11,076 research outputs found
Some effects of small-scale metallicity variations in cooling flows
In an attempt to reconcile recent spectral data with predictions of the
standard cooling flow model, it has been suggested that the metals in the
intracluster medium (ICM) might be distributed inhomogeneously on small scales.
We investigate the possible consequences of such a situation within the
framework of the cooling flow scenario. Using the standard isobaric cooling
flow model, we study the ability of such metallicity variations to
preferentially suppress low-temperature line emission in cooling flow spectra.
We then use simple numerical simulations to investigate the temporal and
spatial evolution of the ICM when the metals are distributed in such a fashion.
Simulated observations are used to study the constraints real data can place on
conditions in the ICM. The difficulty of ruling out abundance variations on
small spatial scales with current observational limits is emphasized. We find
that a bimodal distribution of metals may give rise to interesting effects in
the observed abundance profile, in that apparent abundance gradients with
central abundance drops and off-centre peaks, similar to those seen recently in
some clusters, are produced. Different elements behave in different fashion as
governed by the temperature dependence of their equivalent widths. Our overall
conclusion is that, whilst this process alone seems unlikely to be able to
account for the sharp reduction in low temperature emission lines seen in
current spectral data, a contribution at some level is possible and difficult
to rule out. The possibility of small-scale metallicity variations should be
considered when analysing high resolution cluster X-ray spectra.Comment: 14 pages, 10 figures. Accepted for publication in MNRA
Public policies for the working poor: The earned income tax credit versus minimum wage legislation
This paper documents the declining relationship between low hourly wages and low household income over the last half-century and how this has reduced the share of minimum wage workers who live in poor households. It then compares recent and prospective increases in the earned income tax credit (EITC) and the minimum wage as methods of increasing the labor earnings of poor workers. Data from the Current Population Survey (CPS) are used to simulate the effects of both programs. Increases in the EITC between 1989 and 1992 delivered a much larger proportion of a given dollar of benefits to the poor than did increases in the minimum wage from 4.25. Scheduled increases in the EITC through 1996 will also do far more for the working poor than raising the minimum wage.
Inflation and the User Cost of Capital: Does Inflation Still Matter?
In the late 1970s, many economists argued that the deleterious effects of inflation on the user cost of capital for U.S. firms were large. Since that time, the tax code has changed, the level of inflation has dropped significantly, and the of investment has evolved considerably. In this paper, we demonstrate that the net effect of these changes has--under reasonable assumptions--not relegated inflation to the sidelines. Indeed, we conclude that: (1) inflation, even at its relatively low current rates, continues to increase the user cost of capital significantly; (2) the marginal gain in investment in response to a percentage-point reduction in inflation is larger for lower levels of inflation; (3) the beneficial effects for steady-state consumption of lowering inflation even further than has been achieved to date would likely be significant; and (4) inflation has only a small impact on intratemporal distortion in the allocation of capital within the domestic business sector. We also show that the magnitude of the inflation effect on the user cost of capital is likely much smaller in open economies.
Computational Methods and Results for Structured Multiscale Models of Tumor Invasion
We present multiscale models of cancer tumor invasion with components at the
molecular, cellular, and tissue levels. We provide biological justifications
for the model components, present computational results from the model, and
discuss the scientific-computing methodology used to solve the model equations.
The models and methodology presented in this paper form the basis for
developing and treating increasingly complex, mechanistic models of tumor
invasion that will be more predictive and less phenomenological. Because many
of the features of the cancer models, such as taxis, aging and growth, are seen
in other biological systems, the models and methods discussed here also provide
a template for handling a broader range of biological problems
Parameterization of Dark-Energy Properties: a Principal-Component Approach
Considerable work has been devoted to the question of how to best
parameterize the properties of dark energy, in particular its equation of state
w. We argue that, in the absence of a compelling model for dark energy, the
parameterizations of functions about which we have no prior knowledge, such as
w(z), should be determined by the data rather than by our ingrained beliefs or
familiar series expansions. We find the complete basis of orthonormal
eigenfunctions in which the principal components (weights of w(z)) that are
determined most accurately are separated from those determined most poorly.
