Showing papers on "White dwarf published in 1977"

Atoms in high magnetic fields (white dwarfs)

Abstract: High magnetic fields have been of interest for some time in solid-state physics. Recent spectroscopic experiments on highly excited barium atoms and the discovery of very large magnetic fields in white dwarf stars and neutron stars have stimulated renewed interest in the spectroscopy of free atoms in high magnetic fields. The various spectroscopic regimes ranging from the low-field Zeeman effect to the high-field Landau regime are reviewed. Calculations of the energy levels of hydrogen, helium and their isoelectronic ions in fields of any strength are surveyed. Observations of the quadratic Zeeman effect in the alkalis and high-field effects in the neutral barium spectrum are described. Finally, recent discoveries of continuum polarization in white dwarf stars, its interpretation in terms of the presence of high magnetic fields, the confirmation of such fields spectroscopically, and some remaining mysteries in these remarkable stellar spectra are reviewed.

Stars with degenerate neutron cores. I. structure of equilibrium models

Abstract: Stars with massive envelopes (M/sub env/approximately-greater-than1 M/sub sun/) and degenerate neutron cores (M/sub core/approx.1 M/sub sun/, R/sub core/approx.10 km) are analyzed theoretically: General-relativistic equations of structure are derived under the assumptions of hydrostatic and thermal equilibrium, spherical symmetry, no rotation and no magnetic field. Numerical models are constructed, and analytic expressions are derived for the stellar structure in various interior regions. It is argued that all nonrotating, equilibrium models probably resemble qualitatively those constructed in this paper. Brief discussions are given of the stability and evolution of the models, and of prospects for identifying such stars observationally.Viewed externally, our models are extreme M supergiants (Lapprox.3 x 10/sup 4/ to 1.3 x 10/sup 5/ L/sub sun/, T/sub photosphere/approx.2600 to 3100 K, R/sub photosphere/approx. =1000 R/sub sun/). The large, diffuse envelope of each model is separated from its compact core by a thin (approx.40 meter) energy-generation layer called the ''halo.'' The envelope convects from the outer edge of the halo all the way out to the photosphere. Matter contracts from the envelope through the halo and into the core at a rate of approx.1 x 10/sup -8/ M/sub sun/ yr/sup -1/. The contracting matter releases its gravitational energy and burns its hydrogenmore » and helium while passing through the halo. When the envelope mass exceeds approx.10 M/sub sun/, the hydrogen-burning shell occurs at the halo-envelope interface, and the products of hot (Tapprox. =1 x 10/sup 9/ K) nonequilibrium hydrogen burning are convected directly from the burning shell out to the photosphere, where they should be observable.« less

Accretion torques in X-ray pulsars

Abstract: An analysis of the accretion process in an X-ray pulsar, whereby angular momentum is transferred to the star and its rotation period is changed, is presented, and an expression for the fractional rate of change of the pulse period in terms of X-ray luminosity and other star parameters is derived. It is shown that observed characteristic spin-up time scales for seven X-ray pulsars strongly support the view that in every source (1) the pulse period reflects the rotation period of a compact object, (2) the accretion is mediated by a disk surrounding the compact object and rotating in the same sense, and (3) the compact object is a neutron star rather than a white dwarf.

X-rays from spherical accretion onto degenerate dwarfs

Abstract: I reconsider spherically symmetric accretion onto degenerate dwarfs. The problem is divided into three parameter regimes on the basis of the accretion rate. In the regime of highest accretion rates, the flow is optically thick. For the middle regime, previous calculations by Aizu are extended to include Compton cooling of the shocked material by thermal radiation from the photosphere. If the accreted material is hydrogen-rich and burns, this effect greatly reduces the efficiency of X-ray production. In the lowest regime of accretion rate, the shock occurs far from the star, and the accretional energy is radiated through a thick layer. Graphs of the radiation spectrum and other quantities of interest are presented. Accreting degenerate dwarfs are expected to be characterized by very hard X-ray spectra, which are nearly independent of other variations in the properties of the source. Applications of these results include arguments against the degenerate dwarf hypothesis for Sco X-1 and 3U 0900--40, and suggested observations of DQ Her.

Mira variables, mass loss, and the fate of red giant stars.

