Showing papers on "White dwarf published in 1987"

Existence and Nature of Dark Matter in the Universe

Abstract: Ann. Rev. Astron. Astrophys. 1987. 25: 425-72 Copyright © 1987 by Annual Reviews Inc. All rights reserved EXISTENCE AND NATURE OF DARK MATTER IN THE UNIVERSE Virginia T rimble Astronomy Program, University of Maryland, College Park, Maryland 20742, and Department of Physics, University of California, Irvine, California 92717 1. HISTORICAL INTRODUCTION AND THE SCOPE OF THE PROBLEM The first detection of nonluminous matter from its gravitational effects occurred in 1844, when Friedrich Wilhelm Bessel announced that several decades of positional measurements of Sirius and Procyon implied that each was in orbit with an invisible companion of mass comparable to its own. The companions ceased to be invisible in 1862, when Alvan G. Clark turned his newly-ground 18%” objective toward Sirius and resolved the 10q‘ of the photons from the system emitted by the white dwarf Sirius B. Studies of astrometric and single-line spectroscopic binaries are the modern descendants of Bessel’s work. A couple of generations later, data implying nonluminous matter on two very different scales surfaced almost simultaneously. First, Oort (498, 499) analyzed numbers and velocities of stars near the Sun and concluded that visible stars fell shy by 30-50% of adding up to the amount of gravitating matter implied by the velocities. Then, in 1933, Zwicky (777) concluded that the velocity dispersions in rich clusters of galaxies required 10 to 100 times more mass to keep them bound than could be accounted for by the luminous galaxies themselves. The former result was taken much more seriously than the latter by contemporary and succeeding astronomers (being dignified by the name “the Oort limit”), which is perhaps more a statement about the personalities of Oort and Zwicky than about anything else. 425 0066-4146/87/0915—0425$02.00 © Annual Reviews Inc. - Provided by the NASA Astrophysics Data System

A Catalog of Spectroscopically Identified White Dwarfs

Abstract: A catalog of 2249 white dwarfs which have been identified spectroscopically is presented complete through 1996 April. This compilation is the fourth edition of the Villanova Catalog of Spectroscopically Identified White Dwarfs. For each degenerate star, the following data entries with references are provided: (1) a catalog coordinate designation or WD number, in order of right ascension; (2) the right ascension and declination for epoch 1950.0; (3) the spectral type based upon the new system; (4) a catalog symbol denoting binary membership; (5) a list of most names known to exist for a given star; (6) proper motion and position angle; (7) broadband UBV photometry, V, B-V, U-B; (8) multichannel spectrophotometry, v(MC), g-r; (9) Stromgren narrowband photometry, y, b-y, u-b; (10) an absolute visual magnitude based upon the best available color-magnitude calibration or trigonometric parallax; (11) the observed radial velocity uncorrected for gravitational redshift or solar motion; and (12) the trigonometric parallax with mean error when available. Notes for unusual or peculiar stars and a coded Reference Key alphabetized by the first author's last name are presented, as well as an expanded table cross-referencing all names to the catalog WD number. An introduction and full descriptions of the entries are provided in the text.

An independent method for determining the age of the universe

Abstract: An age of 9.3 + or - 2.0 Gyr is derived for the Galactic disk on the basis of comparisons between the sudden drop in the observed luminosity distribution and theoretical evolutionary white dwarf models and allowance for a mean prewhite-dwarf lifetime of 0.3 Gyr. To obtain the age of the universe, the time between the big bang and the first appearance of stars in the Galactic disk is added. The age of the universe is estimated to be 10.3 + or - 2.2 Gyr.

Excess infrared radiation from a white dwarf—an orbiting brown dwarf?

