1,675 research outputs found
Nonequilibrium perturbation theory for spin-1/2 fields
A partial resummation of perturbation theory is described for field theories
containing spin-1/2 particles in states that may be far from thermal
equilibrium. This allows the nonequilibrium state to be characterized in terms
of quasiparticles that approximate its true elementary excitations. In
particular, the quasiparticles have dispersion relations that differ from those
of free particles, finite thermal widths and occupation numbers which, in
contrast to those of standard perturbation theory evolve with the changing
nonequilibrium environment. A description of this kind is essential for
estimating the evolution of the system over extended periods of time. In
contrast to the corresponding description of scalar particles, the structure of
nonequilibrium fermion propagators exhibits features which have no counterpart
in the equilibrium theory.Comment: 16 pages; no figures; submitted to Phys. Rev.
Irradiated brown dwarfs
We have observed the post common envelope binary WD0137-349 in the near
infrared , and bands and have determined that the photometry varies
on the system period (116 min). The amplitude of the variability increases with
increasing wavelength, indicating that the brown dwarf in the system is likely
being irradiated by its 16500 K white dwarf companion. The effect of the
(primarily) UV irradiation on the brown dwarf atmosphere is unknown, but it is
possible that stratospheric hazes are formed. It is also possible that the
brown dwarf (an L-T transition object) itself is variable due to patchy cloud
cover. Both these scenarios are discussed, and suggestions for further study
are made.Comment: 5 pages, 2 figures. Proceedings from "Brown dwarfs come of age"
meeting in Fuerteventura 201
WD0837+185:the formation and evolution of an extreme mass ratio white dwarf-brown dwarf binary in Praesepe
There is a striking and unexplained dearth of brown dwarf companions in close
orbits (< 3AU) around stars more massive than the Sun, in stark contrast to the
frequency of stellar and planetary companions. Although rare and relatively
short-lived, these systems leave detectable evolutionary end points in the form
of white dwarf - brown dwarf binaries and these remnants can offer unique
insights into the births and deaths of their parent systems. We present the
discovery of a close (orbital separation ~ 0.006 AU) substellar companion to a
massive white dwarf member of the Praesepe star cluster. Using the cluster age
and the mass of the white dwarf we constrain the mass of the white dwarf
progenitor star to lie in the range 3.5 - 3.7 Msun (B9). The high mass of the
white dwarf means the substellar companion must have been engulfed by the B
star's envelope while it was on the late asymptotic giant branch (AGB). Hence,
the initial separation of the system was ~2 AU, with common envelope evolution
reducing the separation to its current value. The initial and final orbital
separations allow us to constrain the combination of the common envelope
efficiency (alpha) and binding energy parameters (lambda) for the AGB star to
alpha lambda ~3. We examine the various formation scenarios and conclude that
the substellar object was most likely to have been captured by the white dwarf
progenitor early in the life of the cluster, rather than forming in situ.Comment: Accepted for publication in ApJ
Scaling in high-temperature superconductors
A Hartree approximation is used to study the interplay of two kinds of
scaling which arise in high-temperature superconductors, namely critical-point
scaling and that due to the confinement of electron pairs to their lowest
Landau level in the presence of an applied magnetic field. In the neighbourhood
of the zero-field critical point, thermodynamic functions scale with the
scaling variable , which differs from the variable
suggested by the gaussian approximation.
Lowest-Landau-level (LLL) scaling occurs in a region of high field surrounding
the upper critical field line but not in the vicinity of the zero-field
transition. For YBaCuO in particular, a field of at least 10 T is needed to
observe LLL scaling. These results are consistent with a range of recent
experimental measurements of the magnetization, transport properties and,
especially, the specific heat of high- materials.Comment: 22 pages + 1 figure appended as postscript fil
Photometric Variability and Rotation in Magnetic White Dwarfs
We present a search for long term (months—years) photometric variability in a sample of ten isolated magnetic white dwarfs using observations taken with the Liverpool Robotic Telescope between March 2005 and January 2007. These stars had previously been found to be photometrically stable on short (hours—one week) timescales [1]. We construct differential light curves for each target and then use CLEAN and Lomb‐Scargle periodograms to determine any periodicity that may be present. Photometric variability is detected in two of the targets during the observed timescale—G 240–72 and G 227–28. We find no variability in the remaining eight targets above the 1% level. Finally, we search for any correlations between the spin periods and intrinsic physical properties of magnetic white dwarfs, such as the magnetic field strength, temperature, mass and age
Component masses of young, wide, non-magnetic white dwarf binaries in the SDSS DR7
We present a spectroscopic component analysis of 18 candidate young, wide,
non-magnetic, double-degenerate binaries identified from a search of the Sloan
Digital Sky Survey Data Release 7 (DR7). All but two pairings are likely to be
physical systems. We show SDSS J084952.47+471247.7 + SDSS J084952.87+471249.4
to be a wide DA+DB binary, only the second identified to date. Combining our
measurements for the components of 16 new binaries with results for three
similar, previously known systems within the DR7, we have constructed a mass
distribution for the largest sample to date (38) of white dwarfs in young,
wide, non-magnetic, double-degenerate pairings. This is broadly similar in form
to that of the isolated field population with a substantial peak around M~0.6
Msun. We identify an excess of ultra-massive white dwarfs and attribute this to
the primordial separation distribution of their progenitor systems peaking at
relatively larger values and the greater expansion of their binary orbits
during the final stages of stellar evolution. We exploit this mass distribution
to probe the origins of unusual types of degenerates, confirming a mild
preference for the progenitor systems of high-field-magnetic white dwarfs, at
least within these binaries, to be associated with early-type stars.
Additionally, we consider the 19 systems in the context of the stellar initial
mass-final mass relation. None appear to be strongly discordant with current
understanding of this relationship.Comment: 20 pages, 5 Tables, 7 figures. accepted for publication in MNRA
Critical-point scaling function for the specific heat of a Ginzburg-Landau superconductor
If the zero-field transition in high temperature superconductors such as
YBa_2Cu_3O_7-\delta is a critical point in the universality class of the
3-dimensional XY model, then the general theory of critical phenomena predicts
the existence of a critical region in which thermodynamic functions have a
characteristic scaling form. We report the first attempt to calculate the
universal scaling function associated with the specific heat, for which
experimental data have become available in recent years. Scaling behaviour is
extracted from a renormalization-group analysis, and the 1/N expansion is
adopted as a means of approximation. The estimated scaling function is
qualitatively similar to that observed experimentally, and also to the
lowest-Landau-level scaling function used by some authors to provide an
alternative interpretation of the same data. Unfortunately, the 1/N expansion
is not sufficiently reliable at small values of N for a quantitative fit to be
feasible.Comment: 20 pages; 4 figure
Critical Casimir Effect in 3He-4He films
Universal aspects of the thermodynamic Casimir effect in wetting films of
3He-4He mixtures near their bulk tricritical point are studied within suitable
models serving as representatives of the corresponding universality class. The
effective forces between the boundaries of such films arising from the
confinement are calculated along isotherms at several fixed concentrations of
3He. Nonsymmetric boundary conditions impose nontrivial concentration profiles
leading to repulsive Casimir forces which exhibit a rich behavior of the
crossover between the tricritical point and the line of critical points. The
theoretical results agree with published experimental data and emphasize the
importance of logarithmic corrections.Comment: 12 pages, 4 figures, submitted to the Phys. Rev. Let
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