Low-mass White Dwarfs Research Articles

Optical and radio observations of the binary pulsar 1855 + 09 - Evolution of pulsar magnetic fields and low-mass white dwarf cooling

view Abstract Citations (34) References (36) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Optical and Radio Observations of the Binary Pulsar 1855+09: Evolution of Pulsar Magnetic Fields and Low-Mass White Dwarf Cooling Kulkarni, S. R. ; Djorgovski, S. ; Klemola, A. R. Abstract New radio and optical observations of the 5.4 ms binary pulsar 1855 + 09 are reported. The visual extinction of the system is shown to be about 1.5 mag. Only one star brighter than R about 24.6 is found that, on the basis of positional coincidence, can be plausibly the optical counterpart of the secondary star. The spectrum of this candidate is inconsistent with a low-mass main-sequence star. Neither is it a white dwarf because the spectroscopic distance modulus is inconsistent with the lower limit on distance obtained from timing observations. It is concluded that the companion of PSR 1855 + 09 must be a low-mass cold white dwarf. The inferred cooling age supports the hypothesis that magnetic field strengths of millisecond pulsars are essentially constant and that millisecond pulsars are long-lived objects. Publication: The Astrophysical Journal Pub Date: January 1991 DOI: 10.1086/169621 Bibcode: 1991ApJ...367..221K Keywords: Binary Stars; Pulsars; Radio Observation; Stellar Magnetic Fields; Visual Observation; White Dwarf Stars; Neutron Stars; Radio Sources (Astronomy); Stellar Evolution; Stellar Mass; Astrophysics; PULSARS; RADIO SOURCES: IDENTIFICATIONS; STARS: BINARIES; STARS: NEUTRON; STARS: WHITE DWARFS full text sources ADS | data products SIMBAD (5)

On the consequences of low-mass white dwarf mergers

The theory of binary star evolution suggests that about 10 percent of all main-sequence binary systems should evolve into a close pair of light white dwarfs which merge within a Hubble time. This paper explores the consequences of such mergers on the assumption that a merger can be approximated by a mass-transfer event which occurs on a time scale shorter than that given by the Eddington accretion limit. The evolution of He + He mergers and of CO + He and of hybrid + He mergers are discussed. The birthrate of helium degenerate pairs which merge in less than a Hubble time is estimated, and the space density of low-luminosity merger products currently present in the Galaxy is predicted. It is shown that the evolutionary tracks of models of simulated mergers pass through the region in the H-R diagram occupied by subdwarfs, but that the predicted space density of merger products exceeds by over a factor of three the space density of subdwarf estimated form the known sample of such stars.

On the origin of hot subdwarfs in globular clusters

Recent observations have unveiled the existence of a large population of subdwarf B stars in several globular clusters. It is suggested that these stars are the consequences of mergers of low-mass helium white dwarf pairs that were formed from primordial main-sequence binaries. Such mergers may be hastened by close encounters between the precursor double degenerate pairs and other cluster members; these encounters serve to harden the binary sufficiently that gravitational wave radiation will force contact in much less than a Hubble time. The presence of mergers caused by encounters may explain the variations in central concentration and numbers of these stars in different clusters. 29 refs.

Optical observations of the eclipsing binary radio pulsar PSR1957 + 20

The recently discovered1 1.6 ms binary radio pulsar PSR1957 + 20 shows radio eclipses whose duration indicates that the occulting body is substantially larger than the Roche lobe of the low-mass (∼10–2M⊙) companion star. This suggests1–4 that this companion is evaporating through the action of a strong pulsar energy flux5. An optical counterpart has been detected which shows brightness variations in phase with the 9.2 h orbital cycle6. We have obtained optical charge coupled device (CCD) images which show that the counterpart is one component of a close visual pair separated by ∼0.7 arcsec. At maximum both are equally bright with combined V magnitude of 19.9, while at minimum PSR1957 + 20 is invisible. From spectroscopic observations we find that the contaminating star is a normal G star. The spectrum of PSR1957 + 20 shows intermittent Hα emission. We confirm that the optical brightness of PSR1957 + 20 varies in phase with the radio Doppler velocity curve1, and find that the amplitude is probably more than 3 magnitudes, minimum light coinciding with the radio eclipse. The optical light curve is consistent with heating of a hydrogen-rich low-mass white dwarf by high-energy radiation from the nearby millisecond pulsar.

Orbital motion of the secondary in three AM Her systems: evidence for low-mass white dwarfs

Detection spectroscopique des secondaires dans les trois systemes AM Her: CW1103+254, PG 1550+191 et E 2003+225

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

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

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.

The nearby quadruple system G 107-69/70

view Abstract Citations (32) References (6) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The nearby quadruple system G 107-69/70 Harrington, R. S. ; Christy, J. W. ; Strand, K. A. Abstract The binary system G 107-69/70 is nearby, having a relative parallax of 0.0896 + or - 0.0014 arcsec. The primary (G 107-69) is a newly discovered unresolved astrometric binary, with a best-period estimate of approximately 0.94 yr, suggesting that the faint component might be a low-mass red dwarf. The secondary (G 107-70) is a partially resolved binary, having a period of 20.5 yr, and consisting of low-mass white dwarfs. Publication: The Astronomical Journal Pub Date: June 1981 DOI: 10.1086/112967 Bibcode: 1981AJ.....86..909H Keywords: Astrometry; Binary Stars; Dwarf Stars; Star Clusters; Stellar Motions; Astronomical Photography; Stellar Parallax; White Dwarf Stars; Astronomy full text sources ADS | data products SIMBAD (2)