Abstract: We have discovered that the white dwarf star Giclas 29 – 38 appears to emit substantial radiation at wavelengths between 2 and 5 μm, far in excess of that expected from an extrapolation of the visual and near-infrared spectrum of the star The infrared colour temperature of the excess radiation is 1,200±200 K and, at the distance of G29 – 38, corresponds to a total luminosity of 5 × 10−5 solar luminosities (L⊙) If the excess 35-μm radiation is emitted by a single spherical body at 1,200 K, then its radius is 015 solar radii (R⊙) These characteristics are similar to those that have been calculated for substellar objects called brown dwarfs The most natural interpretation of our observations is that there is a substellar, somewhat Jupiter-like brown dwarf in orbit around G29–38

The discovery of a 685 second orbital period from the X-ray source 4U 1820-30 in the globular cluster NGC 6624

Abstract: The discovery of a coherent 685 s periodicity in the X-ray source 4U 1820 -30, which is located in the globular cluster NGC 6624, is reported. The modulation has a peak-to-peak amplitude of up to 3 percent and was independently detected in three observations by the EXOSAT Observatory. This period is unlikely to represent the rotation of a neutron star, because accretion torques should cause it to change by at least 10 s/yr, but observations constrain any change to less than 0.4 s/yr. The period is thus identified as the orbital period of 4U 1820 - 30, the first to be found for a globular cluster X-ray source and the shortest yet identified for any binary system. In this compact system the mass-losing star is probably a low-mass (0.055 solar mass) helium white dwarf. 37 references.

The nature of the recurrent novae

Abstract: The basic properties of individual recurrent nova systems are reviewed, and the mechanisms of their outbursts are studied. Some general properties associated with accretion events and thermonuclear runaway models are briefly examined, and detailed models for the recurrent novae T CrB, RS Oph, T Pyx, U Sco, and V1017 Sgr and the possible recurrent novae WZ Sge, V616 Mon, VY Aqr, RZ Leo, V1195 Oph, and V529 Ori are discussed. The results suggest that the outbursts of U Sco and T Pyx are caused by thermonuclear runaways on the surface of massive white dwarfs, while the outbursts of T CrB and RS Oph are very probably accretion events, initiated by a burst of mass transferred from a giant companion onto a main sequence star. V1017 Sgr fails the model criteria for a recurrent nova, being more properly considered a symbiotic star. V1195 Oph, RZ Leo, VY Aqr, and WZ Sge are assigned to the class of dwarf novae. No evidence is found that V529 Ori has recurred. 228 references.

The early spectral phase of type Ib supernovae - Evidence for helium

Abstract: A near-infrared spectrum and several optical spectra of SN 1983N obtained over a period from 10 days before maximum light to about a week after maximum and of SN 1984L from about maximum light to about 60 days later are presented. On the basis of these spectra it is argued that the first definite identification of helium in supernova ejecta near maximum light has been made. The emergence of oxygen emission lines which presage the dramatic transition to the supernebular phase is identified. Supernova atmosphere models which are consistent with the observations are presented, and it is argued that a strong departure of helium level populations from LTE is necessary. The current status of SNe Ib, the arguments that they result from core collapse in moderately massive stars, and some of the immediate work necessary to further clarify the nature of these events are summarized.

The formation of ultra-short period binaries in globular clusters

Abstract: Different formation mechanisms are considered for the formation in globular clusters of ultra-short period binaries containing a neutron star and a white dwarf. Collisions between neutron stars and (red or horizontal-branch) giants are the most efficient mechanism. Only one ultra-short binary presently active as an X-ray source is expected among the 12 bright sources known in globular clusters. The possibility of forming a wide wind-driven X-ray binary is also discussed. 17 references.

Evolutionary Scenarios for Intermediate-Mass Stars in Close Binaries

Abstract: The consequences are explored of assuming that, in common envelope events involving mass transfer in close binaries, the energy required to eject the envelope from the system is comparable with the change in orbital energy of the system. Situations in which the primary fills its Roche lobe for the first time (1) after it has exhausted central hydrogen, but before it has ignited helium, and (2) after it has already developed an electron-degenerate carbon-oxygen core are studied in detail. The properties of a system consisting of a CO degenerate dwarf and a helium star, with angular momentum loss due to gravitational wave radiation maintaining the helium star in Roche lobe contact, are explored. Finally, the formation frequency of non-Roche lobe filling helium stars with companions of various types is examined to see whether they could account for some fraction of observed sdB and sdO stars. 71 references.

