2,402 research outputs found
Observations of the Crab Nebula and its pulsar in the far-ultraviolet and in the optical
We present HST/STIS far-UV observations of the Crab nebula and its pulsar.
Broad, blueshifted absorption arising in the nebula is seen in C IV 1550,
reaching about 2500 km/s. This can be interpreted as evidence for a fast outer
shell, and we adopt a spherically symmetric model to constrain the properties
of this. We find that the density appears to decrease outward in the shell. A
lower limit to the mass is 0.3 solar masses with an accompanying kinetic energy
of 1.5EE{49} ergs. A massive 10^{51} erg shell cannot be excluded, but is less
likely if the density profile is much steeper than R^{-4} and the velocity is
<6000 km/s. The observations cover the region 1140-1720 A. With the time-tag
mode of the spectrograph we obtain the pulse profile. It is similar to that in
the near-UV, although the primary peak is marginally narrower. Together with
the near-UV data, and new optical data from NOT, our spectrum of the pulsar
covers the entire region from 1140-9250 A. Dereddening the spectrum gives a
flat spectrum for E(B-V)=0.52, R=3.1. This dereddened spectrum of the Crab
pulsar can be fitted by a power law with spectral index alpha_{\nu} = 0.11 +/-
0.04. The main uncertainty is the amount and characteristics of the interstel-
lar reddening, and we have investigated the dependence of \alpha_{\nu} on
E(B-V) and R. In the extended emission covered by our 25" x 0.5" slit in the
far-UV, we detect C IV 1550 and He II 1640 emission lines from the Crab nebula.
Several interstellar absorption lines are detected toward the pulsar. The Ly
alpha absorption indicates a column density of 3.0+/-0.5\EE{21} cm^{-2} of
neutral hydrogen, which agrees well with our estimate of E(B-V)=0.52 mag. Other
lines show no evidence of severe depletion of metals in atomic gas.Comment: 18 pages emulateapj style, including 10 figures. ApJ, accepte
Variation in wood properties among five full-sib families of Norway spruce (Picea abies)
Genetic- and environmental variation and correlation patterns were
characterized for modulus of elasticity (MOE), modulus of rupture (MOR) and
related wood traits: latewood proportion, wood density, spiral grain,
microfibril angle and lignin content in five full-sib families of Norway
spruce. The families were evaluated on the basis of clearwood specimens from
the juvenile -mature wood transition zone of 93 sampled trees at age 30 year
from seed. Family-means varied significantly ( < 0.05) for all wood traits
studied except lignin content. MOE varied between 7.9–14.1 GPa among trees
and 9.4–11.0 GPa among families. MOR varied between 47–87 MPa among trees
and 61–71 MPa among families. Families remained significantly different in
an analysis of specific MOE (MOE/density) and MOR (MOR/density). Hence,
solely relying on wood density as a wood quality trait in tree breeding
would not fully yield the potential genetic gain for MOE and MOR.
Correlations between wood structural traits and specific MOE and MOR are
presented and discussed.Variabilité des propriétés du bois pour
cinq familles de pleins-frères d'épicéa commun
(Picea abies). La variabilité
génétique et environnementale pour le module d'élasticité
(MOE), le module de rupture (MOR) et certaines autres propriétés de
base du bois (proportion de bois final, densité du bois, angle du fil,
angle des microfibrilles et teneur en lignine) ont été
étudiées au sein de cinq familles de pleins-frères
d'épicéa commun ainsi que la liaison entre caractères. L'analyse
a été réalisée à partir d'échantillons sans
défaut issus de la zone de transition entre bois juvenile et bois mature
de 93 arbres âgés de 30 ans (depuis la graine). Des différences
significatives ( < 0.05) entre familles ont été observées pour
tous les caractères sauf pour la teneur en lignine. Les valeurs de MOE
variaient entre 7.9–14.1 GPa entre arbres et entre 9.4–11.0 GPa entre
familles. Pour MOR, ces valeurs s'échelonnaient entre 47–87 MPa entre
arbres et entre 61–71 MPa entre familles. Les différences entre familles
pour MOE et MOR sont restées significatives après normalisation pour
la densité du bois. Il est noté que l'utilisation seule de la
densité du bois comme critère de sélection ne permettrait pas
d'obtenir les gains génétiques potentiels escomptés d'une
sélection directe pour MOE et MOR. Les corrélations entre
propriétés de base du bois et les valeurs normalisées de MOE et
MOR sont présentées et discutées
Temperature dependent effective mass renormalization in a Coulomb Fermi liquid
We calculate numerically the quasiparticle effective mass (m*)
renormalization as a function of temperature and electron density in two- and
three-dimensional electron systems with long-range Coulomb interaction. In two
dimensions, the leading temperature correction is linear and positive with the
slope being a universal density independent number in the high density limit.
