3,901 research outputs found
The Kinematics of Kepler's Supernova Remnant as revealed by Chandra
I determine the expansion of the supernova remnant of SN1604 (Kepler's
supernova) based on archival Chandra ACIS-S observations made in 2000 and 2006.
The measurements were done in several distinct energy bands, and were made for
the remnant as a whole, and for six individual sectors. The average expansion
parameter indicates that the remnant expands as , but there
are significant differences in different parts of the remnant: the bright
northwestern part expands as , whereas the rest of the
remnant's expansion shows an expansion . The latter is
consistent with an explosion in which the outer part of the ejecta has a
negative power law slope for density () of , or with
an exponential density profile(). The expansion
parameter in the southern region, in conjunction with the shock radius,
indicate a rather low value (<5E50 erg) for the explosion energy of SN1604 for
a distance of 4 kpc. An higher explosion energy is consistent with the results,
if the distance is larger.
The filament in the eastern part of the remnant, which is dominated by X-ray
synchrotron radiation seems to mark a region with a fast shock speed , corresponding to a shock velocity of v= 4200 km/s, for a distance to
SN1604 of 4 kpc. This is consistent with the idea that X-ray synchrotron
emission requires shock velocities in excess of ~2000 km/s.
The X-ray based expansion measurements reported are consistent with results
based on optical and radio measurements, but disagree with previous X-ray
measurements based on ROSAT and Einstein observations.Comment: Accepted for publication in ApJ. This new version is the accepted
version, which differs mainly in the discussion sectio
Simulation and theory of fluid demixing and interfacial tension of mixtures of colloids and non-ideal polymers
An extension of the Asakura-Oosawa-Vrij model of hard sphere colloids and
non-adsorbing polymers, that takes polymer non-ideality into account through a
repulsive stepfunction pair potential between polymers, is studied with grand
canonical Monte Carlo simulations and density functional theory. Simulation
results validate previous theoretical findings for the shift of the bulk fluid
demixing binodal upon increasing strength of polymer-polymer repulsion,
promoting the tendency to mix. For increasing strength of the polymer-polymer
repulsion, simulation and theory consistently predict the interfacial tension
of the free colloidal liquid-gas interface to decrease significantly for fixed
colloid density difference in the coexisting phases, and to increase for fixed
polymer reservoir packing fraction.Comment: 10 pages, 4 figure
Advances in mass-loss predictions
We present the results of Monte Carlo mass-loss predictions for massive stars
covering a wide range of stellar parameters. We critically test our predictions
against a range of observed mass-loss rates -- in light of the recent
discussions on wind clumping. We also present a model to compute the
clumping-induced polarimetric variability of hot stars and we compare this with
observations of Luminous Blue Variables, for which polarimetric variability is
larger than for O and Wolf-Rayet stars. Luminous Blue Variables comprise an
ideal testbed for studies of wind clumping and wind geometry, as well as for
wind strength calculations, and we propose they may be direct supernova
progenitors.Comment: 3 pages, 3 figures, to appear in the proceedings of workshop
'Clumping in Hot Star Winds', eds. W.-R. Hamann, A. Feldmeier, & L. Oskinov
Proper Motions of H-alpha filaments in the Supernova Remnant RCW 86
We present a proper motion study of the eastern shock-region of the supernova
remnant RCW 86 (MSH 14-63, G315.4-2.3), based on optical observations carried
out with VLT/FORS2 in 2007 and 2010. For both the northeastern and southeastern
regions, we measure an average proper motion of H-alpha filaments of 0.10 +/-
0.02 arcsec/yr, corresponding to 1200 +/- 200 km/s at 2.5kpc. There is
substantial variation in the derived proper motions, indicating shock
velocities ranging from just below 700 km/s to above 2200 km/s.
