6,002 research outputs found
Discovery of Enhanced Germanium Abundances in Planetary Nebulae with FUSE
We report the discovery of Ge III 1088.46 in the planetary nebulae
(PNe) SwSt 1, BD+303639, NGC 3132, and IC 4593, observed with the Far
Ultraviolet Spectroscopic Explorer. This is the first astronomical detection of
this line and the first measurement of Ge (Z = 32) in PNe. We estimate Ge
abundances using S and Fe as reference elements, for a range of assumptions
about gas-phase depletions. The results indicate that Ge, which is synthesized
in the initial steps of the s-process and therefore can be self-enriched in
PNe, is enhanced by factors of > 3-10. The strongest evidence for enrichment is
seen for PNe with Wolf-Rayet central stars, which are likely to contain heavily
processed material.Comment: 11 pages, 1 figure, accepted for publication in ApJ Letter
More evidence for an intracluster planetary nebulae population in the Virgo cluster
We surveyed a 50 sq arcmin region in the Virgo cluster core to search for
intergalactic planetary nebulae, and found 11 candidates in the surveyed area.
The measured fluxes of these unresolved sources are consistent with these
objects being planetary nebulae from an intracluster population of stars. We
compute the cumulative luminosity function of these 11 planetary nebula
candidates. If we assume that they belong to the Virgo cluster, their
cumulative luminosity function is in good agreement with planetary nebula
luminosity function simulations. This comparison allows us to estimate the
surface mass density of the intracluster stellar population at the surveyed
field in the cluster core.Comment: ApJ Letters, in press. A .ps file is also available at:
http://www.usm.uni-muenchen.de:8001/people/mendez/preprints/preprints.htm
Intracluster stars in the Virgo cluster core
We have investigated the properties of the diffuse light in the Virgo cluster
core region, based on the detection of intracluster planetary nebulae (PNe) in
four fields. We eliminate the bias from misclassified faint continuum objects,
using improved Monte Carlo simulations, and the contaminations by high redshift
Ly galaxies, using the Ly luminosity function in blank fields.
Recent spectroscopic observations confirm that our photometric PN samples are
well-understood. We find that the diffuse stellar population in the Virgo core
region is inhomogeneous on scales of 30'-90': there exist significant
field-to-field variations in the number density of PNe and the inferred amount
of intracluster light, with some empty fields, some fields dominated by
extended Virgo galaxy halos, and some fields dominated by the true intracluster
component. There is no clear trend with distance from M87. The mean surface
luminosity density, its rms variation, and the mean surface brightness of
diffuse light in our 4 fields are L
arcmin, L arcmin, and
mag arcsec respectively. Our results indicate that
the Virgo cluster is a dynamically young environment, and that the intracluster
component is associated at least partially with local physical processes like
galaxy interactions or harassment. We also argue, based on kinematic evidence,
that the so-called 'over-luminous' PNe in the halo of M84 are dynamically
associated with this galaxy, and must thus be brighter than and part of a
different stellar population from the normal PN population in elliptical
galaxies.Comment: 31 pages, 6 figure. In press on the Astronomical Journa
Padrões tecnológicos e econômicos do arroz de terras altas.
O presente trabalho teve como objetivo atualizar as informações sobre a cultura do arroz no Mato Grosso, visando fornecer subsídios às ações de pesquisa e transferência de tecnologia.bitstream/CNPAF/20437/1/comt_52.pd
The open cluster NGC 6520 and the nearby dark molecular cloud Barnard 86
Wide field BVI photometry and CO(10) observations are presen ted
in the region of the open cluster NGC 6520 and the dark molecular cloud
Barnard~86. From the analysis of the optical data we find that the cluster is
rather compact, with a radius of 1.00.5 arcmin, smaller than previous
estimates. The cluster age is 15050 Myr and the reddening
E=0.420.10. The distance from the Sun is estimated to be
1900100 pc, and it is larger than previous estimates. We finally derive
basic properties of the dark nebula Barnard 86 on the assumption that it lies
at the same distance of the cluster.Comment: 21 pages, 8 eps figures (a few degraded in resolution), accepted for
publication in the Astronomical Journa
Suppression of hole-hole scattering in GaAs/AlGaAs heterostructures under uniaxial compression
Resistance, magnetoresistance and their temperature dependencies have been
investigated in the 2D hole gas at a [001] p-GaAs/AlGaAs
heterointerface under [110] uniaxial compression. Analysis performed in the
frame of hole-hole scattering between carriers in the two spin splitted
subbands of the ground heavy hole state indicates, that h-h scattering is
strongly suppressed by uniaxial compression. The decay time of the
relative momentum reveals 4.5 times increase at a uniaxial compression of 1.3
kbar.Comment: 5 pages, 3 figures. submitted to Phys.Rev.
