4,818 research outputs found
Spin susceptibility of interacting electrons in one dimension: Luttinger liquid and lattice effects
The temperature-dependent uniform magnetic susceptibility of interacting
electrons in one dimension is calculated using several methods. At low
temperature, the renormalization group reaveals that the Luttinger liquid spin
susceptibility approaches zero temperature with an infinite slope
in striking contrast with the Fermi liquid result and with the behavior of the
compressibility in the absence of umklapp scattering. This effect comes from
the leading marginally irrelevant operator, in analogy with the Heisenberg spin
1/2 antiferromagnetic chain. Comparisons with Monte Carlo simulations at higher
temperature reveal that non-logarithmic terms are important in that regime.
These contributions are evaluated from an effective interaction that includes
the same set of diagrams as those that give the leading logarithmic terms in
the renormalization group approach. Comments on the third law of thermodynamics
as well as reasons for the failure of approaches that work in higher dimensions
are given.Comment: 21 pages, latex including 5 eps figure
Pure-hydrogen 3D model atmospheres of cool white dwarfs
A sequence of pure-hydrogen CO5BOLD 3D model atmospheres of DA white dwarfs
is presented for a surface gravity of log g = 8 and effective temperatures from
6000 to 13,000 K. We show that convective properties, such as flow velocities,
characteristic granulation size and intensity contrast of the granulation
patterns, change significantly over this range. We demonstrate that these 3D
simulations are not sensitive to numerical parameters unlike the 1D structures
that considerably depend on the mixing-length parameters. We conclude that 3D
spectra can be used directly in the spectroscopic analyses of DA white dwarfs.
We confirm the result of an earlier preliminary study that 3D model spectra
provide a much better characterization of the mass distribution of white dwarfs
and that shortcomings of the 1D mixing-length theory are responsible for the
spurious high-log g determinations of cool white dwarfs. In particular, the 1D
theory is unable to account for the cooling effect of the convective overshoot
in the upper atmospheres.Comment: 14 pages, 17 figures, accepted for publication in Astronomy and
Astrophysic
Spectroscopic analysis of DA white dwarfs with 3D model atmospheres
We present the first grid of mean three-dimensional (3D) spectra for
pure-hydrogen (DA) white dwarfs based on 3D model atmospheres. We use CO5BOLD
radiation-hydrodynamics 3D simulations instead of the mixing-length theory for
the treatment of convection. The simulations cover the effective temperature
range of 6000 < Teff (K) < 15,000 and the surface gravity range of 7 < log g <
9 where the large majority of DAs with a convective atmosphere are located. We
rely on horizontally averaged 3D structures (over constant Rosseland optical
depth) to compute spectra. It is demonstrated that our spectra can be
smoothly connected to their 1D counterparts at higher and lower Teff where the
3D effects are small. Analytical functions are provided in order to convert
spectroscopically determined 1D effective temperatures and surface gravities to
3D atmospheric parameters. We apply our improved models to well studied
spectroscopic data sets from the Sloan Digital Sky Survey and the White Dwarf
Catalog. We confirm that the so-called high-log g problem is not present when
employing spectra and that the issue was caused by inaccuracies in the 1D
mixing-length approach. The white dwarfs with a radiative and a convective
atmosphere have derived mean masses that are the same within ~0.01 Msun, in
much better agreement with our understanding of stellar evolution. Furthermore,
the 3D atmospheric parameters are in better agreement with independent Teff and
log g values from photometric and parallax measurements.Comment: 15 pages, 18 figures, 10 pages online appendix, accepted for
publication in Astronomy and Astrophysic
Granulation properties of giants, dwarfs, and white dwarfs from the CIFIST 3D model atmosphere grid
3D model atmospheres for giants, dwarfs, and white dwarfs, computed with the
CO5BOLD code and part of the CIFIST grid, have been used for spectroscopic and
asteroseismic studies. Unlike existing plane-parallel 1D structures, these
simulations predict the spatially and temporally resolved emergent intensity so
that granulation can be analysed, which provides insights on how convective
energy transfer operates in stars. The wide range of atmospheric parameters of
the CIFIST 3D simulations (3600 < Teff (K) < 13,000 and 1 < log g < 9) allows
the comparison of convective processes in significantly different environments.
We show that the relative intensity contrast is correlated with both the Mach
and Peclet numbers in the photosphere. The horizontal size of granules varies
between 3 and 10 times the local pressure scale height, with a tight
correlation between the factor and the Mach number of the flow. Given that
convective giants, dwarfs, and white dwarfs cover the same range of Mach and
Peclet numbers, we conclude that photospheric convection operates in a very
similar way in those objects.Comment: 16 pages, 17 figures, 37 pages online appendix, accepted for
publication in Astronomy and Astrophysic
3D Model Atmospheres for Extremely Low-Mass White Dwarfs
We present an extended grid of mean three-dimensional (3D) spectra for
low-mass, pure-hydrogen atmosphere DA white dwarfs (WDs). We use CO5BOLD
radiation-hydrodynamics 3D simulations covering Teff = 6000-11,500 K and logg =
5-6.5 (cgs units) to derive analytical functions to convert spectroscopically
determined 1D temperatures and surface gravities to 3D atmospheric parameters.