On interacting helium star-white dwarf pairs as supernova precursors

Abstract: Possibilite que certaines supernovae de type Ib et Ia puissent etre la consequence du transfert de masse d'une etoile a helium a une naine degeneree carbone-oxygene

Origin of millisecond pulsars

Abstract: The recent discovery1,2 of a 3-millisecond pulsar in the globular cluster M28 (NGC6626) suggests that a few per cent of type I super novae (of which there are several subcategories, all associated with old stellar populations) form neutron stars. If the contact-binary white dwarf model for type Is is invoked3, it seems probable that millisecond pulsars of low magnetic field would be formed in binary systems which might survive the supernova event.

A search for brown dwarfs and late M dwarfs in the Hyades and the Pleiades

Abstract: The J and K colors of 14 white dwarfs that are believed to be single stars and members of either the Hyades or Pleiades clusters or the Hyades supercluster were measured, and no indication of any excess 2.2 micron (K) emission above that expected from the white dwarf was found. Based on recently published theoretical cooling curves for brown dwarfs, the existence of any cool companion stars, with masses greater than approximately 0.03 solar mass within a radius of 6 arcsec of eight white dwarfs in the Hyades cluster and greater than approximately 0.015 solar mass toward the single white dwarf in the Pleiades, is ruled out. This latter limit, only 15 Jupiter masses, is probably the lowest that has yet been established for any star by purely infrared techniques. 21 references.

The exotic helium variable PG 1346 + 082

Abstract: The results of an extensive study of the blue object PG 1346 + 082, which is both a photometric and a spectroscopic variable, are presented. The system spans the B magnitude range 13.6-17.2, is brighter than m(pg) of about 14.0 roughly 74 percent of the time, and has a photometric quasi-period of four to five days. The rapid photometric flickering at minimum light is identified as the signature of mass transfer in a binary sytsem, and the He I emission at minimum light as indicating the presence of an accretion disk. The inability to detect high-energy X-rays, the absence of the high-excitation lines of He II in the optical spectrum, and the complete absence of hydrogen in the spectrum show that PG 1346 + 082 is an interacting binary white dwarf system. This identification lends strong support to the interacting twin-degenerate interpretation for AM CVn. The outbursts of the system may be similar in physical origin to dwarf novae outbursts.

Discovery of radio emission from AE Aquarii

Abstract: VLA 1.4-GHz and 4.9-GHz observations of six DQ Her cataclysmic variables, obtained in the C/D hybrid configuration with 50-MHz bandwidth, 7-sec time resolution, and limiting flux density about 200 microJy on July 21, 1984, are reported. Variable radio emission with time scale less than 5 min, circular polarization less than 15 percent, and flux density 3-5 mJy at 1.4 GHz and 8-16 mJy at 4.9 GHz is detected from AE Aqr. This emission is tentatively attributed to synchrotron emission from mildly relativistic electrons, powered by the MHD torque coupling the magnetic white dwarf to either (1) a secondary with a strong magnetic field or (2) an accretion disk.

A determination of the white-dwarf masses in wide binary radio-pulsar systems

Abstract: The recent discovery of two new binary pulsars, PS R1831–00 (ref. 1) and PSR1855 + 09 (ref. 2), brings the total number of these objects to seven. By observations of pulse arrival time one can determine the mass function and al sin i, the projected orbital separation of the pulsar from the system's centre of gravity. By adopting a mass for the pulsar the mass of the secondary can be calculated as a function of the unknown orbital inclination angle i. Here I show that one can make use of the fact that four of these binary systems appear to fit neatly into the evolutionary scheme for bright X-ray sources described by Webbink et al.3. This implies that one can apply another constraint to the orbital solution, namely that the (sub)giant progenitor of the present white dwarf secondary should have filled its Roche lobe while spinning up the pulsar companion by mass transfer. With this extra constraint one can determine the mass of the present white dwarf in these systems4,5,6. It also allows one to set an upper limit of ∼ 1.2 M⨀ to the mass of the neutron star PSR1855+09.