Effect of nonlinearity on the dynamics of a particle in dc field-induced systems
Dynamics of a particle in a perfect chain with one nonlinear impurity and in
a perfect nonlinear chain under the action of dc field is studied numerically.
The nonlinearity appears due to the coupling of the electronic motion to
optical oscillators which are treated in adiabatic approximation.
We study for both the low and high values of field strength. Three different
range of nonlinearity is obtained where the dynamics is different. In low and
intermediate range of nonlinearity, it reduces the localization. In fact in the
intermediate range subdiffusive behavior in the perfect nonlinear chain is
obtained for a long time. In all the cases a critical value of nonlinear
strength exists where self-trapping transition takes place. This critical value
depends on the system and the field strength. Beyond the self-trapping
transition nonlinearity enhances the localization.Comment: 9 pages, Revtex, 6 ps figures include
Resonant Tunneling through Linear Arrays of Quantum Dots
We theoretically investigate resonant tunneling through a linear array of
quantum dots with subsequent tunnel coupling. We consider two limiting cases:
(i) strong Coulomb blockade, where only one extra electron can be present in
the array (ii) limit of almost non-interacting electrons. We develop a density
matrix description that incorporates the coupling of the dots to reservoirs. We
analyze in detail the dependence of the stationary current on the electron
energies, tunnel matrix elements and rates, and on the number of dots. We
describe interaction and localization effects on the resonant current. We
analyze the applicability of the approximation of independent conduction
channels. We find that this approximation is not valid when at least one of the
tunnel rates to the leads is comparable to the energy splitting of the states
in the array. In this case the interference of conduction processes through
different channels suppresses the current.Comment: 12 pages, 5 figure
Temperature constraints on the coldest brown dwarf known WISE 0855-0714
Context. Nearby isolated planetary mass objects are beginning to be
discovered, but their individual properties are poorly constrained because
their low surface temperatures and strong molecular self-absorption make them
extremely faint.
Aims. We aimed to detect the near infrared emission of the coldest brown
dwarf (BD) found so far, WISE08550714, located 2.2 pc away, and to
improve its temperature estimate (T= 225-260 K) from a comparison
with state-of-the-art models of BD atmospheres.
Methods. We observed the field containing WISE0855-0714 with HAWK-I at the
VLT in the band. For BDs with T500\,K theoretical models
predict strong signal (or rather less molecular absorption) in this band.
Results. WISE0855-0714 was not detected in our Y-band images, thus placing an
upper limit on its brightness to Y>24.4 mag at 3- level, leading to
Y-[4.5]>10.5. Combining this limit with previous detections and upper limits at
other wavelengths, WISE08550714 is confirmed as the reddest BD detected,
further supporting its status as the coldest known brown dwarf. We applied
spectral energy distribution fitting with collections of models from two
independent groups for extremely cool BD atmospheres leading to an effective
temperature of T250\,K,.Comment: 4 pages, 4 figures. A&A letter Accepte
Discovery of a ~1 Hz quasi-periodic oscillation in the low-mass X-ray binary 4U 1746-37
We have discovered a ~1 Hz quasi-periodic oscillation (QPO) in the persistent
X-ray emission and during type I X-ray bursts of the globular cluster source,
dipper and low-mass X-ray binary (LMXB) 4U 1746-37. The QPO properties resemble
those of QPOs found recently in the LMXB dippers 4U 1323-62, and EXO 0748-676,
which makes 4U 1746-37 the third source known to exhibit this type of QPOs. We
present evidence for X-ray spectral changes in this source similar to those
observed in LMXBs referred to as atoll sources. We detect two states, a low
intensity and spectrally hard state, and a higher intensity and spectrally soft
state. This may explain the different spectral characteristics reported for 4U
1746-37 earlier. The high intensity state resembles the banana branch state of
atoll sources. The QPOs are only seen in the low intensity state, and are
absent when the source is in the banana branch. This strongly suggests that
either the accretion disk or an extended central source change shape between
the low intensity state and the banana branch. Twelve bursts were detected, of
which 5 took place while the source was on the banana branch and 7 when the
source was in the low intensity state. The bursts occurring on the banana
branch had an e-folding time ~3 times longer than those which occurred in the
low intensity state. Whereas previously detected dips showed only a decrease in
count rate of ~15%, we found in one observation a dip in which the count rate
dropped from ~200 counts per second to ~20 counts per second. This dip lasted
only ~250 seconds, during which clear spectral hardening occured. This is the
first time strong evidence for spectral changes during a dip are reported for
this source.Comment: 17 pages, accepted for publication in Ap
- …