3,020 research outputs found
Optical absorption of divalent metal tungstates: Correlation between the band-gap energy and the cation ionic radius
We have carried out optical-absorption and reflectance measurements at room
temperature in single crystals of AWO4 tungstates (A = Ba, Ca, Cd, Cu, Pb, Sr,
and Zn). From the experimental results their band-gap energy has been
determined to be 5.26 eV (BaWO4), 5.08 eV (SrWO4), 4.94 eV (CaWO4), 4.15 eV
(CdWO4), 3.9-4.4 eV (ZnWO4), 3.8-4.2 eV (PbWO4), and 2.3 eV (CuWO4). The
results are discussed in terms of the electronic structure of the studied
tungstates. It has been found that those compounds where only the s electron
states of the A2+ cation hybridize with the O 2p and W 5d states (e.g BaWO4)
have larger band-gap energies than those where also p, d, and f states of the
A2+ cation contribute to the top of the valence band and the bottom of the
conduction band (e.g. PbWO4). The results are of importance in view of the
large discrepancies existent in prevoiusly published data.Comment: 16 pages, 3 figures, 1 tabl
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The control of near-wall turbulence through surface texturing
Certain surfaces that exhibit small textured features can interact with near-wall turbulence and reduce drag, which is of great interest to industries in the aerospace, naval, transport and energy sectors. This paper reviews and discusses the dynamic mechanisms at play in that interaction. General principles of application across different technologies are discussed, and the parameters of interest and relevance are identified. It is argued that the main effect of these surfaces can be expressed as an offset between the positions of the virtual, equivalent smooth walls perceived by different parts of the flow, namely by the mean velocity profile and by the background turbulence, which remains otherwise smooth-like. Individual technologies are also reviewed, including superhydrophobic surfaces, riblets and permeable substrates, with particular emphasis on physical mechanisms that are specific to each technology. We discuss the capillary waves that form in superhydrophobic surfaces and the spanwise-elongated rollers that form over riblets and permeable surfaces
A Failed Gamma-Ray Burst with Dirty Energetic Jets Spirited Away? New Implications for the GRB-SN Connection from Supernova 2002ap
(Abridged) SN 2002ap is an interesting event with broad spectral features
like the famous SN 1998bw / GRB 980425. Here we examine the recently proposed
jet hypothesis from SN 2002ap by a spectropolarimetric observation. We show
that jets should be moving at about 0.23c with a jet kinetic energy of ~5 x
10^{50} erg, a similar energy scale to the GRB jets. The weak radio emission
from SN 2002ap has been used to argue against the jet hypothesis, but we show
that this problem can be avoided. However, the jet cannot be kept ionized
because of adiabatic cooling without external photoionization or heating
source. We found that only the radioactivity of 56Ni is a possible source,
indicating that the jet is formed and ejected from central region of the core
collapse. Then we point out that the jet will eventually sweep up enough
interstellar medium and generate shocks in a few to 10 years, producing strong
radio emission that can be spatially resolved, giving us a clear test for the
jet hypothesis. Discussions are given on possible implications for the GRB-SN
connection in the case that the jet is real. We suggest existence of two
distinct classes of GRBs from similar core-collapse events but by completely
different mechanisms. Cosmologically distant GRBs (~10^{50} erg) are collimated
jets generated by central activity of core collapses. SN 2002ap could be a
failed GRB of this type with a large baryon load. On the other hand, much less
energetic ones like GRB 980425 are rather isotropic, which may be produced by
hydrodynamical shock acceleration at the outer envelope. We propose that the
radioactive ionization for the SN 2002ap jet may give a new explanation also
for the X-ray line features often observed in GRB afterglows.Comment: 14 pages, 5 figures. Version accepted to Ap
Compton Echoes from Gamma-Ray Bursts: Unveiling Misaligned Jets in Nearby Type Ib/c Supernovae
There is now compelling evidence of a link between long-duration gamma-ray
bursts (GRBs) and Type Ib/c supernovae (SNe). These core-collapse explosions
are conjectured to radiate an anisotropic, beamed component associated with a
decelerating, relativistic outflow and an unbeamed, isotropic component
associated with the slowly expanding stellar debris. The anisotropic emission
remains at a very low level until the Doppler cone of the beam intersects the
observer's line of sight, making off-axis GRB jets directly detectable only at
long wavelengths and late times. Circumstellar material, however, will Compton
scatter the prompt gamma-ray and afterglow radiation flux and give rise to a
reflection echo. We show that the Compton echo of a misaligned GRB carries an
X-ray luminosity that may exceed by many orders of magnitude that produced by
the underlying subrelativistic SN during the first few weeks. Bright scattering