Spin-up and mixing in accreting white dwarfs

Abstract: It is demonstrated that existing theories of mixing in accreting white dwarfs encounter difficulties when confronted with observations of enrichments in nova ejecta. Arguments are presented, based on the Ekman spin-up process, which suggest that angular momentum transport from the accreted material to the white dwarf is more efficient than previously thought. This should lead to matter spreading over the entire white dwarf surface, as well as inward mixing. It is shown that when efficient transfer of angular momentum is taken into account, the gross features of nova outbursts can be reproduced, with the runaway occuring in a mixed layer. Some implications of the results for DQ Her, the hibernation model of novae, recurrent novae, and soft X-ray emission are discussed. 63 references.

Magnetic Field Evolution in White Dwarfs

Abstract: The evolution of magnetic fields in a star experiencing radial contraction and cooling into a 0.6 solar mass white dwarf phase is examined analytically. An MHD equation is defined for a radially contracting star with a weak poloidal field that does not disturb the structure or evolution of the star. The distribution of conductivity within the star is approximated. A Newton-Raphson finite difference code is used to track the evolution of the star by iterative solution of the radial equation. A deconvolution algorithm is defined to track the decay eigenmodes. The central conductivity of a star increases rapidly, decreasing the decay time scale beyond the age of the star. Dominance of successively lower eigenmodes can cause multiple reversals in the surface field. 43 references.

Stellar accretion of matter possessing angular momentum

Abstract: Observations of novae and theoretical results, in which the authors studied the nova outburst by radially accreting H-rich matter onto an He white dwarf, have convinced them that realistic nova models must be based on accretion that includes the angular momentum of the accreting matter. In the present paper the authors develop the theory of accretion onto stars of matter possessing angular momentum, including the shear forces between the accreting matter and the star (assumed to be nonrotating), distribution of accreted matter and its angular momentum by turbulent mixing, thermalizing of rotational kinetic energy, and loss of energy by horizontal expansion. 25 references.

Nuclear runaways in a C/O white dwarf accreting H-rich material possessing angular momentum

Abstract: The nova outburst phenomenon is presently studied in light of the results of numerical calculations for the accretion of H-rich material onto 1-solar mass C/O white dwarfs, with arrival angular momentums of the order of 10-100 percent of the Keplerian values. The range of models investigated lies between the two limiting cases of radial accretion and accretion with Keplerian angular momentum. It is judged in view of the results obtained that certain important physical effects are being excluded; one of these may be the loss of angular momentum from the white dwarf's surface layers, either inwardly to the core or outwardly through the disk. 25 references.

The ecology of rotators

Abstract: A generalized physical object, a gravimagnetic rotator (or simply, rotator), is considered. It is specified by the three characteristics: a mass, a magnetic field, and an angular momentum. A rotator interacts with its environment (plasma) via two kinds of the physical fields, the electromagnetic and the gravitational ones. This rotator is a useful theoretical consideration for quite different astrophysical objects: neutron stars, white dwarfs, magnetic stars, super-mass stars (spinars), etc. The ecology of rotators involves the classification of rotators concerning their interaction with the environment, determination of the evolutionary laws and their energetic interpretation, the analysis of stochastic fluctuations, and statistical description of evolution of rotator ensemble. These problems resemble those considered in synergetics. Possible implications for some particular astronomical objects are briefly described: X-ray and radiopulsars, single white dwarfs, X-ray bursters, cataclysmic variables, magnetic stars, active galactic nuclei, and quasars. A number of self-similar solutions are derived.

