10,862 research outputs found
On the Independence Jeffreys prior for skew--symmetric models with applications
We study the Jeffreys prior of the skewness parameter of a general class of
scalar skew--symmetric models. It is shown that this prior is symmetric about
0, proper, and with tails under mild regularity conditions.
We also calculate the independence Jeffreys prior for the case with unknown
location and scale parameters. Sufficient conditions for the existence of the
corresponding posterior distribution are investigated for the case when the
sampling model belongs to the family of skew--symmetric scale mixtures of
normal distributions. The usefulness of these results is illustrated using the
skew--logistic model and two applications with real data
Evaluating the impact of conditional cash transfer programs : lessons from Latin America
Unlike most development initiatives, conditional cash transfer programs recently introduced in the Latin America and the Caribbean region have been subject to rigorous evaluations of their effectiveness. These programs provide money to poor families, conditional on certain behavior, usually investments in human capital-such as sending children to school or bringing them to health centers on a regular basis. Rawlings and Rubio review the experience in evaluating the impact of these programs, exploring the application of experimental and quasi-experimental evaluation methods and summarizing results from programs launched in Brazil, Honduras, Jamaica, Mexico, and Nicaragua. Evaluation results from the first generation of programs in Brazil, Mexico, and Nicaragua show that conditional cash transfer programs are effective in promoting human capital accumulation among poor households. There is clear evidence of success in increasing enrollment rates, improving preventive health care, and raising household consumption. Despite this promising evidence, many questions remain unanswered about the impact of conditional cash transfer programs, including those concerning their effectiveness under different country conditions and the sustainability of the welfare impacts.Public Health Promotion,Poverty Monitoring&Analysis,Poverty Impact Evaluation,Health Monitoring&Evaluation,Health Systems Development&Reform,Poverty Monitoring&Analysis,Poverty Impact Evaluation,Health Monitoring&Evaluation,Poverty Assessment,Health Systems Development&Reform
A search for radio pulsars and fast transients in M31 using the WSRT
We present the results of the most sensitive and comprehensive survey yet
undertaken for radio pulsars and fast transients in the Andromeda galaxy (M31)
and its satellites, using the Westerbork Synthesis Radio Telescope (WSRT) at a
central frequency of 328 MHz. We used the WSRT in a special configuration
called 8gr8 (eight-grate) mode, which provides a large instantaneous
field-of-view, about 5 square degrees per pointing, with good sensitivity, long
dwell times (up to 8 hours per pointing), and good spatial resolution (a few
arc minutes) for locating sources. We have searched for both periodicities and
single pulses in our data, aiming to detect bright, persistent radio pulsars
and rotating radio transients (RRATs) of either Galactic or extragalactic
origin. Our searches did not reveal any confirmed periodic signals or bright
single bursts from (potentially) cosmological distances. However, we do report
the detection of several single pulse events, some repeating at the same
dispersion measure, which could potentially originate from neutron stars in
M31. One in particular was seen multiple times, including a burst of six pulses
in 2000 seconds, at a dispersion measure of 54.7 pc cm^-3, which potentially
places the origin of this source outside of our Galaxy. Our results are
compared to a range of hypothetical populations of pulsars and RRATs in M31 and
allow us to constrain the luminosity function of pulsars in M31. They also show
that, unless the pulsar population in M31 is much dimmer than in our Galaxy,
there is no need to invoke any violation of the inverse square law of the
distance for pulsar fluxes.Comment: 18 pages, 14 figures, 8 tables. Accepted for publication in the main
journal of MNRA
Imaging Spectropolarimetry with IBIS: Evolution of Bright Points in the Quiet Sun
We present the results from first spectropolarimetric observations of the
solar photosphere acquired at the Dunn Solar Telescope with the Interferometric
Bidimensional Spectrometer. Full Stokes profiles were measured in the Fe I
630.15 nm and Fe I 630.25 nm lines with high spatial and spectral resolutions
for 53 minutes, with a Stokes V noise of 0.003 the continuum intensity level.
The dataset allows us to study the evolution of several magnetic features
associated with G-band bright points in the quiet Sun. Here we focus on the
analysis of three distinct processes, namely the coalescence, fragmentation and
cancellation of G-band bright points. Our analysis is based on a SIR inversion
of the Stokes I and V profiles of both Fe I lines. The high spatial resolution
of the G-band images combined with the inversion results helps to interpret the
undergoing physical processes. The appearance (dissolution) of high-contrast
G-band bright points is found to be related to the local increase (decrease) of
the magnetic filling factor, without appreciable changes in the field strength.
The cancellation of opposite-polarity bright points can be the signature of
either magnetic reconnection or the emergence/submergence of magnetic loops.Comment: 4 pages, 5 figures, accepted for publication in ApJ Letter
Granular Scale Magnetic Flux Cancellations in the Photosphere
We investigate the evolution of 5 granular-scale magnetic flux cancellations
just outside the moat region of a sunspot by using accurate spectropolarimetric
measurements and G-band images with the Solar Optical Telescope aboard Hinode.