Comments on the evolution and origin of cataclysmic binaries

Some aspects of the observational data on cataclysmic binaries are discussed and some possible correlations between type of behaviour and binary period are noted. A gap between 2 and 3 hours in the histogram of binary periods is estimated to be real. A numerical procedure for following the evolution of Roche-lobe-fiUing stars using simplified equations is described. This procedure Is applied to white/red dwarf binaries for a variety of initial conditions, and of mass loss and angular momentum loss mechanisms. The results of these calculations, in which we ignore the short timescale behaviour of the systems, are classified into four modes of evolution: normal, nuclear evolution dominated, angular momentum loss dominated and hydrodynamical. The results are discussed in connection with cataclysmic binaries. The clustering in period below 2 hours is Interpreted in terms of evolution following the hydrodynamical mode, and it is suggested that such systems contain low mass white dwarfs as well as low mass secondaries. These may be the most common type of cataclysmic binary. A possible explanation of the clustering of classical novae systems to binary periods of 3 to 5 hours is mentioned, and evolutionary scenarios for cataclysmic binaries are outlined. We suggest, following Ritter and Webbink, that the short period systems (≲ 2 hrs) arise mainly from late Case B mass transfer in the original binary (original primary mass 1.5 to 3M ⊙ ) and the longer period systems arise mainly from Case C mass transfer. Full text to be published in Monthly Notices of the Royal Astronomical Society.

SS 433 – The Ultimate Cataclysmic Variable?

Several authors (Begelman et al. 1979; Martin and Rees 1979; Bandwagon 1979) suggest that the precessing relativistic gas jets observed in SS 433 (Abell and Margon 1979) arise from de Sitter and/or Lense-Thirring precession of one member of a close binary system. For a semi-detached system of a few solar masses, we find the mean density of the lobe-filling secondary, ρ̄2 ∽ 3 × 104 gm cm-3. The secondary may thus be a low mass white dwarf, transferring material to the vicinity of its companion at a supercritical rate because of gravitational radiation losses.

Comments on the Evolution and Origin of cataclysmic Binaries

AbstractSome aspects of the observational data on cataclysmic binaries are discussed and some possible correlations between type of behaviour and binary period are noted. A gap between 2 and 3 hours in the histogram of binary periods is estimated to be real. A numerical procedure for following the evolution of Roche-lobe-fiUing stars using simplified equations is described. This procedure Is applied to white/red dwarf binaries for a variety of initial conditions, and of mass loss and angular momentum loss mechanisms. The results of these calculations, in which we ignore the short timescale behaviour of the systems, are classified into four modes of evolution: normal, nuclear evolution dominated, angular momentum loss dominated and hydrodynamical. The results are discussed in connection with cataclysmic binaries. The clustering in period below 2 hours is Interpreted in terms of evolution following the hydrodynamical mode, and it is suggested that such systems contain low mass white dwarfs as well as low mass secondaries. These may be the most common type of cataclysmic binary. A possible explanation of the clustering of classical novae systems to binary periods of 3 to 5 hours is mentioned, and evolutionary scenarios for cataclysmic binaries are outlined. We suggest, following Ritter and Webbink, that the short period systems (≲ 2 hrs) arise mainly from late Case B mass transfer in the original binary (original primary mass 1.5 to 3M⊙) and the longer period systems arise mainly from Case C mass transfer.Full text to be published in Monthly Notices of the Royal Astronomical Society.

Equations of State for Stellar Partial Ionization Zones

A composite numerical equation of state has been developed to compute thermodynamic surfaces covering the density-temperature domain of interest for stellar ionization zones. Emphasis has been placed on the thermodynamic constraints of stability and consistency in obtaining the ionization equilibrium conditions for nonideal, multicomponent gases. Two different theoretical models have been used. First, in the low-denisty regime, the ionization equilibrium has been obtained by the free-energy minimization technique. This method explicitly minimizes the free energy of a statistical mechanical model by finding the equilibrium concentrations of all atomic and ionic species. This particular model includes electron degeneracy, Coulomb interactions, fluid interactions, and electronic perturbations. Second, in the high-density regime where material properties are dominated by the electron gas, a hot Thomas-Fermi model has been employed. Both these models become inaccurate in the low-temperature, intermediate-density domain, and interpolation was used to approximately describe the properties of matter in this complex region. However, by making use of Maxwell's relations, thermodynamic consistency was preserved even in this interpolation region.The results are presented in the form of extensive tables listing the pressure, the internal energy, the adiabatic gradient, and the pressure derivatives as functions of the temperature and the density. Other thermodynamic quantities of more » astrophysical interest such as specific heats and adiabatic exponents can easily be derived from the listed data. Different methods of composition interpolation have been investigated and it is found that an ''additive volume'' law is to be preferred. The present results are relevant to studies of pulsational properties and outer layers of white dwarfs, and also to evolutionary calculations of low-mass main-sequence stars, red giants, and white dwarfs. « less

Mass and stability of rigidly rotating relativistic polytropes by energy method

The energy of a rigidly rotating star is written in the first and second post-Newtonian approximation. Conditions for equilibrium and stability are derived, as well as evolutionary paths for stars shedding angular momentum or mass. For polytropic index n > 1, these analytic results agree with exact numerical results to within a few percent as long as the general relativity index ( P ϱc 2 ) c < 0.1 . The energy method is applied to low mass white dwarfs, to semirelativistic neutron stars and to supermassive stars.

Low-Mass White Dwarfs and the Cooling Sequences in the Hyades Cluster

Evolutionary cooling sequences and lifetimes for low mass white dwarfs in Hyades cluster, considering near surface convection in models with hydrogen envelopes