An ancient planetary nebula surrounding the old nova GK Persei

Abstract: Classical novae are a subset of the cataclysmic variable (CV) class of objects which undergo outbursts with peak luminosities ˜5×l037–5×l038 erg s–1 every 104–105 years. Around 10–5–10–4M⊙ of material is ejected at velocities typically 1,000 km s–1 at outburst. The central system is a semi-detached binary containing a white dwarf and (usually) a late-type, main-sequence companion. The companion fills its Roche lobe, and loses material, via an accretion disk, onto the surface of the white dwarf. Orbital periods are normally found to be in the range 3–10 h with CV orbits generally of low eccentricity (see ref. 1 for a review). As part of a continuing programme of observations of the old nova GK Persei, we examined Infrared Astronomical Satellite (IRAS) images in the region of the nova, and discovered extended emission in the far infrared. These observations are interpreted in terms of an ancient planetary nebula ejected from the central binary system. If this interpretation is correct, then several unique phenomena associated with GK Per may be explained. In addition, GK Per would then provide valuable clues to the evolutionary status of classical novae, and the later stages of planetary nebula formation.

On the evolution of tidal capture X-ray binaries - 4U 2127+12 (M15) to 4U 1820-30 (NGC 6624)

Abstract: A new evolutionary scenario for X-ray binaries in globular clusters, which begins with a tidal capture of a main-sequence star by a neutron star and ends with a white dwarf-neutron star system, is presented. For tidal captures of main-sequence stars into orbits too wide to begin mass transfer immediately, the subsequent evolution of the secondary can lead to a common envelope binary similar to what the 9 hr X-ray binary 4U 2127+12 in M15 is suspected to be. If the common envelope is thick enough, it may cause the neutron star and the white dwarf core of the secondary to spiral in, producing a detached white dwarf-neutron star system. Subsequently, gravitational radiation losses may evolve this into the configuration seen in the 11 minute X-ray binary 4U 1820-30 in NGC 6624. This model appears more likely on statistical grounds than formation by collision of a neutron star and a red giant. In some circumstances, the latter process may result in unstable mass transfer, which would result in coalescence rather than a binary system like 4U 1820-30.

Sustained magnetic fields in binary millisecond pulsars

Abstract: The two known binary millisecond pulsars1,2 PSR1953 + 29 and PSR1855 + 09 with periods P =6.1ms and 5.4ms, respectively, and the long orbital-period binary radio pulsars3 are believed to have evolved from low-mass X-ray binaries (LMXBs)4. The neutron stars in these binaries are believed to have formed from the accretion-induced collapse of white dwarfs with shorter orbital periods5. These neutron stars have weak magnetic fields (3×108 G≲B≲6×1011 G) and rapid spin (5 ms≲P≲1s), compared with typical isolated pulsars, and are believed to have been spun up by accretion after their magnetic fields had decayed (see ref. 6 for review) on a timescale tD ˜9×106 yr (ref. 7). If correct, the fraction of pulsars which are binary millisecond pulsars is too large8 by a factor of >100. This severe discrepancy is removed if, as we propose here, the magnetic fields of neutron stars do not decay either in such binaries or in general. We also show that, if such neutron stars are formed from the accretion-induced magnetic flux and angular momentum-conserving collapse of white dwarfs, most of them are likely to have been born, and remain, spinning rapidly and to have weak magnetic fields, in agreement with observations of binary millisecond pulsars and LMXBs.

The Eclipses of Cataclysmic Variables. II. U Geminorum

Abstract: U Gem is an eclipsing dwarf nova with an orbital period of 4 h 15 m. High-speed, multicolor photometric observations of U Gem in its quiescent state were obtained. A program was used that synthesizes the light curves of cataclysmic variables to derive the properties of U Gem from its eclipses. Using radial velocity curves published by Wade (1981) and by Stover (1981), it was found that i = 69.7 + or - 0.7 deg, M1 = 1.12 + or - 0.13 solar masses, and M2 = 0.53 + or - 0.06 solar mass. The radial temperature distribution across the accretion disk in U Gem shows that the disk is a hollow ring around the white dwarf with R(out) = 0.30 + or - 0.04 and R(in) = 0.12 + or - 0.05 a, where a is the separation of the two stars. The temperature of the ring is 4800 + or - 300 K. The model also reproduces the published infrared light curves and ultraviolet spectral distributions of U Gem. A mass transfer rate of 7.8 x 10 to the -10th solar mass/yr is derived. The structure of the ring around the white dwarf is consistent with the currentmore » theories of accretion disk instabilities in dwarf novae. 39 references.« less