The optical proper motion is lower than the previously measured X-ray proper
motion of northeastern region. The new measurements are consistent with the
previously measured proton temperature of 2.3 +/- 0.3 keV, assuming no
cosmic-ray acceleration. However, within the uncertainties, moderately
efficient (< 27 per cent) shock acceleration is still possible. The combination
of optical proper motion and proton temperature rule out the possibility that
RCW 86 has a distance less than 1.5kpc.
The similarity of the proper motions in the northeast and southeast is
peculiar, given the different densities and X-ray emission properties of the
regions. The northeastern region has lower densities and the X-ray emission is
synchrotron dominated, suggesting that the shock velocities should be higher
than in the southeastern, thermal X-ray dominated, region. A possible solution
is that the H-alpha emitting filaments are biased toward denser regions, with
lower shock velocities. Alternatively, in the northeast the shock velocity may
have decreased rapidly during the past 200yr, and the X-ray synchrotron
emission is an afterglow from a period when the shock velocity was higher.Comment: Accepted for publication in MNRA
Accelerated Electrons in Cassiopeia A: An Explanation for the Hard X-ray Tail
We propose a model for the hard X-ray (> 10 keV) emission observed from the
supernova remnant Cas A. Lower hybrid waves are generated in strong (mG)
magnetic fields, generally believed to reside in this remnant, by shocks
reflected from density inhomogeneities. These then accelerate electrons to
energies of several tens of keV. Around 4% of the x-ray emitting plasma
electrons need to be in this accelerated distribution, which extends up to
electron velocities of order the electron Alfven speed, and is directled along
magnetic field lines. Bremsstrahlung from these electrons produces the observed
hard x-ray emission. Such waves and accelerated electrons have been observed in
situ at Comet Halley, and we discuss the viability of the extrapolation from
this case to the parameters relevant to Cas A.Comment: 20 pages, 3 figures, aasTeX502, accepted in Ap
Critical phenomena in colloid-polymer mixtures: interfacial tension, order parameter, susceptibility and coexistence diameter
The critical behavior of a model colloid-polymer mixture, the so-called AO
model, is studied using computer simulations and finite size scaling
techniques. Investigated are the interfacial tension, the order parameter, the
susceptibility and the coexistence diameter. Our results clearly show that the
interfacial tension vanishes at the critical point with exponent 2\nu ~ 1.26.
This is in good agreement with the 3D Ising exponent. Also calculated are
critical amplitude ratios, which are shown to be compatible with the
corresponding 3D Ising values. We additionally identify a number of subtleties
that are encountered when finite size scaling is applied to the AO model. In
particular, we find that the finite size extrapolation of the interfacial
tension is most consistent when logarithmic size dependences are ignored. This
finding is in agreement with the work of Berg et al.[Phys. Rev. B, V47 P497
(1993)]Comment: 13 pages, 16 figure
The Becklin-Neugebauer Object as a Runaway B Star, Ejected 4000 years ago from the theta^1C system
We attempt to explain the properties of the Becklin-Neugebauer (BN) object as
a runaway B star, as originally proposed by Plambeck et al. (1995). This is one
of the best-studied bright infrared sources, located in the Orion Nebula
Cluster -- an important testing ground for massive star formation theories.
From radio observations of BN's proper motion, we trace its trajectory back
to Trapezium star theta^1C, the most massive (45 Msun) in the cluster and a
relatively tight (17 AU) visual binary with a B star secondary. This origin
would be the most recent known runaway B star ejection event, occurring only
\~4000 yr ago and providing a unique test of models of ejection from multiple
systems of massive stars. Although highly obscured, we can constrain BN's mass
(~7 Msun) from both its bolometric luminosity and the recoil of theta^1C.
