraviolet, except at more than 5000 A, where a blackbody near 7000 K is seen.
Abstract: The continuum spectrum of the flickering blue variable RW Sex was observed from 10,000 to 1150 A. The star is a cataclysmic variable currently stabilized at maximum, and the spectrum is dominated by an accretion disk, with flat spectrum in the ultraviolet, except at more than 5000 A, where a blackbody near 7000 K is seen. A distance of 400 pc is derived, if the latter arises from an F type main sequence star. The accretion rate required is near 10 to the -8th solar masses per year. Only weak emission is seen, except for Lyman alpha; strong, broad UV absorption lines are seen with centers displaced up to -3000 km/s, with terminal velocities up to -4500 km/s, the velocity of escape from a white dwarf. The low X-ray flux may arise from absorption within an unusually dense, hot wind from the innermost portions of the disk. The estimated mass loss rate is nearly 10 to the -12th solar masses per year.
TL;DR: In this paper, the authors investigated the local interstellar medium in the directions of four white dwarfs, G191-B2B, W1346, HD 149499B, and Sirius B, in the spectral range from about 1150 to 3200 A with the International Ultraviolet Explorer (IUE).
Abstract: We have investigated the local interstellar medium in the directions of four white dwarfs, G191-B2B, W1346, HD 149499B, and Sirius B. All the observational data were obtained at the high-resolution mode (lambda/..delta..lambdaroughly-equal10/sup 4/) in the spectral range from about 1150 to 3200 A with the International Ultraviolet Explorer (IUE). Interstellar absorption lines of several elements in various stages of ionization are seen against the continuum of the white dwarfs. Low average hydrogen number densities (n-bar/sub HtsI/) are found. They range from n-bar/sub HtsI/ = 0.08 cm/sup -3/ for Sirius B, the nearest white dwarf (2.7 pc), to n-bar/sub HtsI/ = 0.006 cm/sup -3/ for G191-B2B, the most distant white dwarf (48 pc) studied. The results show, when combined with other recent ultraviolet, EUV, and diffuse X-ray observations, that: (a) the Sun is located inside a low-density (n-bar/sub HtsI/roughly-equal0.1 cm/sup -3/) cloud; (b) beyond 2--3 pc from the Sun, this cloud is surrounded, at least in most directions, by an extended region of hot (Troughly-equal10/sup 5en-dash6/ K) thin (nroughly-equal10/sup -2/ to 10/sup -3/ cm/sup -3/) interstellar plasma with no evidence for additional clouds in the lines of sight studied; (c) the elemental depletions of C, N, O, Si, Mg, and possiblymore » Fe are low in the solar vicinity as previously found toward ..cap alpha.. Vir, (d) the Sun is moving through this cloud at a relative velocity of about 20 km s/sup -1/; and (e) the current results, which are quite consistent with previous ultraviolet, EUV, and diffuse X-ray observations, have significant bearings on the theoretical modeling of the interstellar medium. Subject headings: interstellar: abundances: interstellar: matter: stars: white dwarfs: ultraviolet: spectra« less
TL;DR: A high-resolution ultraviolet spectrum of the binary white dwarf binary Feige 24 (DA + M1-2 V) obtained with the International Ultraviolet Explorer satellite (IUE) reveals two sets of absorption features in the species C IV, Si IV and N V as discussed by the authors.
Abstract: A high-resolution ultraviolet spectrum of the nearby (90 pc) white dwarf binary Feige 24 (DA + M1-2 V) obtained with the International Ultraviolet Explorer satellite (IUE) reveals two sets of absorption features in the species C IV, Si IV, and N V. The low-velocity component (V approximately 1 km/s) arises from the local interstellar gas apparently ionized by the white dwarf. The second component, at high velocities (V approximately +83 km/s), arises in the atmosphere of the white dwarf in the Feige 24 system. The presence of this large velocity shift supplies the first strong evidence for narrow absorption cores due to high-temperature species in a white dwarf atmosphere. Observations around the orbital period will provide a direct measurement of the gravitational redshift and the mass of the white dwarf. The inferred hydrogen density is 0.008 per cu cm toward this source.
