1,173 research outputs found
MSFC Skylab electrical power systems mission evaluation
The design, development, and operation of the Skylab electrical power system are discussed. The electrical systems for the airlock module of the orbital workshop and the Apollo telescope mount are described. Skylab is considered an integral laboratory, however, both cluster and module hardware distinct sections are included. Significant concept and requirement evolution, testing, and modifications resulting from tests are briefly summarized to aid in understanding the launch configuration description and the procedures and performance discussed for in-orbit operation. Specific problems encountered during Skylab orbital missions are analyzed
Surface detonation in type Ia supernova explosions?
We explore the evolution of thermonuclear supernova explosions when the
progenitor white dwarf star ignites asymmetrically off-center. Several
numerical simulations are carried out in two and three dimensions to test the
consequences of different initial flame configurations such as spherical
bubbles displaced from the center, more complex deformed configurations, and
teardrop-shaped ignitions. The burning bubbles float towards the surface while
releasing energy due to the nuclear reactions. If the energy release is too
small to gravitationally unbind the star, the ash sweeps around it, once the
burning bubble approaches the surface. Collisions in the fuel on the opposite
side increase its temperature and density and may -- in some cases -- initiate
a detonation wave which will then propagate inward burning the core of the star
and leading to a strong explosion. However, for initial setups in two
dimensions that seem realistic from pre-ignition evolution, as well as for all
three-dimensional simulations the collimation of the surface material is found
to be too weak to trigger a detonation.Comment: 5 pages, 3 figures, in: Proceedings of the SciDAC 2006 Meeting,
Denver June 25-26 2006, also available at
http://herald.iop.org/jpcs46/m51/gbr//link/40
Dynamical Bar-Mode Instability in Differentially Rotating Magnetized Neutron Stars
This paper presents a numerical study over a wide parameter space of the
likelihood of the dynamical bar-mode instability in differentially rotating
magnetized neutron stars. The innovative aspect of this study is the
incorporation of magnetic fields in such a context, which have thus far been
neglected in the purely hydrodynamical simulations available in the literature.
The investigation uses the Cosmos++ code which allows us to perform three
dimensional simulations on a cylindrical grid at high resolution. A sample of
Newtonian magneto-hydrodynamical simulations starting from a set of models
previously analyzed by other authors without magnetic fields has been
performed, providing estimates of the effects of magnetic fields on the
dynamical bar-mode deformation of rotating neutron stars. Overall, our results
suggest that the effect of magnetic fields are not likely to be very
significant in realistic configurations. Only in the most extreme cases are the
magnetic fields able to suppress growth of the bar mode.Comment: 12 pages, 16 figures. References added and minor edits made to match
published versio
Chandra Observations of the X-ray Environs of SN 1998bw/GRB 980425
(Abrigded) We report X-ray studies of the environs of SN 1998bw and GRB
980425 using the Chandra X-Ray Observatory 1281 days after the GRB. Combining
our observation of the supernova with others of the GRB afterglow, a smooth
X-ray light curve, spanning ~1300 days, is obtained by assuming the burst and
supernova were coincident at 35.6 Mpc. When this X-ray light curve is compared
with those of the X-ray ``afterglows'' of ordinary GRBs, X-ray Flashes, and
ordinary supernovae, evidence emerges for at least two classes of lightcurves,
perhaps bounding a continuum. By three to ten years, all these phenomena seem
to converge on a common X-ray luminosity, possibly indicative of the supernova
underlying them all. This convergence strengthens the conclusion that SN 1998bw
and GRB 980425 took place in the same object. One possible explanation for the
two classes is a (nearly) standard GRB observed at different angles, in which
case X-ray afterglows with intermediate luminosities should eventually be
discovered. Finally, we comment on the contribution of GRB afterglows to the
ULX source population.Comment: 26 pages, 5 figues, submitted to Ap
Modeling of the Interaction of GRB Prompt Emission with the Circumburst Medium
We present methodology and results of numerical modeling of the interaction
of GRB prompt emission with the circumburst medium using a modified version of
the multi-group radiation hydrocode STELLA. The modification includes the
nonstationary photoionization, the photoionization heating and the Compton
heating along with the hydrodynamics and radiation transfer. The lightcurves
and spectra of the outcoming gamma-ray, X-ray and optical emission are
calculated for a set of models (shells) of the circumburst environment, which
differ in dimensions, density, density profile, composition, temperature. In
some cases total bolometric and optical luminosities can reach 10^47 and 10^43
erg/s respectively. These effects can be responsible for irregularities which
