19,306 research outputs found
Simultaneous observations of solar protons inside and outside the magnetosphere Progress report
Simultaneous observations of solar protons inside and outside magnetosphere by Explorer XXXIII AND Injun I
Solar particle observations inside the magnetosphere during the 7 July 1966 event
Injun satellite observations of solar particles inside magnetospher
Influence of an Internal Magnetar on Supernova Remnant Expansion
Most of the proposed associations between magnetars and supernova remnant
suffer from age problems. Usually, supernova remnants ages are determined from
an approximation of the Sedov-Taylor phase relation between radius and age, for
a fixed energy of the explosion ~ 10^{51} erg. Those ages do not generally
agree with the characteristic ages of the (proposed) associated magnetars. We
show quantitatively that, by taking into account the energy injected on the
supernova remnant by magnetar spin-down, a faster expansion results, improving
matches between characteristic ages and supernova remnants ages. However, the
magnetar velocities inferred from observations would inviabilize some
associations. Since characteristic ages may not be good age estimators, their
influence on the likelihood of the association may not be as important.
In this work we present simple numerical simulations of supernova remnants
expansion with internal magnetars, and apply it to the observed objects. A
short initial spin period, thought to be important for the very generation of
the magnetic field, is also relevant for the modified expansion of the remnant.
We next analyze all proposed associations case-by-case, addressing the
likelyhood of each one, according to this perspective. We consider a larger
explosion energy and reasses the characteristic age issue, and conclude that
about 50% of the associations can be true ones, provided SGRs and AXPs are
magnetars.Comment: 30 pages, AAStex, 5 figures, format fixe
Chemical Segregation in Hot Cores With Disk Candidates: An investigation with ALMA
In the study of high-mass star formation, hot cores are empirically defined
stages where chemically rich emission is detected toward a massive YSO. It is
unknown whether the physical origin of this emission is a disk, inner envelope,
or outflow cavity wall and whether the hot core stage is common to all massive
stars. We investigate the chemical make up of several hot molecular cores to
determine physical and chemical structure. We use high spectral and spatial
resolution Cycle 0 ALMA observations to determine how this stage fits into the
formation sequence of a high mass star. We observed the G35.20-0.74N and
G35.03+0.35 hot cores at 350 GHz. We analyzed spectra and maps from four
continuum peaks (A, B1, B2 and B3) in G35.20, separated by 1000-2000 AU, and
one continuum peak in G35.03. We made all possible line identifications across
8 GHz of spectral windows of molecular emission lines and determined column
densities and temperatures for as many as 35 species assuming local
thermodynamic equilibrium. In comparing the spectra of the four peaks, we find
each has a distinct chemical composition expressed in over 400 different
transitions. In G35.20, B1 and B2 contain oxygen- and sulfur-bearing organic
and inorganic species but few nitrogen-bearing species whereas A and B3 are
strong sources of O, S, and N-bearing species (especially those with the
CN-bond). CHDCN is clearly detected in A and B3 with D/H ratios of 8 and
13, respectively, but is much weaker at B1 and undetected at B2. No
deuterated species are detected in G35.03, but similar molecular abundances to
G35.20 were found in other species. We also find co-spatial emission of HNCO
and NHCHO in both sources indicating a strong chemical link between the two
species. The chemical segregation between N-bearing organic species and others
in G35.20 suggests the presence of multiple protostars, surrounded by a disk or
torus.Comment: 14 pages with 13 figures main text, 54 pages appendi
Back-flow ripples in troughs downstream of unit bars: Formation, preservation and value for interpreting flow conditions
Back-flow ripples are bedforms created within the lee-side eddy of a larger bedform with migration directions opposed or oblique to that of the host bedform. In the flume experiments described in this article, back-flow ripples formed in the trough downstream of a unit bar and changed with mean flow velocity; varying from small incipient back-flow ripples at low velocities, to well-formed back-flow ripples with greater velocity, to rapidly migrating transient back-flow ripples formed at the greatest velocities tested. In these experiments back-flow ripples formed at much lower mean back-flow velocities than predicted from previously published descriptions. This lower threshold mean back-flow velocity is attributed to the pattern of velocity variation within the lee-side eddy of the host bedform. The back-flow velocity variations are attributed to vortex shedding from the separation zone, wake flapping and increases in the size of, and turbulent intensity within, the flow separation eddy controlled by the passage of superimposed bedforms approaching the crest of the bar. Short duration high velocity packets, whatever their cause, may form back-flow ripples if they exceed the minimum bed shear stress for ripple generation for long enough or, if much faster, may wash them out. Variation in back-flow ripple cross-lamination has been observed in the rock record and, by comparison with flume observations, the preserved back-flow ripple morphology may be useful for interpreting formative flow and sediment transport dynamics
Efficient Reactive Brownian Dynamics
We develop a Split Reactive Brownian Dynamics (SRBD) algorithm for particle
simulations of reaction-diffusion systems based on the Doi or volume reactivity
model, in which pairs of particles react with a specified Poisson rate if they
are closer than a chosen reactive distance. In our Doi model, we ensure that
the microscopic reaction rules for various association and disassociation
reactions are consistent with detailed balance (time reversibility) at
thermodynamic equilibrium. The SRBD algorithm uses Strang splitting in time to
separate reaction and diffusion, and solves both the diffusion-only and
reaction-only subproblems exactly, even at high packing densities. To
efficiently process reactions without uncontrolled approximations, SRBD employs
an event-driven algorithm that processes reactions in a time-ordered sequence
over the duration of the time step. A grid of cells with size larger than all
of the reactive distances is used to schedule and process the reactions, but
unlike traditional grid-based methods such as Reaction-Diffusion Master
Equation (RDME) algorithms, the results of SRBD are statistically independent
of the size of the grid used to accelerate the processing of reactions. We use
the SRBD algorithm to compute the effective macroscopic reaction rate for both
reaction- and diffusion-limited irreversible association in three dimensions.
