1,898 research outputs found
Flash ionization of the partially ionized wind of the progenitor of SN 1987A
The H II region created by the progenitor of SN 1987A was further heated and
ionized by the supernova flash. Prior to the flash, the temperature of the gas
was 4000 - 5000 K, and helium was neutral, while the post-flash temperature was
only slightly less than 10^5 K, with the gas being ionized to helium-like
ionization stages of C, N and O. We have followed the slow post-flash cooling
and recombination of the gas, as well as its line emission, and find that the
strongest lines are N V 1240 and O VI 1034. Both these lines are good probes
for the density of the gas, and suitable instruments to detect the lines are
STIS on HST and FUSE, respectively. Other lines which may be detectable are N
IV] 1486 and [O III] 5007, though they are expected to be substantially weaker.
The relative strength of the oxygen lines is found to be a good tracer of the
color temperature of the supernova flash. From previous observations, we put
limits on the hydrogen density, n_H, of the H II region. The early N V 1240
flux measured by IUE gives an upper limit which is n_H ~ 180 \eta^{-0.40}
cm^{-3}, where \eta is the filling factor of the gas. The recently reported
emission in [O III] 5007 at 2500 days requires n_H = (160\pm12) \eta^{-0.19}
cm^{-3}, for a supernova burst similar to that in the 500full1 model of Ensman
& Burrows (1992). For the more energetic 500full2 burst the density is n_H =
(215\pm15) \eta^{-0.19} cm^{-3}. These values are much higher than in models of
the X-ray emission from the supernova (n_H ~ 75 cm^{-3}), and it seems
plausible that the observed [O III] emission is produced primarily elsewhere
than in the H II region. We also discuss the type of progenitor consistent with
the H II region. In particular, it seems unlikely that its spectral type was
much earlier than B2 Ia.Comment: LaTeX, 23 pages including 4 figures. To appear in ApJ (Main Journal
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
Dipole Interactions and Electrical Polarity in Nanosystems -- the Clausius-Mossotti and Related Models
Point polarizable molecules at fixed spatial positions have solvable
electrostatic properties in classical approximation, the most familiar being
the Clausius-Mossotti (CM) formula. This paper generalizes the model and
imagines various applications to nanosystems. The behavior is worked out for a
sequence of octahedral fragments of simple cubic crystals, and the crossover to
the bulk CM law is found. Some relations to fixed moment systems are discussed
and exploited. The one-dimensional dipole stack is introduced as an important
model system. The energy of interaction of parallel stacks is worked out, and
clarifies the diverse behavior found in different crystal structures. It also
suggests patterns of self-organization which polar molecules in solution might
adopt. A sum rule on the stack interaction is found and tested. Stability of
polarized states under thermal fluctuations is discussed, using the
one-dimensional domain wall as an example. Possible structures for polar hard
ellipsoids are considered. An idea is formulated for enhancing polarity of
nanosystems by intentionally adding metallic coatings.Comment: 18 pages (includes 6 embedded figures and 3 tables). New references,
and other small improvements. Scheduled for publication by J. Chem. Phys.,
Jan. 200
Three-dimensional distribution of ejecta in Supernova 1987A at 10 000 days
Due to its proximity, SN 1987A offers a unique opportunity to directly
observe the geometry of a stellar explosion as it unfolds. Here we present
spectral and imaging observations of SN 1987A obtained ~10,000 days after the
explosion with HST/STIS and VLT/SINFONI at optical and near-infrared
wavelengths. These observations allow us to produce the most detailed 3D map of
H-alpha to date, the first 3D maps for [Ca II] \lambda \lambda 7292, 7324, [O
I] \lambda \lambda 6300, 6364 and Mg II \lambda \lambda 9218, 9244, as well as
new maps for [Si I]+[Fe II] 1.644 \mu m and He I 2.058 \mu m. A comparison with
previous observations shows that the [Si I]+[Fe II] flux and morphology have
not changed significantly during the past ten years, providing evidence that it
is powered by 44Ti. The time-evolution of H-alpha shows that it is
predominantly powered by X-rays from the ring, in agreement with previous
findings. All lines that have sufficient signal show a similar large-scale 3D
structure, with a north-south asymmetry that resembles a broken dipole. This
structure correlates with early observations of asymmetries, showing that there
is a global asymmetry that extends from the inner core to the outer envelope.
On smaller scales, the two brightest lines, H-alpha and [Si I]+[Fe II] 1.644
\mu m, show substructures at the level of ~ 200 - 1000 km/s and clear
differences in their 3D geometries. We discuss these results in the context of
explosion models and the properties of dust in the ejecta.Comment: Accepted for publication in Ap
Attentional bias for negative expressions depends on previous target location: replicable effect but unreliable measures
Observability of threat-related spatial attentional biases may require previous-trial responses associated with threat-related locations. This carryover effect might affect reliability and correlations. In Study 1, a diagonalized Visual Probe Task was completed online (N=131) with colour, anger, fear and disgust stimuli, with questionnaires on aggression, anxiety, depression and impulsivity. Bias towards negative stimuli was found only following previous targets on the negative location. Study 2 aimed to test an interpretation in terms of cue-evoked attention. Task variants were completed (N=101) with and without removal of the cue when targets appeared. Anger and disgust stimuli and aggression, anxiety and depression scales were used. Carryover was replicated with no interaction with cue offset. Over both tasks, reliability was low and no robust correlations with questionnaires were found. Carryover thus determined whether attentional bias to negative facial expressions was observed, but analyses taking this into account did not improve reliability or reveal correlations
Radio Emission and Particle Acceleration in SN 1993J
The radio light curves of SN 1993J are found to be well fit by a synchrotron
spectrum, suppressed by external free-free absorption and synchrotron
self-absorption. A standard r^-2 circumstellar medium is assumed, and found to
be adequate. The magnetic field and number density of relativistic electrons
behind the shock are determined. The strength of the magnetic field argues
strongly for turbulent amplification behind the shock. The ratio of the
magnetic and thermal energy density behind the shock is ~0.14. Synchrotron and
Coulomb cooling dominate the losses of the electrons. The injected electron
spectrum has a power law index -2.1, consistent with diffusive shock
acceleration, and the number density scales with the thermal electron energy
density. The total energy density of the relativistic electrons is, if
extrapolated to gamma ~ 1, ~ 5x10^-4 of the thermal energy density. The
free-free absorption required is consistent with previous calculations of the
circumstellar temperature of SN 1993J, T_e ~ (2-10)x10^5 K. The relative
importance of free-free absorption, Razin suppression, and the synchrotron
self-absorption effect for other supernovae are briefly discussed. Guidelines
for the modeling and interpretation of VLBI observations are given.Comment: accepted for Ap.
MTMDAT: Automated analysis and visualization of mass spectrometry data for tertiary and quaternary structure probing of proteins
Summary: In structural biology and -genomics, nuclear magnetic resonance (NMR) spectroscopy and crystallography are the methods of choice, but sample requirements can be hard to fulfil. Valuable structural information can also be obtained by using a combination of limited proteolysis and mass spectrometry, providing not only knowledge of how to improve sample conditions for crystallization trials or NMR spectrosopy by gaining insight into subdomain identities but also probing tertiary and quaternary structure, folding and stability, ligand binding, protein interactions and the location of post-translational modifications. For high-throughput studies and larger proteins, however, this experimentally fast and easy approach produces considerable amounts of data, which until now has made the evaluation exceedingly laborious if at all manually possible. MTMDAT, equipped with a browser-like graphical user interface, accelerates this evaluation manifold by automated peak picking, assignment, data processing and visualization
- âŠ