2,833 research outputs found
Learning Agility: In Search of Conceptual Clarity and Theoretical Grounding
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/92376/1/j.1754-9434.2012.01444.x.pd
Cross-linkable azido C60-fullerene derivatives for efficient thermal stabilization of polymer bulk-heterojunction solar cells
International audienceOriginal [60]PCBM-inspired fullerene derivatives bearing an azidofunctional group were synthesized. By incorporating an optimizedquantity of this thermal cross-linker additive in the P3HT:[60]PCBMphotoactive layer, an impressive stabilization of the bulk-heterojunctionmorphology at its optimal photovoltaic performance wasachieved
Learning Agility: Many Questions, a Few Answers, and a Path Forward
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/92383/1/j.1754-9434.2012.01465.x.pd
Microlensing by Compact Objects associated to Gas Clouds
We investigate gravitational microlensing of point-like lenses surrounded by
diffuse gas clouds. Besides gravitational bending, one must also consider
refraction and absorption phenomena. According to the cloud density, the light
curves may suffer small to large deviations from Paczynski curves, up to
complete eclipses. Moreover, the presence of the cloud endows this type of
microlensing events with a high chromaticity and absorption lines recognizable
by spectral analysis. It is possible that these objects populate the halo of
our galaxy, giving a conspicuous contribution to the fraction of the baryonic
dark matter. The required features for the extension and the mass of the cloud
to provide appreciable signatures are also met by several astrophysical
objects.Comment: 11 pages with 4 figures. Accepted by A&
Optical Gravitational Lensing Experiment. OGLE-1999-BUL-19: The First Multi-Peak Parallax Event
We describe a highly unusual microlensing event, OGLE-1999-BUL-19, which
exhibits multiple peaks in its light curve. The Einstein radius crossing time
for this event is approximately one year, which is unusually long. We show that
the motion of the Earth induces these multiple peaks in the light curve, since
the relative transverse velocity of the lens projected into the observer plane
is very small (v = 12.5 km/s). This is the lowest velocity so far published and
we believe that this is the first multiple-peak parallax event ever observed.
We also believe that this event may be exhibiting slight binary-source
signatures in addition to these parallax-induced multiple peaks. With
spectroscopic observations it is possible to test this `parallax plus
binary-source' hypothesis and (if this hypothesis turns out to be correct) to
simultaneously fit both models and obtain a measurement of the lens mass.
Furthermore, spectroscopic observations could also supply information regarding
the lens properties, possibly providing another avenue for determining the lens
mass. We found that most of the I-band blending is probably caused by light
from the lens or a binary companion to the source. However, in the V-band,
there appears to be a second blended source 0.35" away from the lensed source.
HST observations will be very useful for understanding the nature of the
blends. We also suggest that a radial velocity survey of all parallax events
will be very useful for further constraining the lensing kinematics and
understanding the origins of these events and the excess of long events toward
the bulge.Comment: 36 pages, 7 figures. Accepted for publication in MNRA
The MACHO project: Microlensing Optical Depth towards the Galactic Bulge from Difference Image Analysis
We present the microlensing optical depth towards the Galactic bulge based on
the detection of 99 events found in our Difference Image Analysis (DIA) survey.
This analysis encompasses three years of data, covering ~ 17 million stars in ~
4 deg^2, to a source star baseline magnitude limit of V = 23. The DIA technique
improves the quality of photometry in crowded fields, and allows us to detect
more microlensing events with faint source stars. We find this method increases
the number of detection events by 85% compared with the standard analysis
technique. DIA light curves of the events are presented and the microlensing
fit parameters are given. The total microlensing optical depth is estimated to
be tau_(total)= 2.43^(+0.39/-0.38) x 10^(-6) averaged over 8 fields centered at
l=2.68 and b=-3.35. For the bulge component we find
tau_(bulge)=3.23^(+0.52/-0.50) x 10^(-6) assuming a 25% stellar contribution
from disk sources. These optical depths are in good agreement with the past
determinations of the MACHO Alcock et al. (1997) and OGLE Udalski et al. (1994)
groups, and are higher than predicted by contemporary Galactic models. We show
that our observed event timescale distribution is consistent with the
distribution expected from normal mass stars, if we adopt the stellar mass
function of Scalo (1986) as our lens mass function. However, we note that as
there is still disagreement about the exact form of the stellar mass function,
there is uncertainty in this conclusion. Based on our event timescale
distribution we find no evidence for the existence of a large population of
brown dwarfs in the direction of the Galactic bulge.Comment: Updated references and corrected optical depth values. tau_tot=
[2.91(+0.47/-0.45) -> 2.43^(+0.39/-0.38)] x 10^(-6) tau_bul =
[3.88(+0.63/-0.60) -> 3.23^(+0.52/-0.50)] x 10^(-6
A precision measurement of direct CP violation in the decay of neutral kaons into two pions
The direct CP violation parameter Re(epsilon'/epsilon) has been measured from
the decay rates of neutral kaons into two pions using the NA48 detector at the
CERN SPS. The 2001 running period was devoted to collecting additional data
under varied conditions compared to earlier years (1997-99). The new data yield
the result: Re(epsilon'/epsilon) = (13.7 +/- 3.1) times 10^{-4}. Combining this
result with that published from the 1997, 98 and 99 data, an overall value of
Re(epsilon'/epsilon) = (14.7 +/- 2.2) times 10^{-4} is obtained from the NA48
experiment.Comment: 19 pages, 5 figures, to be published in Physics Letters
Search for CP violation in K0 -> 3 pi0 decays
Using data taken during the year 2000 with the NA48 detector at the CERN SPS,
a search for the CP violating decay K_S -> 3 pi0 has been performed. From a fit
to the lifetime distribution of about 4.9 million reconstructed K0/K0bar -> 3
pi0 decays, the CP violating amplitude eta_000 = A(K_S -> 3 pi0)/A(K_L -> 3
pi0) has been found to be Re(eta_000) = -0.002 +- 0.011 +- 0.015 and
Im(eta_000) = -0.003 +- 0.013 +- 0.017. This corresponds to an upper limit on
the branching fraction of Br(K_S -> 3 pi0) < 7.4 x 10^-7 at 90% confidence
level. The result is used to improve knowledge of Re(epsilon) and the CPT
violating quantity Im(delta) via the Bell-Steinberger relation.Comment: 18 pages, 7 figures, submitted to Phys. Lett.
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