We also study long-time tails in the time correlation functions for reversible
association at thermodynamic equilibrium. Finally, we compare different
particle and continuum methods on a model exhibiting a Turing-like instability
and pattern formation. We find that for models in which particles diffuse off
lattice, such as the Doi model, reactions lead to a spurious enhancement of the
effective diffusion coefficients.Comment: To appear in J. Chem. Phy
Technisch rapport WO-Monitor 1998
In 1998 is onder auspiciën van de VSNU de WO-Monitor van start gegaan. Aanleiding hiervoor was de behoefte om over landelijke gegevens te beschikken met betrekking tot de arbeidsmarktintrede van afgestudeerden. Weliswaar werd op diverse plaatsen reeds onderzoek verricht naar de overgang van het WO naar de arbeidsmarkt, maar door de uiteenlopende designs en vraagstellingen leverde dit geen onderling vergelijkbare gegevens op. Het Algemeen Bestuur van de VSNU heeft daarom besloten tot invoering van een landelijke WO-Monitor, waarin alle universiteiten participeren.labour market entry and occupational careers;
Speckle interferometry and radiative transfer modelling of the Wolf-Rayet star WR 118
WR 118 is a highly evolved Wolf-Rayet star of the WC10 subtype surrounded by
a permanent dust shell absorbing and re-emitting in the infrared a considerable
fraction of the stellar luminosity. We present the first diffraction-limited
2.13micron speckle interferometric observations of WR 118 with 73 mas
resolution. The speckle interferograms were obtained with the 6m telescope at
the Special Astrophysical Observatory. The two-dimensional visibility function
of the object does not show any significant deviation from circular symmetry.
The visibility curve declines towards the diffraction cut-off frequency to 0.66
and can be approximated by a linear function. Radiative transfer calculations
have been carried out to model the spectral energy distribution, given in the
range of 0.5-25micron, and our 2.13micron visibility function, assuming
spherical symmetry of the dust shell. Both can be fitted with a model
containing double-sized grains (``small'' and ``large'') with the radii of a =
0.05micron and 0.38micron, and a mass fraction of the large grains greater than
65%. Alternatively, a good match can be obtained with the grain size
distribution function n(a)~a^-3, with a ranging between 0.005micron and
0.6micron. At the inner boundary of the modelled dust shell (angular diameter
(17 +/- 1)mas), the temperature of the smallest grains and the dust shell
density are 1750K +/- 100K and (1 +/- 0.2)x10^-19 g/cm^3, respectively. The
dust formation rate is found to be (1.3 +/- 0.5)x10^-7 Msol/yr assuming Vwind =
1200 km/s.Comment: 6 pages including 4 PostScript figures, also available from
http://www.mpifr-bonn.mpg.de/div/ir-interferometry/publications.html;
accepted for publication in Astronomy & Astrophysic
New Observations and Analysis of the Bright Semi-Detached Eclipsing Binary mu1 Sco
Using new and published photometric observations of mu1 Sco (HR 6247),
spanning 70 years, a period of 1.4462700(5) days was determined. It was found
that the epoch of primary minimum suggested by Shobbrook at HJD 2449534.178
requires an adjustment to HJD 2449534.17700(9) to align all the available
photometric datasets. Using the resulting combined-data light-curve, radial
velocities derived from IUE data and the modelling software PHOEBE, a new
system solution for this binary was obtained. It appears that the secondary is
close to, or just filling, its Roche-lobe.Comment: 4 figures, 6 tables, 9 pages, uses mn2e.sty, to be published in MNRA
A multi-epoch spectrophotometric atlas of symbiotic stars
A multi-epoch, absolute-fluxed spectral atlas extending from about 3200 to
9000 Ang is presented for 130 symbiotic stars, including members of the LMC,
SMC and Draco dwarf galaxies. The fluxes are accurate to better than 5% as
shown by comparison with Tycho and ground-based photometric data. The spectra
of 40 reference objects (MKK cool giant standards, Mira and Carbon stars,
planetary nebulae, white dwarfs, hot sub-dwarfs, Wolf-Rayet stars, classical
novae, VV Cep and Herbig Ae/Be objects) are provided to assist the
interpretation of symbiotic star spectra. Astrometric positions and
counterparts in astrometric catalogues are derived for all program symbiotic
stars.Comment: A&A, in press (264 pages, 3 tables, 256 figures). The spectra are
available in electronic form from the authors. Only a sample of the whole
paper is given here. The full text can be downloaded from
http://ulisse.pd.astro.it/symbio-atlas/ where the spectra in electronic form
of the 40 reference objects can be found to
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