The opposite polarity magnetic elements approach a junction of the
intergranular lanes and then they collide with each other there. The
intergranular junction has strong red shifts, darker intensities than the
regular intergranular lanes, and surface converging flows. This clearly
confirms that the converging and downward convective motions are essential for
the approaching process of the opposite-polarity magnetic elements. However,
motion of the approaching magnetic elements does not always match with their
surrounding surface flow patterns in our observations. This suggests that, in
addition to the surface flows, subsurface downward convective motions and
subsurface magnetic connectivities are important for understanding the approach
and collision of the opposite polarity elements observed in the photosphere. We
find that the horizontal magnetic field appears between the canceling opposite
polarity elements in only one event. The horizontal fields are observed along
the intergranular lanes with Doppler red shifts. This cancellation is most
probably a result of the submergence (retraction) of low-lying photospheric
magnetic flux. In the other 4 events, the horizontal field is not observed
between the opposite polarity elements at any time when they approach and
cancel each other. These approaching magnetic elements are more concentrated
rather than gradually diffused, and they have nearly vertical fields even while
they are in contact each other. We thus infer that the actual flux cancellation
is highly time dependent events at scales less than a pixel of Hinode SOT
(about 200 km) near the solar surface.Comment: Accepted for publication in the Astrophysical Journa
High Excitation Molecular Gas in the Magellanic Clouds
We present the first survey of submillimeter CO 4-3 emission in the
Magellanic Clouds. The survey is comprised of 15 6'x6' maps obtained using the
AST/RO telescope toward the molecular peaks of the Large and Small Magellanic
Clouds. We have used these data to constrain the physical conditions in these
objects, in particular their molecular gas density and temperature. We find
that there are significant amounts of molecular gas associated with most of
these molecular peaks, and that high molecular gas temperatures are pervasive
throughout our sample. We discuss whether this may be due to the low
metallicities and the associated dearth of gas coolants in the Clouds, and
conclude that the present sample is insufficient to assert this effect.Comment: 18 pages, 3 figures, 5 tables. To appear in Ap
Scanning tunneling microscopy simulations of poly(3-dodecylthiophene) chains adsorbed on highly oriented pyrolytic graphite
We report on a novel scheme to perform efficient simulations of Scanning
Tunneling Microscopy (STM) of molecules weakly bonded to surfaces. Calculations
are based on a tight binding (TB) technique including self-consistency for the
molecule to predict STM imaging and spectroscopy. To palliate the lack of
self-consistency in the tunneling current calculation, we performed first
principles density-functional calculations to extract the geometrical and
electronic properties of the system. In this way, we can include, in the TB
scheme, the effects of structural relaxation upon adsorption on the electronic
structure of the molecule. This approach is applied to the study of
regioregular poly(3-dodecylthiophene) (P3DDT) polymer chains adsorbed on highly
oriented pyrolytic graphite (HOPG). Results of spectroscopic calculations are
discussed and compared with recently obtained experimental datComment: 15 pages plus 5 figures in a tar fil
Switching the Conductance of a Molecular Junction using a Proton Transfer Reaction
A novel mechanism for switching a molecular junction based on a proton
transfer reaction triggered by an external electrostatic field is proposed. As
a specific example to demonstrate the feasibility of the mechanism, the
tautomers [2,5-(4-hydroxypyridine)] and {2,5-[4(1H)-pyridone]} are considered.
Employing a combination of first-principles electronic structure calculations
and Landauer transport theory, we show that both tautomers exhibit very
different conductance properties and realize the "on" and "off" states of a
molecular switch. Moreover, we provide a proof of principle that both forms can
be reversibly converted into each other using an external electrostatic field.Comment: 14 pages, 5 figure
Properties of Umbral Dots from Stray Light Corrected Hinode Filtergrams
High resolution blue continuum filtergrams from Hinode are employed to study
the umbral fine structure of a regular unipolar sunspot. The removal of
scattered light from the images increases the rms contrast by a factor of 1.45
on average. Improvement in image contrast renders identification of short
filamentary structures resembling penumbrae that are well separated from the
umbra-penumbra boundary and comprise bright filaments/grains flanking dark
filaments. Such fine structures were recently detected from ground based
telescopes and have now been observed with Hinode. A multi-level tracking
algorithm was used to identify umbral dots in both the uncorrected and
corrected images and to track them in time. The distribution of the values
describing the photometric and geometric properties of umbral dots are more
easily affected by the presence of stray light while it is less severe in the
case of kinematic properties. Statistically, umbral dots exhibit a peak
intensity, effective diameter, lifetime, horizontal speed and a trajectory
length of 0.29 I_QS, 272 km, 8.4 min, 0.45 km/s and 221 km respectively. The 2
hr 20 min time sequence depicts several locations where umbral dots tend to
appear and disappear repeatedly with various time intervals. The correction for
scattered light in the Hinode filtergrams facilitates photometry of umbral fine
structure which can be related to results obtained from larger telescopes and
numerical simulations.Comment: Accepted for publication in ApJ : 10 pages, 10 figures, 3 table
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