2,623 research outputs found
Interaction of Supernova Ejecta with Nearby Protoplanetary Disks
The early Solar System contained short-lived radionuclides such as 60Fe (t1/2
= 1.5 Myr) whose most likely source was a nearby supernova. Previous models of
Solar System formation considered a supernova shock that triggered the collapse
of the Sun's nascent molecular cloud. We advocate an alternative hypothesis,
that the Solar System's protoplanetary disk had already formed when a very
close (< 1 pc) supernova injected radioactive material directly into the disk.
We conduct the first numerical simulations designed to answer two questions
related to this hypothesis: will the disk be destroyed by such a close
supernova; and will any of the ejecta be mixed into the disk? Our simulations
demonstrate that the disk does not absorb enough momentum from the shock to
escape the protostar to which it is bound. Only low amounts (< 1%) of mass loss
occur, due to stripping by Kelvin-Helmholtz instabilities across the top of the
disk, which also mix into the disk about 1% of the intercepted ejecta. These
low efficiencies of destruction and injectation are due to the fact that the
high disk pressures prevent the ejecta from penetrating far into the disk
before stalling. Injection of gas-phase ejecta is too inefficient to be
consistent with the abundances of radionuclides inferred from meteorites. On
the other hand, the radionuclides found in meteorites would have condensed into
dust grains in the supernova ejecta, and we argue that such grains will be
injected directly into the disk with nearly 100% efficiency. The meteoritic
abundances of the short-lived radionuclides such as 60Fe therefore are
consistent with injection of grains condensed from the ejecta of a nearby (< 1
pc) supernova, into an already-formed protoplanetary disk.Comment: 57 pages, 16 figure
SALT Spectropolarimetry and Self-Consistent SED and Polarization Modeling of Blazars
We report on recent results from a target-of-opportunity program to obtain
spectropolarimetry observations with the Southern African Large Telescope
(SALT) on flaring gamma-ray blazars. SALT spectropolarimetry and
contemporaneous multi-wavelength spectral energy distribution (SED) data are
being modelled self-consistently with a leptonic single-zone model. Such
modeling provides an accurate estimate of the degree of order of the magnetic
field in the emission region and the thermal contributions (from the host
galaxy and the accretion disk) to the SED, thus putting strong constraints on
the physical parameters of the gamma-ray emitting region. For the specific case
of the -ray blazar 4C+01.02, we demonstrate that the combined SED and
spectropolarimetry modeling constrains the mass of the central black hole in
this blazar to .Comment: Submitted to Galaxies - Proceedings of "Polarized Emission from
Astrophysical Jets", Ierapetra, Crete, June 12 - 16, 201
A VLA Search for Water Masers in Six HII Regions: Tracers of Triggered Low-Mass Star Formation
We present a search for water maser emission at 22 GHz associated with young
low-mass protostars in six HII regions -- M16, M20, NGC 2264, NGC 6357, S125,
and S140. The survey was conducted with the NRAO Very Large Array from 2000 to
2002. For several of these HII regions, ours are the first high-resolution
observations of water masers. We detected 16 water masers: eight in M16, four
in M20, three in S140, and one in NGC 2264. All but one of these were
previously undetected. No maser emission was detected from NGC 6357 or S125.
There are two principle results to our study. (1) The distribution of water
masers in M16 and M20 does not appear to be random but instead is concentrated
in a layer of compressed gas within a few tenths of a parsec of the ionization
front. (2) Significantly fewer masers are seen in the observed fields than
expected based on other indications of ongoing star formation, indicating that
the maser-exciting lifetime of protostars is much shorter in HII regions than
in regions of isolated star formation. Both of these results confirm
predictions of a scenario in which star formation is first triggered by shocks
driven in advance of ionization fronts, and then truncated approximately 10^5
years later when the region is overrun by the ionization front.Comment: 30 pages, 20 figures, 3 tables. Accepted for publication by ApJ. Full
resolution figures and PS and PDF versions with full-res figures available at
http://eagle.la.asu.edu/healy/preprints/hhc0
GPS Position and Heading Circuitry for Ships
Circuit boards that contain radio-frequency (RF) and digital circuitry have been developed by NASA to satisfy a requirement of the Port of Houston Authority for relatively inexpensive Global Positioning System (GPS) receivers that indicate the azimuthal headings as well as the positions of ships. The receiver design utilizes the unique architecture of the Mitel commercial chip-set, which provides for an accurate GPS-based heading-determination device. The major components include two RF front ends (each connected to a separate antenna), a surface-acoustic-wave intermediate-frequency filter between second- and third-stage mixers, a correlator, and a reduced-instruction- set computer. One of the RF front ends operates as a master, the other as a slave. Both RF front ends share a 10-MHz sinusoidal clock oscillator, which provides for more accurate carrier phase measurements between the two antennas. The outputs of the RF front ends are subjected to conventional GPS processing. The commercial-based chip-set design approach provides an inexpensive open architecture GPS platform, which can be used in developing and implementing unique GPS-heading and attitude-determination algorithms for specific applications. The heading is estimated from the GPS position solutions of the two antennas by an algorithm developed specifically for this application. If a third (and preferably a fourth) antenna were added, it would be possible to estimate the attitude of the GPS receiver in three dimensions instead of only its heading in a horizontal plane
Rationale and study design of the Prospective comparison of Angiotensin Receptor neprilysin inhibitor with Angiotensin receptor blocker MEasuring arterial sTiffness in the eldERly (PARAMETER) study.