echoes may therefore provide a means for detecting a population of misaligned
GRBs associated with nearby Type Ib/c SNe and yield crucial information on the
environment surrounding a massive star at the time of its death. The question
of whether the interpretation of GRB980425 as an ordinary GRB observed off-axis
is consistent with the lack of an X-ray echo is addressed, along with the
constraints derived on the possible existence of misaligned GRB jets in
SN1993J, SN1994I, SN1999em, and SN2002ap.Comment: 9 pages, 2 figures, to appear in the ApJ Letter
Physical Structure of Small Wolf-Rayet Ring Nebulae
We have selected the seven most well-defined WR ring nebulae in the LMC (Br
2, Br 10, Br 13, Br 40a, Br 48, Br 52, and Br 100) to study their physical
nature and evolutionary stages. New CCD imaging and echelle observations have
been obtained for five of these nebulae; previous photographic imaging and
echelle observations are available for the remaining two nebulae. Using the
nebular dynamics and abundances, we find that the Br 13 nebula is a
circumstellar bubble, and that the Br 2 nebula may represent a circumstellar
bubble merging with a fossil main-sequence interstellar bubble. The nebulae
around Br 10, Br 52, and Br 100 all show influence of the ambient interstellar
medium. Their regular expansion patterns suggest that they still contain
significant amounts of circumstellar material. Their nebular abundances would
be extremely interesting, as their central stars are WC5 and WN3-4 stars whose
nebular abundances have not been derived previously. Intriguing and tantalizing
implications are obtained from comparisons of the LMC WR ring nebulae with ring
nebulae around Galactic WR stars, Galactic LBVs, LMC LBVs, and LMC BSGs;
however, these implications may be limited by small-number statistics. A SNR
candidate close to Br 2 is diagnosed by its large expansion velocity and
nonthermal radio emission. There is no indication that Br 2's ring nebula
interacts dynamically with this SNR candidate.Comment: 20 pages, Latex (aaspp4.sty), 2 figures, accepted by the Astronomical
Journal (March 99 issue
The Diversity of Gamma-Ray Bursts and the Surroundings of Massive Stars
The finding of a Type Ic supernova connected with GRB 030329 showed a massive
star origin for this burst, supporting evidence for this association in
previous bursts with lightcurve bumps at the appropriate time for a supernova.
Here, we explore the possibility that all long bursts have massive star
progenitors, interacting with either the freely expanding wind of the
progenitor or the shocked wind. We present models for the afterglows of GRB
020405 and GRB 021211, which are a challenge to wind interaction models.
Considering sources for which wind interaction models are acceptable, a range
of wind densities is required, from values typical of Galactic Wolf-Rayet stars
to values ~100 times smaller. The reason for the low densities is unclear, but
may involve low progenitor masses and/or low metallicities. If mass is a
factor, a low density event should be associated with a low mass supernova. The
interpretation of bursts apparently interacting with constant density media as
interaction with a shocked wind requires both a range of mass loss densities
and a range of external pressures. The highest pressures, p/k > 10^8 cm^{-3} K,
may be due to an extreme starburst environment, which would imply that the
burst is superposed on an active star forming region. Although the range of
observed events can be accomodated by the shocked wind theory, special
circumstances are necessary to bring this about. Finally, we consider the high
velocity, high ionization absorption features observed in some afterglow
spectra. If the features are circumstellar, the presence of the burst in a
starburst region may be important for the formation of clumps near the burst.Comment: 31 pages, 1 figure, ApJ, submitte
FUSE Observations of Nebular O VI Emission from NGC 6543
NGC 6543 is one of the few planetary nebulae (PNe) whose X-ray emission has
been shown to be extended and originate from hot interior gas. Using FUSE
observations we have now detected nebular O VI emission from NGC 6543. Its
central star, with an effective temperature of ~50,000 K, is too cool to
photoionize O V, so the O VI ions must have been produced by thermal collisions
at the interface between the hot interior gas and the cool nebular shell. We
modeled the O VI emission incorporating thermal conduction, but find that
simplistic assumptions for the AGB and fast wind mass loss rates overproduce
X-ray emission and O VI emission. We have therefore adopted the pressure of the
interior hot gas for the interface layer and find that expected O VI emission
to be comparable to the observations.Comment: 4 pages, 4 figures, 1 table, using emulateapj.cls style. Accepted for
publication in ApJ Letter
A new radiative cooling curve based on an up to date plasma emission code
This work presents a new plasma cooling curve that is calculated using the
SPEX package. We compare our cooling rates to those in previous works, and
implement the new cooling function in the grid-adaptive framework `AMRVAC'.
