2,912 research outputs found
The star cluster survivability after gas expulsion is independent of the impact of the Galactic tidal field
We study the impact of the tidal field on the survivability of star clusters
following instantaneous gas expulsion. Our model clusters are formed with a
centrally-peaked star-formation efficiency profile as a result of
star-formation taking place with a constant efficiency per free-fall time. We
define the impact of the tidal field as the ratio of the cluster half-mass
radius to its Jacobi radius immediately after gas expulsion, . We vary by varying either the Galactocentric distance,
or the size (hence volume density) of star clusters.
We propose a new method to measure the violent relaxation duration, in which
we compare the total mass-loss rate of star clusters with their stellar
evolutionary mass-loss rate. That way, we can robustly estimate the bound mass
fraction of our model clusters at the end of violent relaxation. The duration
of violent relaxation correlates linearly with the Jacobi radius, when
considering identical clusters at different Galactocentric distances. In
contrast, it is nearly constant for the solar neighbourhood clusters, slightly
decreasing with . The violent relaxation does not last longer than 50
Myr in our simulations.
Identical model clusters placed at different Galactocentric distances have
the same final bound fraction, despite experiencing different impacts of the
tidal field. The solar neighbourhood clusters with different densities
experience only limited variations of their final bound fraction.
In general, we conclude that the cluster survivability after instantaneous
gas expulsion, as measured by their bound mass fraction at the end of violent
relaxation, , is independent of the impact of the tidal field,
.Comment: accepted for publication in MNRAS, 8 pages, 5 figures,3 table
The Welfare Impacts of Commodity Price Fluctuations: Evidence from Rural Ethiopia
Many governments try to stabilize commodity prices based on the widespread belief that households value price stability and that the poor especially benefit from food price stabilization. We derive an exact measure of multivariate price risk aversion and of associated household willingness to pay for price stabilization across multiple commodities. Using data from a panel of Ethiopian households, we estimate that the average household would be willing to pay 6-32 percent of its income to eliminate fluctuations in the prices of the seven primary food commodities. But not everyone benefits from price stabilization. Contrary to conventional wisdom, the welfare gains from eliminating price fluctuations would be concentrated in the upper 40 percent of the income distribution, making food price stabilization a distributionally regressive policy in this context.Price Fluctuations; Price Stabilization; Price Risk; Risk and Uncertainty
Preferential antiferromagnetic coupling of vacancies in graphene on SiO_2: Electron spin resonance and scanning tunneling spectroscopy
Monolayer graphene grown by chemical vapor deposition and transferred to
SiO_2 is used to introduce vacancies by Ar^+ ion bombardment at a kinetic
energy of 50 eV. The density of defects visible in scanning tunneling
microscopy (STM) is considerably lower than the ion fluence implying that most
of the defects are single vacancies. The vacancies are characterized by
scanning tunneling spectroscopy (STS) on graphene and HOPG exhibiting a peak
close to the Fermi level. The peak persists after air exposure up to 180 min,
albeit getting broader. After air exposure for less than 60 min, electron spin
resonance (ESR) at 9.6 GHz is performed. For an ion flux of 10/nm^2, we find a
signal corresponding to a g-factor of 2.001-2.003 and a spin density of 1-2
spins/nm^2. The ESR signal consists of a mixture of a Gaussian and a Lorentzian
of equal weight exhibiting a width down to 0.17 mT, which, however, depends on
details of the sample preparation. The g-factor anisotropy is about 0.02%.
Temperature dependent measurements reveal antiferromagnetic correlations with a
Curie-Weiss temperature of -10 K. Albeit the electrical conductivity of
graphene is significantly reduced by ion bombardment, the spin resonance
induced change in conductivity is below 10^{-5}.Comment: 10 pages, 5 figures, discussion on STM images in the literature of
defects in graphene adde
Heating and Turbulence Driving by Galaxy Motions in Galaxy Clusters
Using three-dimensional hydrodynamic simulations, we investigate heating and
turbulence driving in an intracluster medium (ICM) by orbital motions of
galaxies in a galaxy cluster. We consider Ng member galaxies on isothermal and
isotropic orbits through an ICM typical of rich clusters. An introduction of
the galaxies immediately produces gravitational wakes, providing perturbations
that can potentially grow via resonant interaction with the background gas.