We predict an enhancement of the effective mass at low temperatures and a
non-monotonic temperature dependence at higher temperatures (T/T_F ~ 0.1) with
the peak shifting toward higher temperatures as density decreases. In three
dimensions, we find that the effective mass temperature dependence is nonlinear
and non-universal, and depends on the electron density in a complicated way. At
very high densities, the leading correction is positive, while at lower
densities it changes sign and the effective mass decreases monotonically from
its zero temperature value with increasing temperature
Spectral evolution and polarization of variable structures in the pulsar wind nebula of PSR B0540-69.3
We present high spatial resolution optical imaging and polarization
observations of the PSR B0540-69.3 and its highly dynamical pulsar wind nebula
(PWN) performed with HST, and compare them with X-ray data obtained with the
Chandra X-ray Observatory. We have studied the bright region southwest of the
pulsar where a bright "blob" is seen in 1999. We show that it may be a result
of local energy deposition around 1999, and that the emission from this then
faded away. Polarization data from 2007 show that the polarization properties
show dramatic spatial variations at the 1999 blob position arguing for a local
process. Several other positions along the pulsar-"blob" orientation show
similar changes in polarization, indicating previous recent local energy
depositions. In X-rays, the spectrum steepens away from the "blob" position,
faster orthogonal to the pulsar-"blob" direction than along this axis of
orientation. This could indicate that the pulsar-"blob" orientation is an axis
along where energy in the PWN is mainly injected, and that this is then
mediated to the filaments in the PWN by shocks. We highlight this by
constructing an [S II]-to-[O III]-ratio map. We argue, through modeling, that
the high [S II]/[O III] ratio is not due to time-dependent photoionization
caused by possible rapid Xray emission variations in the "blob" region. We have
also created a multiwavelength energy spectrum for the "blob" position showing
that one can, to within 2sigma, connect the optical and X-ray emission by a
single power law. We obtain best power-law fits for the X-ray spectrum if we
include "extra" oxygen, in addition to the oxygen column density in the
interstellar gas of the Large Magellanic Cloud and the Milky Way. This oxygen
is most naturally explained by the oxygen-rich ejecta of the supernova remnant.
The oxygen needed likely places the progenitor mass in the 20 - 25 Msun range.Comment: Accepted by MNRAS on December 6th 2010, 18 pages, 15 figures. The
article with full resolution figures is available here
ftp://ftp.astro.su.se/pub/peter/papers/pwn0540_2010_corrected.pd
The on-top pair-correlation density in the homogeneous electron liquid
The ladder theory, in which the Bethe-Goldstone equation for the effective
potential between two scattering particles plays a central role, is well known
for its satisfactory description of the short-range correlations in the
homogeneous electron liquid. By solving exactly the Bethe-Goldstone equation in
the limit of large transfer momentum between two scattering particles, we
obtain accurate results for the on-top pair-correlation density , in both
three dimensions and two dimensions. Furthermore, we prove, in general, the
ladder theory satisfies the cusp condition for the pair-correlation density
at zero distance .Comment: 8 pages, 4 figure
Adaptive Optics Discovery of Supernova 2004ip in the Nuclear Regions of the Luminous Infrared Galaxy IRAS 18293-3413
We report a supernova discovery in Ks-band images from the NAOS CONICA
adaptive optics (AO) system on the ESO Very Large Telescope (VLT). The images
were obtained as part of a near-infrared search for highly-obscured supernovae
in the nuclear regions of luminous and ultraluminous infrared galaxies. SN
2004ip is located within a circumnuclear starburst at 1.4 arcsec (or 500 pc)
projected distance from the K-band nucleus of the luminous infrared galaxy IRAS
18293-3413. The supernova luminosity and light curve are consistent with a
core-collapse event suffering from a host galaxy extinction of up to about 40
magnitudes in V-band which is as expected for a circumnuclear starburst
environment. This is the first supernova to be discovered making use of AO
correction and demonstrates the potential of the current 8-meter class
telescopes equipped with AO in discovering supernovae from the innermost
nuclear regions of luminous and ultraluminous infrared galaxies.Comment: 4 pages, 3 figures, ApJ Letters (accepted
Late-Time Optical and UV Spectra of SN 1979C and SN 1980K
A low-dispersion Keck I spectrum of SN 1980K taken in August 1995 (t = 14.8
yr after explosion) and a November 1997 MDM spectrum (t = 17.0 yr) show broad
5500 km s^{-1} emission lines of H\alpha, [O I] 6300,6364 A, and [O II]
7319,7330 A. Weaker but similarly broad lines detected include [Fe II] 7155 A,
[S II] 4068,4072 A, and a blend of [Fe II] lines at 5050--5400 A. The presence
of strong [S II] 4068,4072 A emission but a lack of [S II] 6716,6731 A emission
suggests electron densities of 10^{5-6} cm^{-3}. From the 1997 spectra, we
estimate an H\alpha flux of 1.3 \pm 0.2 \times 10^{-15} erg cm^{-2} s^{-1}
indicating a 25% decline from 1987--1992 levels during the period 1994 to 1997,
possibly related to a reported decrease in its nonthermal radio emission.Comment: 21 pages, 8 figures, submitted to the Astronomical Journa
New Pseudo-Phase Structure for -Pu
In this paper we propose a new pseudo-phase crystal structure, based on an
orthorhombic distortion of the diamond structure, for the ground-state
-phase of plutonium. Electronic-structure calculations in the
generalized-gradient approximation give approximately the same total energy for
the two structures. Interestingly, our new pseudo-phase structure is the same
as the Pu -phase structure except with very different b/a and c/a
ratios. We show how the contraction relative to the phase, principally
in the direction, leads to an -like structure in the [0,1,1] plane.
This is an important link between two complex structures of plutonium and opens
new possibilities for exploring the very rich phase diagram of Pu through
theoretical calculations
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