Interaction of a runaway B star with dense ambient gas should produce a compact
wind bow shock. We suggest that X-ray emission from this shocked gas may have
been seen by Chandra: the offset from the radio position is ~300 AU in the
direction of BN's motion. Given this model, we constrain the ambient density,
wind mass-loss rate and wind velocity. BN made closest approach to the massive
protostar, source ``I'', 500 yr ago. This may have triggered enhanced accretion
and thus outflow, consistent with previous interpretations of the outflow being
a recent (~10^3 yr) "explosive" event.Comment: 6 pages, accepted to ApJ Letter
Recommended from our members
Now is the time for a comfort congress
When buying a bed, a chair or a car, taking the train, holding a hand tool or flying across the ocean,
comfort comes into play. Users interact with products and rate their experience. Therefore,
designers and manufacturers of products such as seats, cars, beds, hand tools, and production lines
strive for optimal comfort. If we look at some trends like âattention to healthâ, âageing workforce
(and population)â, âenvironmental awareness and sustainabilityâ and âattention to well-beingâ,
(dis)comfort is an important consideration (Vink & Hallbeck, 2012). This means that in our daily
lives we are confronted with comfort
Predictions for mass-loss rates and terminal wind velocities of massive O-type stars
Mass loss forms an important aspect of the evolution of massive stars, as
well as for the enrichment of the surrounding ISM. Our goal is to predict
accurate mass-loss rates and terminal wind velocities. These quantities can be
compared to empirical values, thereby testing radiation-driven wind models. One
specific issue is that of the "weak-wind problem", where empirically derived
mass-loss rates fall orders of magnitude short of predicted values. We employ
an established Monte Carlo model and a recently suggested new line acceleration
formalism to solve the wind dynamics consistently. We provide a new grid of
mass-loss rates and terminal wind velocities of O stars, and compare the values
to empirical results. Our models fail to provide mass-loss rates for
main-sequence stars below a luminosity of log(L/Lsun) = 5.2, where we run into
a fundamental limit. At luminosities below this critical value there is
insufficient momentum transferred in the region below the sonic point to
kick-start the acceleration. This problem occurs at the location of the onset
of the weak-wind problem. For O dwarfs, the boundary between being able to
start a wind, and failing to do so, is at spectral type O6/O6.5. The direct
cause of this failure is a combination of the lower luminosity and a lack of Fe
V lines at the wind base. This might indicate that another mechanism is
required to provide the necessary driving to initiate the wind. For stars more
luminous than log(L/Lsun) = 5.2, our new mass-loss rates are in excellent
agreement with the mass-loss prescription by Vink et al. 2000. This implies
that the main assumption entering the method of the Vink et al. prescriptions -
i.e. that the momentum equation is not explicitly solved for - does not
compromise the reliability of the Vink et al. results for this part of
parameter space (Abridged).Comment: 10 pages, 10 figures, Astronomy & Astrophysics (in press
SurF: an innovative framework in biosecurity and animal health surveillance evaluation
Surveillance for biosecurity hazards is being conducted by the New Zealand Competent Authority, the Ministry for Primary Industries (MPI) to support New Zealand's biosecurity system. Surveillance evaluation should be an integral part of the surveillance life cycle, as it provides a means to identify and correct problems and to sustain and enhance the existing strengths of a surveillance system. The surveillance evaluation Framework (SurF) presented here was developed to provide a generic framework within which the MPI biosecurity surveillance portfolio, and all of its components, can be consistently assessed. SurF is an innovative, crossâsectoral effort that aims to provide a common umbrella for surveillance evaluation in the animal, plant, environment and aquatic sectors. It supports the conduct of the following four distinct components of an evaluation project: (i) motivation for the evaluation, (ii) scope of the evaluation, (iii) evaluation design and implementation and (iv) reporting and communication of evaluation outputs. Case studies, prepared by MPI subject matter experts, are included in the framework to guide users in their assessment. Three case studies were used in the development of SurF in order to assure practical utility and to confirm usability of SurF across all included sectors. It is anticipated that the structured approach and information provided by SurF will not only be of benefit to MPI but also to other New Zealand stakeholders. Although SurF was developed for internal use by MPI, it could be applied to any surveillance system in New Zealand or elsewhere
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