Abstract: The period distribution of Mira variables has been obtained for a region near the galactic plane, and from this distribution a strip of instability for Mira variables in the mass-luminosity plane has been derived. Mass loss rates that are smaller by factors of 2 to 4 than those obtained by Reimers are required to explain the existence of the high-velocity and halo Miras with periods less than 300 days. Assuming that the high-luminosity edge of the Mira instability strip coincides with ejection of the hydrogen-rich envelope, we derive remnant white dwarf (or planetary nebula nucleus) masses and masses of planetary nebulae as a function of initial stellar mass. Using a mainsequence death rate function, we find that near the plane of the Galaxy the mass distribution of planetary nebula nuclei is highly peaked in the mass range 0.6--0.8 M/sub sun/, while the mass distribution of planetary nebulae is bimodal, with peaks near 0.01 and 0.8 M/sub sun/. The mass distribution of white dwarfs produced near the galactic plane during the lifetime of the Galaxy is also predicted to show a strong peak near 0.75 M/sub sun/. The lower initial mass limit for stars which produce degenerate cores of 1.4 M/submore » sun/ and therefore possibly supernovae lies in the range approx.3.7 to approx.4.7 M/sub sun/.« less

Radial oscillations of zero-temperature white dwarfs and neutron stars below nuclear densities

Abstract: Radial oscillations of zero-temperature degenerate stars with subnuclear densities have been calculated for the fundamental and first two excited modes using recent equations of state. The calculations were made under the assumptions that (a) the dynamical time scale tauvery-much-less-thantau/sub N/, where tau/sub N/ is the nuclear relaxation time, in which case the adiabatic index ..gamma..=..gamma../sub E/ (calculated along the equation of state) and (b) tauvery-much-less-thantau/sub N/ with ..gamma../sub C/ calculated at constant composition. For case a, it is shown that white dwarfs are dynamically stable for rho/sub c/9 or approx. =1.2 x 10/sup 9/ g cm/sup -3/ while neutron stars are dynamically stable for rho/sub c/> or approx. =10/sup 14/ g cm/sup -3/. The instability for white dwarfs in case a arises principally from the phase transitions rather than from the effects of general relativity, conjecture of Baym et al. Low-density neutron stars have for the fundamental (unstable) mode ''e-folding'' times which are a factor 100 less than the typically expected dynamical time scale. It is shown that this is consistent with the unusual decay of the radial eigenfunction, a qualitative explanation of which is given. For densities below the neutron drip point it is shown that ..gamma../sub c/=..gamma../sub E/more » except at a finite number of phase transition points. For densities just above the neutron drip point, it is shown analytically that ..gamma../sub C/ must be greater than 4/3 in the critical region where ..gamma../sub E/ drops below 4/3. For case b, white dwarfs are dynamically stable for rho/sub c/9 or approx. =4 x 10/sup 10/ g cm/sup -3/, while neutron stars are dynamically stable for rho/sub c/> or approx. =7 x 10/sup 12/ g cm/sup -3/ as had been suggested previously by the author. Case b is shown to be more appropriate for infinitesimal perturbations. It is also pointed out that arguments given earlier to indicate that high-density white dwarfs and low-density neutron stars may not exist are lacking in rigor.« less

Consequences of Mass Transfer in Close Binary Systems

Abstract: Of all aspects of stellar evolution the most important for close binary systems is that once a star has condensed out of a protostellar cloud it may repeatedly increase its dimensions by large factors. During such an expansion matter can reach a point where the attraction of a binary companion is strong enough to pull it toward the other star. Since the stellar radii that are reached in this expansion can be of the order of an astronomical unit, this kind of interaction may take place even in rather wide binaries with periods of the order of years. At least during such a stage binary evolution must follow a path different from that of a single star. There are other types of interaction between the two stars that come into play much earlier, such as energy transfer by radiation or by matter, which leaves the. star as a stellar wind, and tidal friction or interaction via magnetic fields, but because of a built-in instability mass transfer is by far the most important factor. In its simplest form this instability is caused by the fact that each gram of transferred matter adds to the gravitational pull of the mass-gaining companion while weakening the attractive properties of the expanding star. Since the components of a binary are not fixed in space, their separation may change as well, depending on the amount of angular momenl:um transferred. From observations of close binaries it is well known that many rotate in synchronism with their revolution. This is expected because of the tidal interaction in these systems. Hence for ratios of masses and radii that are not too extreme the rotational angular momentum of both components can be neglected compared to the orbital angular momentum. Under these conditions the individual contributions to the total angular momentum are fixed by the masses of the two stars. Nevertheless, the one-dimensional picture of single-star evolution cannot be retained, since the initial conditions must include not only mass and chemical composition, but also the distance and mass of the companion star. Further complications are met during the stages of mass transfer. However, we should not forget that binary systems in

A model for bursting X-ray sources: time-dependent accretion by magnetic neutron stars and degenerate dwarfs.