Along with the previously published 3D models, the 1D to 3D corrections are now
available for essentially all known convective DA WDs (i.e., logg = 5-9). For
low-mass WDs, the correction in temperature is relatively small (a few per cent
at the most), but the surface gravities measured from the 3D models are lower
by as much as 0.35 dex. We revisit the spectroscopic analysis of the extremely
low-mass (ELM) WDs, and demonstrate that the 3D models largely resolve the
discrepancies seen in the radius and mass measurements for relatively cool ELM
WDs in eclipsing double WD and WD + milli-second pulsar binary systems. We also
use the 3D corrections to revise the boundaries of the ZZ Ceti instability
strip, including the recently found ELM pulsators.Comment: 11 pages, 8 figures, accepted for publication in the Astrophysical
Journa
Effect of obesity and metabolic syndrome on plasma oxysterols and fatty acids in human
BACKGROUND:
Obesity and the related entity metabolic syndrome are characterized by altered lipid metabolism and associated with increased morbidity risk for cardiovascular disease and cancer. Oxysterols belong to a large family of cholesterol-derived molecules known to play crucial role in many signaling pathways underlying several diseases. Little is known on the potential effect of obesity and metabolic syndrome on oxysterols in human.
OBJECTIVES:
In this work, we questioned whether circulating oxysterols might be significantly altered in obese patients and in patients with metabolic syndrome. We also tested the potential correlation between circulating oxysterols and fatty acids.
METHODS:
60 obese patients and 75 patients with metabolic syndrome were enrolled in the study along with 210 age- and sex-matched healthy subjects, used as control group. Plasma oxysterols were analyzed by isotope dilution GC/MS, and plasma fatty acids profiling was assessed by gas chromatography coupled with flame ionization detection.
RESULTS:
We found considerable differences in oxysterols profiling in the two disease groups that were gender-related. Compared to controls, males showed significant differences only in 4α- and 4ÎČ-hydroxycholesterol levels in obese and metabolic syndrome patients. In contrast, females showed consistent differences in 7-oxocholesterol, 4α-hydroxycholesterol, 25-hydroxycholesterol and triol. Concerning fatty acids, we found minor differences in the levels of these variables in males of the three groups. Significant changes were observed in plasma fatty acid profile of female patients with obesity or metabolic syndrome. We found significant correlations between various oxysterols and fatty acids. In particular, 4ÎČ-hydroxycholesterol, which is reduced in obesity and metabolic syndrome, correlated with a number of saturated and mono-unsaturated fatty acids that are end-products of de novo lipogenesis.
CONCLUSIONS:
Our data provide the first evidence that obesity and metabolic syndrome are associated with major, gender-specific, changes in circulating oxysterols and fatty acids. These findings suggest a metabolic link between oxysterols and fatty acids, and that oxysterols may contribute to the epidemic diseases associated with obesity and metabolic syndrome in female
On The Evolution of Magnetic White Dwarfs
We present the first radiation magnetohydrodynamics simulations of the
atmosphere of white dwarf stars. We demonstrate that convective energy transfer
is seriously impeded by magnetic fields when the plasma-beta parameter, the
thermal to magnetic pressure ratio, becomes smaller than unity. The critical
field strength that inhibits convection in the photosphere of white dwarfs is
in the range B = 1-50 kG, which is much smaller than the typical 1-1000 MG
field strengths observed in magnetic white dwarfs, implying that these objects
have radiative atmospheres. We have then employed evolutionary models to study
the cooling process of high-field magnetic white dwarfs, where convection is
entirely suppressed during the full evolution (B > 10 MG). We find that the
inhibition of convection has no effect on cooling rates until the effective
temperature (Teff) reaches a value of around 5500 K. In this regime, the
standard convective sequences start to deviate from the ones without convection
owing to the convective coupling between the outer layers and the degenerate
reservoir of thermal energy. Since no magnetic white dwarfs are currently known
at the low temperatures where this coupling significantly changes the
evolution, effects of magnetism on cooling rates are not expected to be
observed. This result contrasts with a recent suggestion that magnetic white
dwarfs with Teff < 10,000 K cool significantly slower than non-magnetic
degenerates.Comment: 11 pages, 12 figures, accepted for publication in the Astrophysical
Journa
Constraining planet formation around 6Mâ-8Mâ stars
Identifying planets around O-type and B-type stars is inherently difficult; the most massive known planet host has a mass of only about 3Mâ. However, planetary systems which survive the transformation of their host stars into white dwarfs can be detected via photospheric trace metals, circumstellar dusty and gaseous discs, and transits of planetary debris crossing our line-of-sight. These signatures offer the potential to explore the efficiency of planet formation for host stars with masses up to the core-collapse boundary at â8Mâ, a mass regime rarely investigated in planet formation theory. Here, we establish limits on where both major and minor planets must reside around â6Mâ â 8Mâ stars in order to survive into the white dwarf phase. For this mass range, we find that intact terrestrial or giant planets need to leave the main sequence beyond approximate minimum star-planet separations of respectively about 3 and 6 au. In these systems, rubble pile minor planets of radii 10, 1.0, and 0.1 km would have been shorn apart by giant branch radiative YORP spin-up if they formed and remained within, respectively, tens, hundreds and thousands of au. These boundary values would help distinguish the nature of the progenitor of metal-pollution in white dwarf atmospheres. We find that planet formation around the highest mass white dwarf progenitors may be feasible, and hence encourage both dedicated planet formation investigations for these systems and spectroscopic analyses of the highest mass white dwarfs
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