are seen on lightcurves of some GRB's X-ray and optical afterglows.Comment: 27 pages, 16 colour figures, this version is translated by authors,
so it differs from that, which is published in Astronomy Letter
Resonant Neutrino Spin-Flavor Precession and Supernova Nucleosynthesis and Dynamics
We discuss the effects of resonant spin-flavor precession (RSFP) of Majorana
neutrinos on heavy element nucleosynthesis in neutrino-heated supernova ejecta
and the dynamics of supernovae. In assessing the effects of RSFP, we explicitly
include matter-enhanced (MSW) resonant neutrino flavor conversion effects where
appropriate. We point out that for plausible ranges of neutrino magnetic
moments and proto-neutron star magnetic fields, spin-flavor conversion of
(or ) with a cosmologically significant mass (1--100 eV)
into a light could lead to an enhanced neutron excess in
neutrino-heated supernova ejecta. This could be beneficial for models of
-process nucleosynthesis associated with late-time neutrino-heated ejecta
from supernovae. Similar spin-flavor conversion of neutrinos at earlier epochs
could lead to an increased shock reheating rate and, concomitantly, a larger
supernova explosion energy. We show, however, that such increased neutrino
heating likely will be accompanied by an enhanced neutron excess which could
exacerbate the problem of the overproduction of the neutron number
nuclei in the supernova ejecta from this stage. In all of these scenarios, the
average energy will be increased over those predicted by supernova
models with no neutrino mixings. This may allow the SN1987a data to constrain
RSFP-based schemes.Comment: Latex file, 33 pages including 11 figures, uses psfig.sty, minor
changes about wording and clarification of the text, to be published in Phys.
Rev.
Neutrino-induced neutron spallation and supernova r-process nucleosynthesis
In order to explore the consequences of the neutrino irradiation for the
supernova r-process nucleosynthesis, we calculate the rates of charged-current
and neutral-current neutrino reactions on neutron-rich heavy nuclei, and
estimate the average number of neutrons emitted in the resulting spallation.
Our results suggest that charged-current captures can be important in
breaking through the waiting-point nuclei at N=50 and 82, while still allowing
the formation of abundance peaks. Furthermore, after the r-process freezes out,
there appear to be distinctive neutral-current and charged-current
postprocessing effects. A subtraction of the neutrino postprocessing effects
from the observed solar r-process abundance distribution shows that two mass
regions, A=124-126 and 183-187, are inordinately sensitive to neutrino
postprocessing effects. This imposes very stringent bounds on the freeze-out
radii and dynamic timescales governing the r-process. Moreover, we find that
the abundance patterns within these mass windows are entirely consistent with
synthesis by neutrino interactions. This provides a strong argument that the
r-process must occur in the intense neutrino flux provided by a core-collapse
supernova.Comment: 34 pages, 4 PostScript figures, RevTe
Thermonuclear Burning Regimes and the Use of SNe Ia in Cosmology
The calculations of the light curves of thermonuclear supernovae are carried
out by a method of multi-group radiation hydrodynamics. The effects of spectral
lines and expansion opacity are taken into account. The predictions for UBVI
fluxes are given. The values of rise time for B and V bands found in our
calculations are in good agreement with the observed values. We explain why our
results for the rise time have more solid physical justification than those
obtained by other authors. It is shown that small variations in the chemical
composition of the ejecta, produced in the explosions with different regimes of
nuclear burning, can influence drastically the light curve decline in the B
band and, to a lesser extent, in the V band. We argue that recent results on
positive cosmological constant Lambda, found from the high redshift supernova
observations, could be wrong in the case of possible variations of the
preferred mode of nuclear burning in the earlier Universe.Comment: 20 pages, 5 figures, presented at the conference "Astronomy at the
Eve of the New Century", Puschino, May 17-22, 1999. A few references and a
table added, typos correcte
Supernova 1998bw - The final phases
The probable association with GRB 980425 immediately put SN 1998bw at the
forefront of supernova research. Here, we present revised late-time BVRI light
curves of the supernova, based on template images taken at the VLT. To follow
the supernova to the very last observable phases we have used HST/STIS. Deep
images taken in June and November 2000 are compared to images taken in August
2001. The identification of the supernova is firmly established. This allows us
to measure the light curve to about 1000 days past explosion. The main features
are a rapid decline up to more than 500 days after explosion, with no sign of
complete positron trapping from the Cobolt-56 decay. Thereafter, the light
curve flattens out significantly. One possible explanation is powering by more
long lived radioactive isotopes, if they are abundantly formed in this
energetic supernova.Comment: 13 pages, 5 figures, A&A, In pres
- …