Abstract: We consider spherically symmetric accretion flow onto a strongly magnetic compact star. We show that under certain conditions such flow is intermittent, and that the resultant time-dependent X-ray emission from the stellar surface is akin to that observed in the bursting X-ray sources. The stability properties of the magnetosphere of a compact star provide a natural gating mechanism that can produce burst rise times (approx.0.1--1 s) and durations (approx.1--100 s) which are much shorter than the time (approx.10--10/sup 5/ s) between bursts. A key feature of high-luminosity flows is the interaction of the burst radiation with the accreting matter, which determines the burst shape and the time between bursts.Four distinct types of burstlike flows are possible, depending on the luminosity and temperature of the burst radiation. In each type of flow the bursts are terminated by a different physical mechanism: compressional heating of matter at the magnetospheric boundary, Compton heating of matter at the boundary, Compton heating of matter far from the boundary, or radiation pressure. The possible burst luminosities produced by such flows range from 9 or approx. =10/sup 36/ erg s/sup -1/ for the low-luminosity flow up to 10/sup 38/-10/sup 39/ ergs s/sup -1/ for the high-luminosity flows.Flowsmore » of the type studied here may be able to account both for behavior like that of the rapid burster, MXB 1730--335, and like that of the high-luminosity, long-period bursters, such as 3U1820--30=NGC 6624 and MXB 1728--34.« less

Dipole gravitational radiation in Rosen's theory of gravity - Observable effects in the binary system PSR 1913+16

Abstract: It is shown that Rosen's (1973) bimetric theory of gravity predicts the emission of dipole gravitational radiation from binary systems containing neutron stars, such as the binary pulsar PSR 1913+16, which causes rapid changes in orbital period. The theory also predicts sizable corrections to masses inferred from orbital data and periastron-shift data. It is demonstrated that this prediction is inconsistent with the observed upper limit on period changes unless the system consists of two neutron stars whose masses differ by less than 0.3 solar mass, or a neutron star of mass less than 0.4 solar mass and a companion which must be a rapidly rotating white dwarf or a helium main-sequence star. Because Rosen's theory is in agreement with all solar-system experiments to date, this represents a feasible test of its viability.

Magnetism in white dwarfs

Abstract: Searches have been made for the normal and quadratic Zeeman effect and broad-band circular polarization in white dwarf stars. A positive effect has been found in Grw + 70°8247 whose continuum shows both linear and circular polarization.

Accretion of rotating fluids by barytropes - Numerical results for white-dwarf models

Abstract: Numerical sequences of rotating axisymmetric nonmagnetic equilibrium models are constructed which represent the evolution of a barytropic star as it accretes material from a rotating medium. Two accretion geometries are considered - one approximating accretion from a rotating cloud and the other, accretion from a Keplerian disk. It is assumed that some process, such as Ekman spin-up or nonequilibrium oscillations, maintains nearly constant angular velocity along cylinders about the rotation axis. Transport of angular momentum in the cylindrically radial direction by viscosity is included. Fluid instabilities and other physical processes leading to enhancement of this transport are discussed. Particular application is made to zero-temperature white-dwarf models, using the degenerate electron equation of state. An initially nonrotating 0.566-solar-mass white dwarf is followed during the accretion of more than one solar mass of material. Applications to degenerate stellar cores, to mass-transfer binary systems containing white dwarfs, such as novae and dwarf novae, to Type I supernovae, and to galactic X-ray sources are considered.

Feige 7: a hot, rotating magnetic white dwarf.

Abstract: Results are reported for image-tube-scanner and digicon observations of Feige 7, a faint blue star identified as a probable white dwarf. It is found that this star is a magnetic white dwarf showing a very rich spectrum with Zeeman subcomponents of both hydrogen and neutral helium as well as periodic spectrum and circular-polarization variations. A polarization period of 2.2 hr is computed, and a surface magnetic-field strength of about 18 MG is determined by matching features of the absorption spectrum to Zeeman components. It is suggested that the only reasonable explanation for the periodic variations in circular polarization is an oblique rotator with the spin axis approximately in the plane of the sky and tilted by about 24 deg to the magnetic axis. An effective temperature in the range from 20,000 to 25,000 K is estimated, an absolute magnitude of about 10.5 is derived, and the atmosphere is shown to be helium-dominated. The evolution of Feige 7 is discussed in terms of possible magnetic-field effects on atmospheric composition, rotation velocity (5.5 km/s for a radius of 7000 km), and the origin of white-dwarf magnetic fields.

Masses, radii, and model atmospheres for cool white-dwarf stars

Abstract: Model atmospheres for white-dwarf stars with 4000< or =T/sub eff/< or =8000 K are presented for surface gravities of 10/sup 7/ and 10/sup 8/. A composition of pure hydrogen is assumed; convection and molecular contributions to the opacity and equation of state are considered. The unimportance of helium and metals is justified by the analysis of the spectra of several white dwarfs.The model-atmosphere computations are then applied to observations currently in the literature. Temperatures, masses, and radii for a number of stars are tabulated. There is no systematic dependence of mass on temperature or spectral class. The model calculations cannot reproduce the observed breadth of the Ca II lambda3933 line in DA, F stars like Ross 627 without appealing to an unknown line-broadening mechanism; the calcium abundance in this star is approx.1.2 x 10/sup -10/.

