3,313 research outputs found
SBS 0335-052E+W: deep VLT/FORS+UVES spectroscopy of the pair of the lowest-metallicity blue compact dwarf galaxies
(abridged) We present deep archival VLT/FORS1+UVES spectroscopic observations
of the system of two blue compact dwarf (BCD) galaxies SBS 0335-052E and SBS
0335-052W. Our aim is to derive element abundances in different HII regions of
this unique system of galaxies and to study spatial abundance variations. We
determine abundances of helium, nitrogen, oxygen, neon, sulfur, chlorine, argon
and iron. The oxygen abundance in the brighter eastern galaxy varies in the
range 7.11 to 7.32 in different HII regions supporting previous findings and
suggesting the presence of oxygen abundance variations on spatial scales of
~1-2 kpc. The oxygen abundance in the brightest region No.1 of SBS 0335-052W is
7.22+/-0.07, consistent with previous determinations.Three other HII regions
are much more metal-poor with an unprecedently low oxygen abundance of
12+logO/H=7.01+/-0.07 (region No.2), 6.98+/-0.06 (region No.3), and 6.86+/-0.14
(region No.4). These are the lowest oxygen abundances ever derived in
emission-line galaxies. Helium abundances derived for the brightest HII regions
of both galaxies are mutually consistent. We derive weighted mean He mass
fractions of 0.2485+/-0.0012 and 0.2514+/-0.0012 for two different sets of HeI
emissivities. The N/O abundance ratio in both galaxies is slightly higher than
that derived for other BCDs with 12+logO/H<7.6. This implies that the N/O in
extremely metal-deficient galaxies could increase with decreasing metallicity.Comment: 20 pages, 11 figures, accepted for pulication in Astronomy and
Astrophysic
Morphology and thermal conductivity of model organic aerogels
The intersection volume of two independent 2-level cut Gaussian random fields
is proposed to model the open-cell microstructure of organic aerogels. The
experimentally measured X-ray scattering intensity, surface area and solid
thermal conductivity of both polymeric and colloidal organic aerogels can be
accounted for by the model.Comment: 5 pages. RevTex with 4 encapsulated figures. Higher resolution
figures have been submitted for publication. To be published in Phys. Rev. E
(Rapid Comm.). email, [email protected]
Percolation thresholds and fractal dimensions for square and cubic lattices with long-range correlated defects
We study long-range power-law correlated disorder on square and cubic
lattices. In particular, we present high-precision results for the percolation
thresholds and the fractal dimension of the largest clusters as function of the
correlation strength. The correlations are generated using a discrete version
of the Fourier filtering method. We consider two different metrics to set the
length scales over which the correlations decay, showing that the percolation
thresholds are highly sensitive to such system details. By contrast, we verify
that the fractal dimension is a universal quantity and unaffected
by the choice of metric. We also show that for weak correlations, its value
coincides with that for the uncorrelated system. In two dimensions we observe a
clear increase of the fractal dimension with increasing correlation strength,
approaching . The onset of this change does not seem to
be determined by the extended Harris criterion.Comment: 12 pages, 8 figure
Scaling laws for random walks in long-range correlated disordered media
We study the scaling laws of diffusion in two-dimensional media with
long-range correlated disorder through exact enumeration of random walks. The
disordered medium is modelled by percolation clusters with correlations
decaying with the distance as a power law, , generated with the
improved Fourier filtering method. To characterize this type of disorder, we
determine the percolation threshold by investigating
cluster-wrapping probabilities. At , we estimate the
(sub-diffusive) walk dimension for different correlation
exponents . Above , our results suggest a normal random walk
behavior for weak correlations, whereas anomalous diffusion cannot be ruled out
in the strongly correlated case, i.e., for small .Comment: 11 pages, 6 figure
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