Contributions to the cooling rate by the individual elements are given, to
allow for the creation of cooling curves tailored to specific abundance
requirements. In some situations, it is important to be able to include
radiative losses in the hydrodynamics. The enhanced compression ratio can
trigger instabilities (such as the Vishniac thin-shell instability) that would
otherwise be absent. For gas with temperatures below 10,000 K, the cooling time
becomes very long and does not affect the gas on the timescales that are
generally of interest for hydrodynamical simulations of circumstellar plasmas.
However, above this temperature, a significant fraction of the elements is
ionised, and the cooling rate increases by a factor 1000 relative to lower
temperature plasmas.Comment: 11 pages, 6 figures. Typos fixed to match version on A&A
'forthcoming' website. Tables in text format online available at
http://www.phys.uu.nl/~schure/coolin
A Compact X-ray Source and Possible X-ray Jets within the Planetary Nebula Menzel 3
We report the discovery, by the Chandra X-ray Observatory, of X-ray emission
from the bipolar planetary nebula Menzel 3. In Chandra CCD imaging, Mz 3
displays hot (3-6x10^6 K) gas within its twin, coaxial bubbles of optical
nebulosity, as well as a compact X-ray source at the position of its central
star(s). The brightest diffuse X-ray emission lies along the polar axis of the
optical nebula, suggesting a jet-like configuration. The observed combination
of an X-ray-emitting point source and possible X-ray jet(s) is consistent with
models in which accretion disks and, potentially, magnetic fields shape bipolar
planetary nebulae via the generation of fast, collimated outflows.Comment: 12 pages, 3 figures; to appear in Astrophysical Journal (Letters
The gas turbulence in planetary nebulae: quantification and multi-D maps from long-slit, wide-spectral range echellogram
This methodological paper is part of a short series dedicated to the
long-standing astronomical problem of de-projecting the bi-dimensional,
apparent morphology of a three-dimensional distribution of gas. We focus on the
quantification and spatial recovery of turbulent motions in planetary nebulae
(and other classes of expanding nebulae) by means of long-slit echellograms
over a wide spectral range. We introduce some basic theoretical notions,
discuss the observational methodology, and develop an accurate procedure
disentangling all broadening components of the velocity profile in all spatial
positions of each spectral image. This allows us to extract random, non-thermal
motions at unprecedented accuracy, and to map them in 1-, 2- and 3-dimensions.
We present the solution to practical problems in the multi-dimensional
turbulence-analysis of a testing-planetary nebula (NGC 7009), using the
three-step procedure (spatio-kinematics, tomography, and 3-D rendering)
developed at the Astronomical Observatory of Padua. In addition, we introduce
an observational paradigm valid for all spectroscopic parameters in all classes
of expanding nebulae. Unsteady, chaotic motions at a local scale constitute a
fundamental (although elusive) kinematical parameter of each planetary nebula,
providing deep insights on its different shaping agents and mechanisms, and on
their mutual interaction. The detailed study of turbulence, its stratification
within a target and (possible) systematic variation among different sub-classes
of planetary nebulae deserve long-slit, multi-position angle, wide-spectral
range echellograms containing emissions at low-, medium-, and high-ionization,
to be analyzed pixel-to-pixel with a straightforward and versatile methodology,
extracting all the physical information stored in each frame at best.Comment: 11 page, 10 figures, A&A in pres
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