The remarkable system AM Herculis/3U 1809+50

Abstract: The principal features of the optical radiation, X-radiation, and polarization variations in the binary system AM Her are explained by a model based on a magnetic white dwarf in essentially synchronous rotation and undergoing accretion.

Neutron star and degenerate dwarf models of x‐ray bursts*

Abstract: Burst events with timescales of seconds were discovered by the Vela satellites and reported i n 1973.’ These “soft y-ray” bursts have energies of 150 keV and occur at a rate of 10 yr ’ at the level of sensitivity of the Vela detectors.* T w o bursts detected by the Kosmos 428 satellite were reported in 1975.3 These “hard x-ray” bursts had temporal structures that resembled those of the soft y-ray bursts but softer spectra and much smaller fluxes.$ Independently in 1975, the ANS satellite discovered4 x-ray bursts that originated from the previously known x-ray source 3U 1820-30 in the globular cluster NGC6624. Analysis of earlier SAS-3 observations quickly confirmed this result and led to the remarkable additional discovery that the bursts recurred quasiperiodically on a timescale of 4h4 with a phase jitter of a few percent.5 Since then, there have been many additional reports of x-ray bursts, and more than 20 burst sources are now known. Observations of x-ray bursts have recently been reviewed by Grindlay6 and Lewin’ and are also discussed elsewhere in this volume in the articles by Gursky’ and Lewin.’ F I G U R E 1 shows schematically the light curvc of a burst source. Typically, the bursts have rise times 6 t b <, Is, durations Atb z 3’-IOs, and repetition timescales f b -I h I O h . Peak burst luminosities are L b 103’erg sec-’, and burst energies are Eb 1 0 ~ ~ 1 0 ~ erg. Many , and perhaps all, burst sources have “steady” background luminosities Lo 5 10”erg scc-’, and some have “active” and “inactive” states that last from weeks to months. TABLE I gives repetition timescales. burst energies, and the ratio A of the background luminosity Lo to the time-averaged burst luminosity = Eb/t , , while the source is in an “active” state for the sources for which this information is available at the time of writing. Identification of 3111820-30 as a source of bursts and the subsequent discovery that these bursts recur quasiperiodically represented a breakthrough in the field of fast timescale x-ray and y-ray phenomena: one now knew where and, often, approximately when to look for a burst. Data from successive bursts could be accumulated for a given source, and astrophysics with bursts could begin. I n this review, we shall survey compact star (either a neutron star or a degenerate dwarf) models of the bursts. These models fall into two distinct categories, which we shall discuss separately. Under ACCRETION MODELS, we discuss bursts

The evolution of massive close binaries: V: Systems containing primaries with masses between 10M ⊙ and 15M ⊙

Abstract: The final state of the primaries of binary systems with initial massesM1i=10M⊙ to 15M⊙ is derived from the mass of their C/O-cores. The possibility of a second stage of mass transfer towards the secondary is considered. It turns out that the critical mass for the bifurcation is about 14M⊙: stars with larger masses in this range are the progenitors of neutron stars, while the lower mass stars are the ancestors of white dwarfs.

A self-consistent model-atmosphere analysis of the EUV white dwarf HZ 43

Abstract: Observations of the hot white dwarf HZ 43 from the visible to the EUV are analyzed using model atmospheres. We find that the derived effective temperature is a function of the assumed helium abundance, ranging from 55,000 K for negligible He to 70,000 K for n (He)/n (H) =0.001. Higher helium abundances are inconsistent with the absence of He II lambda4686. The EUV observations imply an interstellar neutral hydrogen density approximately-less-than0.01 cm/sup -3/.

On interstellar accretion and the rejuvenation of white dwarfs.

Abstract: The paper investigates physical conditions which can give rise to thermonuclear runaways in the hydrogen envelopes of low-luminosity white dwarfs. Specifically, calculations are performed for white dwarfs of 0.5, 0.75, 1, and 1.25 solar masses with envelope masses in the range from 0.00001 to 0.001 solar mass and initial luminosities of 0.00001 to 0.01 times the solar value. It is found that envelope masses as low as 0.0001 solar mass are sufficient to initiate thermonuclear runaways on more massive white dwarfs with luminosities as low as about 0.001 the solar value. The runaway time scales under these conditions, typically of the order of a billion years, are comparable to the time scales for cooling of white dwarfs to these low luminosities. Since time-averaged accretion rates as low as a few times 10 to the -14th power solar mass per year are sufficient to reconstitute such envelopes, also on a time scale of several billion years, it is suggested that the association of novalike events with binary systems may not be unique. Accretion of interstellar matter onto isolated white dwarfs may, under some circumstances, be sufficient to rekindle and perhaps rejuvenate the dwarf. Such evolutionary behavior might define a new and distinct class of objects.