1,590 research outputs found
Unordered Canonical Ramsey Numbers
AbstractWe define a weak form of canonical colouring, based on that of P. Erdős and R. Rado (1950, J. London Math. Soc.25, 249–255). This yields a class of unordered canonical Ramsey numbers CR(s, t), again related to the canonical Ramsey numbers ER(2; s) of Erdős and Rado. We present upper and lower bounds (the latter via a construction) for CR(s, t) which are significantly tighter than the best-known corresponding bounds for ER(2; s)
Cell synchronization and dynamic G-banding of equine chromosomes by bromodeoxyurldine
Both dynamic G-banding and cell synchronization produced by bromodeoxyuridine (BrdU), were applied to equine chromosomes. BrdU incorporated during the first half of the S-phase is taken up into the R-bands that are early replicating. These bands, which have incorporated BrdU, cannot contract as usual and remain elongated; only the other regions of the chromosome, i.e., the G-bands, contract normally and are sharply defined. BrdU also can be used for cell synchronization. The addition of BrdU in a high concentration, 15 hours before harvest, and its removal 11 hours later, has two effects: initially the BrdU is incorporated during the first part of the S-phase and then it blocks the cells at mid-S-phase. Within the cell cycle, mid-S-phase appears to be the most vuinerable time to various blocking agents. To differentiate the regions of BrdU incorporation from those that have not been substituted, the fluorescence-photolysis-Glemsa (FPG) technique was applied as modified for horse chromosomes. This dynamic technique, which produces many prometaphase and prophase chromosomes showing very sharp G-bands, is certain to enhance the accuracy of cytogenetic analysis and aid in the standardization of equine chromosome
High resolution R-bands produced in equine chromosomes after incorporation of bromodeoxyuridine
Cell synchronization was used to obtain an adequate percentage of very long chromosomes in equine mitotic spreads. Reported here is our variation, adapted to horse chromosomes, of a method using excess thymidine followed by bromodeoxyuridine incorporation. This technique routinely yields excellent quality cells, predominantly in prometaphase and prophase. Among other differences with the standard technique, this addltion does not use Colcemid, which, in addltion to Inhibiting spindle fiber formation, also increases chromosome contraction resulting in thicker and thus fewer bands. Consequently, horse prometaphase chromosomes, which have incorporated BrdU in the late-S-phase, are very long and display a large number of R-bands after the fluorescence-photolysis Glemsa method. This technique should definitely be useful for the analysis of structural anomalies and the standardization of equine R-band
Proper motion measurements as indicators of binarity in open clusters
We analyze 9 open clusters with ages in the range 70 Myr to 3.2 Gyr using
UCAC2 proper motion data and 2MASS photometry. For each cluster we consider the
projected velocity distributions in the core and off-core regions separately.
In the projected velocity distribution of all sample clusters we find a
well-defined low-velocity peak, as well as an excess in the number of stars at
larger velocities. The low-velocity peak is accounted for by the random motion
of the single stars, while the high-velocity excess can be attributed to the
large velocity changes produced by a significant fraction of unresolved
binaries in a cluster. We derive kinematic parameters of the single-star
distribution, in particular the projected velocity dispersion. The relatively
large velocity dispersions derived in this work may reflect the non-virialized
state of the clusters. Based on the relative number of high-velocity (binary)
and single stars, we inferred for the sample clusters unresolved binary
fractions in the range , for both core and off-core regions. The
present results suggest that care must be taken when applying proper-motion
filters to sort out members, especially binaries in a star cluster. This paper
shows that proper motions turn out to be a useful tool for identifying
high-velocity stars as unresolved binary cluster members, and as a consequence,
map and quantify the binary component in colour-magnitude diagrams.Comment: 9 pages and 9 figures. Astronomy and Astrophysics, accepted
(25/10/2004
Phase transitions in a gas of anyons
We continue our numerical Monte Carlo simulation of a gas of closed loops on
a 3 dimensional lattice, however now in the presence of a topological term
added to the action corresponding to the total linking number between the
loops. We compute the linking number using certain notions from knot theory.
Adding the topological term converts the particles into anyons. Using the
correspondence that the model is an effective theory that describes the
2+1-dimensional Abelian Higgs model in the asymptotic strong coupling regime,
the topological linking number simply corresponds to the addition to the action
of the Chern-Simons term. We find the following new results. The system
continues to exhibit a phase transition as a function of the anyon mass as it
becomes small \cite{mnp}, although the phases do not change the manifestation
of the symmetry. The Chern-Simons term has no effect on the Wilson loop, but it
does affect the {\rm '}t Hooft loop. For a given configuration it adds the
linking number of the 't Hooft loop with all of the dynamical vortex loops to
the action. We find that both the Wilson loop and the 't Hooft loop exhibit a
perimeter law even though there are no massless particles in the theory, which
is unexpected.Comment: 6 pages, 5 figure
Effectiveness of a 10-day melarsoprol schedule for the treatment of late-stage human African trypanosomiasis: confirmation from a multinational study (IMPAMEL II).