Furthermore, we show that keeping a few of the best-measured modes can be an
effective way of obtaining information about w(z).Comment: Unfeasibility of a truly model-independent reconstruction of w at z>1
illustrated. f(z) left out, and w(z) discussed in more detail. Matches the
PRL versio
Convectively driven shear and decreased heat flux
We report on direct numerical simulations of two-dimensional, horizontally
periodic Rayleigh-B\'enard convection, focusing on its ability to drive
large-scale horizontal flow that is vertically sheared. For the Prandtl numbers
() between 1 and 10 simulated here, this large-scale shear can be induced
by raising the Rayleigh number () sufficiently, and we explore the
resulting convection for up to . When present in our simulations,
the sheared mean flow accounts for a large fraction of the total kinetic
energy, and this fraction tends towards unity as . The shear helps
disperse convective structures, and it reduces vertical heat flux; in parameter
regimes where one state with large-scale shear and one without are both stable,
the Nusselt number of the state with shear is smaller and grows more slowly
with . When the large-scale shear is present with , the
convection undergoes strong global oscillations on long timescales, and heat
transport occurs in bursts. Nusselt numbers, time-averaged over these bursts,
vary non-monotonically with for . When the shear is present with
, the flow does not burst, and convective heat transport is
sustained at all times. Nusselt numbers then grow roughly as powers of ,
but the growth rates are slower than any previously reported for
Rayleigh-B\'enard convection without large-scale shear. We find the Nusselt
numbers grow proportionally to when and to when
. Analogies with tokamak plasmas are described.Comment: 25 pages, 12 figures, 5 video
MEASURING SCOPE AND SCALE EFFICIENCY GAINS DUE TO SPECIALIZATION
Using the non-parametric linear programming approach, this study examines overall efficiency gains due to diversification between crop and livestock enterprises for a sample of Kansas farms. Overall efficiency gains were decomposed into scope efficiency gains and scale efficiency gains. Farms with both crops and livestock were found to be less efficient than farms with just crops or just livestock. Operator age, profit margin, and farm size were significantly related to overall efficiency.Farm Management,
L'CO/LFIR Relations with CO Rotational Ladders of Galaxies Across the Herschel SPIRE Archive
We present a catalog of all CO (J=4-3 through J=13-12)), [CI], [NII] lines
available from extragalactic spectra from the Herschel SPIRE Fourier Transform
Spectrometer (FTS) archive combined with observations of the low-J CO lines
from the literature and from the Arizona Radio Observatory. This work examines
the relationships between LFIR, L'CO, and LCO/LCO(1-0). We also present a new
method for estimating probability distribution functions (PDFs) from marginal
signal-to-noise ratio Herschel} FTS spectra, which takes into account the
instrumental "ringing" and the resulting highly correlated nature of the
spectra. The slopes of log(LFIR) vs. log(L'CO) are linear for all mid- to
high-J CO lines and slightly sublinear if restricted to (U)LIRGs. The mid- to
high-J CO luminosity relative to CO J=1-0 increases with increasing LFIR,
indicating higher excitement of the molecular gas, though these ratios do not
exceed ~ 180. For a given bin in LFIR, the luminosities relative to CO J=1-0
remain relatively flat from J=6-5 through J=13-12, across three orders of
magnitude of LFIR. A single component theoretical photon-dominated region (PDR)
model cannot match these flat SLED shapes, though combinations of PDR models
with mechanical heating added qualitatively match the shapes, indicating the
need for further comprehensive modeling of the excitation processes of warm
molecular gas in nearby galaxies.Comment: 17 pages, 4 figures (including appendix), accepted by ApJ. Full
tables will be in VizieR upon publication, email first author for tables in
the meantim
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