71,784 research outputs found
Calculation of the microcanonical temperature for the classical Bose field
The ergodic hypothesis asserts that a classical mechanical system will in
time visit every available configuration in phase space. Thus, for an ergodic
system, an ensemble average of a thermodynamic quantity can equally well be
calculated by a time average over a sufficiently long period of dynamical
evolution. In this paper we describe in detail how to calculate the temperature
and chemical potential from the dynamics of a microcanonical classical field,
using the particular example of the classical modes of a Bose-condensed gas.
The accurate determination of these thermodynamics quantities is essential in
measuring the shift of the critical temperature of a Bose gas due to
non-perturbative many-body effects.Comment: revtex4, 10 pages, 1 figure. v2: updated to published version. Fuller
discussion of numerical results, correction of some minor error
A comprehensive population synthesis study of post-common envelope binaries
We apply population synthesis techniques to calculate the present day
population of post-common envelope binaries (PCEBs) for a range of theoretical
models describing the common envelope (CE) phase. Adopting the canonical energy
budget approach we consider models where the ejection efficiency,
\alpha_{\rmn{CE}} is either a constant, or a function of the secondary mass.
We obtain the envelope binding energy from detailed stellar models of the
progenitor primary, with and without the thermal and ionization energy, but we
also test a commonly used analytical scaling. We also employ the alternative
angular momentum budget approach, known as the -algorithm. We find that
a constant, global value of \alpha_{\rmn{CE}} \ga 0.1 can adequately account
for the observed population of PCEBs with late spectral-type secondaries.
However, this prescription fails to reproduce IK Pegasi, which has a secondary
with spectral type A8. We can account for IK Pegasi if we include thermal and
ionization energy of the giant's envelope, or if we use the -algorithm.
However, the -algorithm predicts local space densities that are 1 to 2
orders of magnitude greater than estimates from observations. In contrast, the
canonical energy budget prescription with an initial mass ratio distribution
that favours unequal initial mass ratios gives a local space density which is
in good agreement with observations, and best reproduces the observed
distribution of PCEBs. Finally, all models fail to reproduce the sharp decline
for orbital periods, P_{\rmn{orb}} \ga 1 d in the orbital period distribution
of observed PCEBs, even if we take into account selection effects against
systems with long orbital periods and early spectral-type secondaries.Comment: Accepted for publication in the Monthly Notices of the Royal
Astronomical Society. 18 pages, 10 figures. Work concerning the
reconstruction of the common envelope phase presented in the previous version
will now be submitted in a separate paper in the near futur
Atomic screening of nuclear transitions
In the analysis of time-reversal and Mössbauer absorption experiments, it is important to consider atomic processes which interfere with the direct nuclear transition. Interaction of the photon with the atomic electrons causes the radiation to acquire a phase shift, specified by the interference parameter ξ(L_π). We present theoretical expressions for ξ and compare our calculated values with experiment. Satisfactory agreement is obtained. In particular, an apparent violation of time-reversal invariance in the 129-keV transition of ^(191)Ir is fully explained by these effects
Gross Job Flows in Ukraine: Size, Ownership and Trade Effects
This paper documents and analyses gross job flows and their determinants in Ukraine using a unique data set of more than 2200 Ukrainian firms operating in both the manufacturing and the non-manufacturing sector for the years 1998-2000. There are several important findings in the paper. Job destruction is dominating job creation in both 1999 and 2000. In connection with other evidence we infer from this that Ukraine is only at the beginning of the restructuring process. The most clear-cut result of our analysis is the strong positive effect of new private firms on net employment growth, a finding established for other transition economies as well. At the same time, we do not find differences in the employment growth of state-owned and privatised firms. Apart from ownership effects we also find, at the firm level, an inverse correlation of size and net employment growth and of size and job reallocation. Finally, we establish that strong foreign trade links force firms to shed labour more aggressively and to engage in more restructuring when trade is directed to and originating from Western economies. This disciplining function is absent when the trade flows are confined to CIS countries.http://deepblue.lib.umich.edu/bitstream/2027.42/39906/3/wp521.pd
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