BACKGROUND: Treatment of late-stage human African trypanosomiasis (HAT) with melarsoprol can be improved by shortening the regimen. A previous trial demonstrated the safety and efficacy of a 10-day treatment schedule. We demonstrate the effectiveness of this schedule in a noncontrolled, multinational drug-utilization study. METHODS: A total of 2020 patients with late-stage HAT were treated with the 10-day melarsoprol schedule in 16 centers in 7 African countries. We assessed outcome on the basis of major adverse events and the cure rate after treatment and during 2 years of follow-up. RESULTS: The cure rate 24 h after treatment was 93.9%; 2 years later, it was 86.2%. However, 49.3% of patients were lost to follow-up. The overall fatality rate was 5.9%. Of treated patients, 8.7% had an encephalopathic syndrome that was fatal 45.5% of the time. The rate of severe bullous and maculopapular eruptions was 0.8% and 6.8%, respectively. CONCLUSIONS: The 10-day treatment schedule was well implemented in the field and was effective. It reduces treatment duration, drug amount, and hospitalization costs per patient, and it increases treatment-center capacity. The shorter protocol has been recommended by the International Scientific Council for Trypanosomiasis Research and Control for the treatment of late-stage HAT caused by Trypanosoma brucei gambiense
Deep near-IR observations of the Globular Cluster M4: Hunting for Brown Dwarfs
We present an analysis of deep HST/WFC3 near-IR (NIR) imaging data of the
globular cluster M4. The best-photometry NIR colour-magnitude diagram (CMD)
clearly shows the main sequence extending towards the expected end of the
Hydrogen-burning limit and going beyond this point towards fainter sources. The
white dwarf sequence can be identified. As such, this is the deepest NIR CMD of
a globular cluster to date. Archival HST optical data were used for
proper-motion cleaning of the CMD and for distinguishing the white dwarfs (WDs)
from brown dwarf (BD) candidates. Detection limits in the NIR are around F110W
approx 26.5 mag and F160W approx27 mag, and in the optical around F775W approx
28 mag. Comparing our observed CMDs with theoretical models, we conclude that
we have reached beyond the H-burning limit in our NIR CMD and are probably just
above or around this limit in our optical-NIR CMDs. Thus, any faint NIR sources
that have no optical counterpart are potential BD candidates, since the optical
data are not deep enough to detect them. We visually inspected the positions of
NIR sources which are fainter than the H-burning limit in F110W and for which
the optical photometry did not return a counterpart. We found in total five
sources for which we did not get an optical measurement. For four of these five
sources, a faint optical counterpart could be visually identified, and an upper
optical magnitude was estimated. Based on these upper optical magnitude limits,
we conclude that one source is likely a WD, one source could either be a WD or
BD candidate, and the remaining two sources agree with being BD candidates. For
only one source no optical counterpart could be detected, which makes this
source a good BD candidate. We conclude that we found in total four good BD
candidates.Comment: ApJ accepted, 28 pages including 16 figure
Quantification of myocardial perfusion based on signal intensity of flow sensitized MRI
A new method to quantify myocardial perfusion was developed based on slice select (M S) and non-select (MG) inversion recovery acquisitions at a single inversion time. A modified Bloch equation was solved to obtain an analytical expression for perfusion (P) in terms of ΔM SG =M S-M G The average myocardial perfusion of healthy C57BL/6 mice measured using this technique (P=5.7±0.4 ml/g/min) agreed with that measured using traditional techniques and it had a high reproducibility with mean standard deviation of 3.6 % between repeated measures. Perfusion maps of ischemia-reperfusion mice showed significantly low perfusion (P=1.6±0.3 ml/g/min)intheinfarctedregionscomparedtothatof remote regions (P=4.1±0.3 ml/g/min,p=0.004). Backgroun
Isochrones and Luminosity Functions for Old White Dwarfs
Using a new grid of models of cooling white dwarfs, we calculate isochrones
and luminosity functions in the Johnson-Kron/Cousins and HST filter sets for
systems containing old white dwarfs. These new models incorporate a non-grey
atmosphere which is necessary to properly describe the effects of molecular
opacity at the cool temperatures of old white dwarfs. The various functions
calculated and extensively tabulated and plotted are meant to be as utilitarian
as possible for observers so all results are listed in quantities that
observers will obtain. The tables and plots developed should eventually prove
critical in interpreting the results of HST's Advanced Camera observations of
the oldest white dwarfs in nearby globular clusters, in understanding the
results of searches for old white dwarfs in the Galactic halo, and in
determining ages for star clusters of all ages using white dwarfs. As a
practical application we demonstrate the use of these results by deriving the
white dwarf cooling age of the old Galactic cluster M67.Comment: 7 pages, 8 tables, accepted for publication in the Astrophysical
Journa
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