Hypertension in elderly people is characterised by elevated systolic blood pressure (SBP) and increased pulse pressure (PP), which indicate large artery ageing and stiffness. LCZ696, a first-in-class angiotensin receptor neprilysin inhibitor (ARNI), is being developed to treat hypertension and heart failure. The Prospective comparison of Angiotensin Receptor neprilysin inhibitor with Angiotensin receptor blocker MEasuring arterial sTiffness in the eldERly (PARAMETER) study will assess the efficacy of LCZ696 versus olmesartan on aortic stiffness and central aortic haemodynamics
New Studies of the Pulsar Wind Nebula in the Supernova Remnant CTB 80
We investigated the kinematics of the pulsar wind nebula (PWN) associated
with PSR B1951+32 in the old supernova remnant CTB 80 using the Fabry-Perot
interferometer of the 6m Special Astrophysical Observatory telescope. In
addition to the previously known expansion of the system of bright filaments
with a velocity of 100-200km/s, we detected weak high-velocity features in the
H-alpha line at least up to velocities of 400-450km/s. We analyzed the
morphology of the PWN in the H-alpha, [SII], and [OIII] lines using HST data
and discuss its nature. The shape of the central filamentary shell, which is
determined by the emission in the [OIII] line and in the radio continuum, is
shown to be consistent with the bow-shock model for a significant (about 60
degrees) inclination of the pulsar's velocity vector to the plane of the sky.
In this case, the space velocity of the pulsar is twice higher than its
tangential velocity, i.e., it reaches ~500 km/s, and PSR B1951+32 is the first
pulsar whose line-of-sight velocity (of about 400 km/s) has been estimated from
the PWN observations. The shell-like H-alpha-structures outside the bow shock
front in the east and the west may be associated with both the pulsar's jets
and the pulsar-wind breakthrough due to the layered structure of the extended
CTB 80 shell.Comment: to appear in Astronomy Letters, 12 pages, 6 postscript figures, two
in colour; for a version with high resolution figures see
http://www.sao.ru/hq/grb/team/vkom/CTB80_fine.pd
Crystal-field splitting for low symmetry systems in ab initio calculations
In the framework of the LDA+U approximation we propose the direct way of
calculation of crystal-field excitation energy and apply it to La and Y
titanates. The method developed can be useful for comparison with the results
of spectroscopic measurements because it takes into account fast relaxations of
electronic system. For titanates these relaxation processes reduce the value of
crystal-field splitting by as compared with the difference of LDA one
electron energies. However, the crystal-field excitation energy in these
systems is still large enough to make an orbital liquid formation rather
unlikely and experimentally observed isotropic magnetism remains unexplained.Comment: 13 pages, 5 figures, 3 table
Fitting Pulsar Wind Tori
CXO imaging has shown that equatorial tori, often with polar jets, are very
common in young pulsar wind nebulae (PWNe). These structures are interesting
both for what they reveal about the relativistic wind itself and for the
(nearly) model-independent measurement of the neutron star spin orientation
they provide. The later is a particularly valuable probe of pulsar emission
models and of neutron star physics.We describe here a procedure for fitting
simple 3-D torus models to the X-ray data which provides robust estimates of
the geometric parameters. An application to 6 PWN tori gives orientations, PWN
shock scales and post-shock wind speeds along with statistical errors. We
illustrate the use of these data by commenting on the implications for kick
physics and for high energy beaming models.Comment: 10 pages, 2 figures; to appear in the Astrophysical Journa
Modes of Multiple Star Formation
This paper argues that star forming environments should be classified into
finer divisions than the traditional isolated and clustered modes. Using the
observed set of galactic open clusters and theoretical considerations regarding
cluster formation, we estimate the fraction of star formation that takes place
within clusters. We find that less than 10% of the stellar population
originates from star forming regions destined to become open clusters,
confirming earlier estimates. The smallest clusters included in the
observational surveys (having at least N=100 members) roughly coincide with the
smallest stellar systems that are expected to evolve as clusters in a dynamical
sense. We show that stellar systems with too few members N < N_\star have
dynamical relaxation times that are shorter than their formation times (1-2
Myr), where the critical number of stars N_\star \approx 100. Our results
suggest that star formation can be characterized by (at least) three principal
modes: I. isolated singles and binaries, II. groups (N<N_\star), and III.
clusters (N>N_\star). Many -- if not most -- stars form through the
intermediate mode in stellar groups with 10<N<100. Such groups evolve and
disperse much more rapidly than open clusters; groups also have a low
probability of containing massive stars and are unaffected by supernovae and
intense ultraviolet radiation fields. Because of their short lifetimes and
small stellar membership, groups have relatively little effect on the star
formation process (on average) compared to larger open clusters.Comment: accepted to The Astrophysical Journa
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