TL;DR: In this article, it is shown that the convective Urca process can prevent a runaway toward a dynamic event, but it is only able to delay the dynamic event until core densities are somewhat larger than they would have been had the process been negelected, and the average energy of potentially detectable neutrinos resulting from electron capture is larger than would otherwise have been the case.
Abstract: Carbon burning in highly electron-degenerate matter is followed in two astrophysically interesting cases; an intermediate-mass star that, on the asymptotic giant branch, has developed a large carbon-oxygen core; and an accreting white dwarf composed primarily of carbon and oxygen. Calculations are continued until heating at the edge of the region within which a dominant Urca pair is assumed to be mixed uniformly makes it impossible to achieve coincidence between this edge and the edge of an associated convectively unstable region within which convection is expected to promote mixing. Thereafter, global heating occurs at a rate that exceeds the rate of cooling by neutrino losses, and it does not appear that the convective Urca process can prevent a runaway toward a dynamic event. The major effect of the convective Urca process is thus only to delay the dynamic event until core densities are somewhat larger than they would have been had the process been negelected. One consequence of this delay is that the average energy of potentially detectable neutrinos resulting from electron capture is larger than would otherwise have been the case.
TL;DR: In this paper, thermal and electrical conductivities due to electron scattering on phonons are calculated for degenerate cores of white dwarfs and envelopes of neutron stars for wide ranges of density, temperature and ion charge.
Abstract: Thermal and electrical conductivities due to electron scattering on phonons are calculated for degenerate cores of white dwarfs and envelopes of neutron stars for wide ranges of density, temperature and ion charge. In the stellar zones, in which T ≳ ħωpi(Z1/3e2/ħυF) (ωpiis the ion plasma frequency and υF the Fermi velocity of electrons), the main contribution into scattering comes from the Umklapp processes. In the zones with lowerT, the Umklapp processes are frozen out, that results in a sharp growth of electrical and thermal conductivities. This, for instance, should make nuclear burning more stable in such zones.
TL;DR: In this paper, the authors investigated the properties of shell flash on a carbon-oxygen white dwarf and found that the shell flash grows up to a strength of supernova explosion when the mass of the helium zone is large enough on a massive white dwarf (> or approx. = 0.7 M/sub sun/).
Abstract: In a close binary system or in a dense cloud, gas may be accreted onto a carbon-oxygen white dwarf and will be processed into helium by hydrogen burning in an accreted envelope. As a result, a helium zone grows in mass, and a helium shell flash takes place just as in cores of red giant stars. Properties of such helium shell flashes are investigated both by a generalized theory of shell flash and numerical computations. It is found that the shell flash grows up to a strength of supernova explosion when the mass of the helium zone is large enough on a massive white dwarf (> or approx. =0.7 M/sub sun/). Otherwise, shell flashes are relatively weak: Even then protons in the envelope are mixed into a helium convective zone, and it becomes a site of s-process nucleosynthesis.
01 Jan 1982
TL;DR: In this paper, the authors interpreted the binary X-ray source AM Herculis from the IUE satellite as two components, a blackbody (blackbody kT of 25-30 eV) which accounts for the x-ray emission below 0.5 keV and the eclipsed part of the UV continuum, and a component which is never eclipsed, whose spectrum is roughly proportional to the inverse of frequency.
Abstract: Ultraviolet observations of the binary X-ray source AM Herculis from the IUE satellite show strong emission lines of ions from O I to N V, probably originating in photoionized gas, and a continuum which is partially eclipsed in phase with the X-ray eclipse. The emission lines have broad (about 600 km/s) and sharp (about 80 km/s) components at different velocities, as has been seen in optical lines. The continuum is interpreted as two components, a blackbody (blackbody kT of 25-30 eV) which accounts for the X-ray emission below 0.5 keV and the eclipsed part of the UV continuum, and a component which is never eclipsed, whose spectrum is roughly proportional to the inverse of frequency. Strong ultraviolet emission due to optically thick cyclotron emission, which has been predicted theoretically, is not observed. This presents a severe difficulty for the theory of accretion onto the magnetic pole of a white dwarf.