When Ng^{1/2}Mg_11 < 100, where Mg_11 is each galaxy mass in units of 10^{11}
Msun, the perturbations are in the linear regime and the resonant excitation of
gravity waves is efficient to generate kinetic energy in the ICM, resulting in
the velocity dispersion sigma_v ~ 2.2 Ng^{1/2}Mg_11 km/s. When Ng^{1/2}Mg_11 >
100, on the other hand, nonlinear fluctuations of the background ICM destroy
galaxy wakes and thus render resonant excitation weak or absent. In this case,
the kinetic energy saturates at the level corresponding to sigma_v ~ 220 km/s.
The angle-averaged velocity power spectra of turbulence driven in our models
have slopes in the range of -3.7 to -4.3. With the nonlinear saturation of
resonant excitation, none of the cooling models considered are able to halt
cooling catastrophe, suggesting that the galaxy motions alone are unlikely to
solve the cooling flow problem.Comment: 12 pages including 3 figures, To appear in ApJ
The Isomorphism Relation Between Tree-Automatic Structures
An -tree-automatic structure is a relational structure whose domain
and relations are accepted by Muller or Rabin tree automata. We investigate in
this paper the isomorphism problem for -tree-automatic structures. We
prove first that the isomorphism relation for -tree-automatic boolean
algebras (respectively, partial orders, rings, commutative rings, non
commutative rings, non commutative groups, nilpotent groups of class n >1) is
not determined by the axiomatic system ZFC. Then we prove that the isomorphism
problem for -tree-automatic boolean algebras (respectively, partial
orders, rings, commutative rings, non commutative rings, non commutative
groups, nilpotent groups of class n >1) is neither a -set nor a
-set
Transfer of autocollimator calibration for use with scanning gantry profilometers for accurate determination of surface slope and curvature of state of the art x ray mirrors
X ray optics, desired for beamlines at free electron laser and diffraction limited storage ring x ray light sources, must have almost perfect surfaces, capable of delivering light to experiments without significant degradation of brightness and coherence. To accurately characterize such optics at an optical metrology lab, two basic types of surface slope profilometers are used the long trace profilers LTPs and nanometer optical measuring NOM like angular deflectometers, based on electronic autocollimator AC ELCOMAT 3000. The inherent systematic errors of the instrument s optical sensors set the principle limit to their measuring performance. Where autocollimator of a NOM like profiler may be calibrated at a unique dedicated facility, this is for a particular configuration of distance, aperture size, and angular range that does not always match the exact use in a scanning measurement with the profiler. Here we discuss the developed methodology, experimental set up, and numerical methods of transferring the calibration of one reference AC to the scanning AC of the Optical Surface Measuring System OSMS , recently brought to operation at the ALS Xray Optics Laboratory. We show that precision calibration of the OSMS performed in three steps, allows us to provide high confidence and accuracy low spatial frequency metrology and not print into measurements the inherent systematic error of tool in use. With the examples of the OSMS measurements with a state of the art x ray aspherical mirror, available from one of the most advanced vendors of X ray optics, we demonstrate the high efficacy of the developed calibration procedure. The results of our work are important for obtaining high reliability data, needed for sophisticated numerical simulations of beamline performance and optimization of beamline usage of the optics. This work was supported by the U. S. Department of Energy under contract number DE AC02 05CH1123
Continued-fraction expansion of eigenvalues of generalized evolution operators in terms of periodic orbits
A new expansion scheme to evaluate the eigenvalues of the generalized
evolution operator (Frobenius-Perron operator) relevant to the
fluctuation spectrum and poles of the order- power spectrum is proposed. The
``partition function'' is computed in terms of unstable periodic orbits and
then used in a finite pole approximation of the continued fraction expansion
for the evolution operator. A solvable example is presented and the approximate
and exact results are compared; good agreement is found.Comment: CYCLER Paper 93mar00
Major QTL for carrot color are positionally associated with carotenoid biosynthetic genes and interact epistatically in a domesticated x wild carrot cross.
We performed QTL analyses for pigment content on a carotenoid biosynthesis function map based on progeny of a wild white carrot (QAL) which accumulates no pigments x domesticated orange carrot (B493), one of the richest sources of carotenoid pigments - mainly provitamin A a - and B-carotenes
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