18,833 research outputs found
On a conjecture regarding the upper graph box dimension of bounded subsets of the real line
Let X \subset R be a bounded set; we introduce a formula that calculates the
upper graph box dimension of X (i.e.the supremum of the upper box dimension of
the graph over all uniformly continuous functions defined on X). We demonstrate
the strength of the formula by calculating the upper graph box dimension for
some sets and by giving an "one line" proof, alternative to the one given in
[1], of the fact that if X has finitely many isolated points then its upper
graph box dimension is equal to the upper box dimension plus one. Furthermore
we construct a collection of sets X with infinitely many isolated points,
having upper box dimension a taking values from zero to one while their graph
box dimension takes any value in [max{2a,1},a + 1], answering this way,
negatively to a conjecture posed in [1]
Photometry of Irregular Satellites of Uranus and Neptune
We present BVR photometric colors of six Uranian and two Neptunian irregular
satellites, collected using the Magellan Observatory (Las Campanas, Chile) and
the Keck Observatory, (Manua Kea, Hawaii). The colors range from neutral to
light red, and like the Jovian and the Saturnian irregulars (Grav et al. 2003)
there is an apparent lack of the extremely red objects found among the Centaurs
and Kuiper belt objects.
The Uranian irregulars can be divided into three possible dynamical families,
but the colors collected show that two of these dynamical families, the Caliban
and Sycorax-clusters, have heterogeneous colors. Of the third possible family,
the 168-degree cluster containing two objects with similar average inclinations
but quite different average semi-major axis, only one object (U XXI Trinculo)
was observed. The heterogeneous colors and the large dispersion of the average
orbital elements leads us to doubt that they are collisional families. We favor
single captures as a more likely scenario. The two neptunians observed (N II
Nereid and S/2002 N1) both have very similar neutral, sun-like colors. Together
with the high collisional probability between these two objects over the age of
the solar system (Nesvorny et al. 2003, Holman et al. 2004), this suggests that
S/2002 N1 be a fragment of Nereid, broken loose during a collision or cratering
event with an undetermined impactor.Comment: 13 pages (including 3 figures and 2 tables). Submitted to ApJ Letter
Continued monitoring of LMXBs with the Faulkes Telescopes
The Faulkes Telescope Project is an educational and research arm of the Las Cumbres Observatory Global Telescope Network (LCOGTN). It has two 2-metre robotic telescopes, located at Haleakala on Maui (FT North) and Siding Spring in Australia (FT South). It is planned for these telescopes to be complemented by a research network of eighteen 1-metre telescopes, along with an educational network of twenty-eight 0.4-metre telescopes, providing 24 hour coverage of both northern and southern hemispheres.
We have been conducting a monitoring project of 13 low-mass X-ray binaries (LMXBs) using FT North since early 2006. The introduction of FT South has allowed us to extend this to monitor a total of 30 LMXBs (see target list, Section 4). New instrumentation will allow us to expand this project to include both infrared wavelengths (z and y band) and spectroscopy. Brighter targets (~ 16 - 18 mag.) are imaged weekly in V, R and i’ bands (SNR ~ 50), while fainter ones (> 18 mag.) are observed only in i’ band (SNR ~ 20). We alter this cadence in response to our own analysis or Astronomers Telegrams (ATels)
Thermal Desorption of Water-Ice in the Interstellar Medium
Water (H2O) ice is an important solid constituent of many astrophysical
environments. To comprehend the role of such ices in the chemistry and
evolution of dense molecular clouds and comets, it is necessary to understand
the freeze-out, potential surface reactivity, and desorption mechanisms of such
molecular systems. Consequently, there is a real need from within the
astronomical modelling community for accurate empirical molecular data
pertaining to these processes. Here we give the first results of a laboratory
programme to provide such data. Measurements of the thermal desorption of H2O
ice, under interstellar conditions, are presented. For ice deposited under
conditions that realistically mimic those in a dense molecular cloud, the
thermal desorption of thin films (~50 molecular layers) is found to occur with
zero order kinetics characterised by a surface binding energy, E_{des}, of 5773
+/- 60 K, and a pre-exponential factor, A, of 10^(30 +/- 2) molecules cm^-2
s^-1. These results imply that, in the dense interstellar medium, thermal
desorption of H2O ice will occur at significantly higher temperatures than has
previously been assumed.Comment: 9 pages, 4 figures, accepted for publication in MNRA
First-passage theory of exciton population loss in single-walled carbon nanotubes reveals micron-scale intrinsic diffusion lengths
One-dimensional crystals have long range translational invariance which
manifests as long exciton diffusion lengths, but such intrinsic properties are
often obscured by environmental perturbations. We use a first-passage approach
to model single-walled carbon nanotube (SWCNT) exciton dynamics (including
exciton-exciton annihilation and end effects) and compare it to results from
both continuous-wave and multi-pulse ultrafast excitation experiments to
extract intrinsic SWCNT properties. Excitons in suspended SWCNTs experience
macroscopic diffusion lengths, on the order of the SWCNT length, (1.3-4.7 um)
in sharp contrast to encapsulated samples. For these pristine samples, our
model reveals intrinsic lifetimes (350-750 ps), diffusion constants (130-350
cm^2/s), and absorption cross-sections (2.1-3.6 X 10^-17 cm^2/atom) among the
highest previously reported.and diffusion lengths for SWCNTs.Comment: 6 pages, 3 figure
Antigen-driven T-cell turnover.
A mathematical model is developed to characterize the distribution of cell turnover rates within a population of T lymphocytes. Previous models of T-cell dynamics have assumed a constant uniform turnover rate; here we consider turnover in a cell pool subject to clonal proliferation in response to diverse and repeated antigenic stimulation. A basic framework is defined for T-cell proliferation in response to antigen, which explicitly describes the cell cycle during antigenic stimulation and subsequent cell division. The distribution of T-cell turnover rates is then calculated based on the history of random exposures to antigens. This distribution is found to be bimodal, with peaks in cell frequencies in the slow turnover (quiescent) and rapid turnover (activated) states. This distribution can be used to calculate the overall turnover for the cell pool, as well as individual contributions to turnover from quiescent and activated cells. The impact of heterogeneous turnover on the dynamics of CD4(+) T-cell infection by HIV is explored. We show that our model can resolve the paradox of high levels of viral replication occurring while only a small fraction of cells are infected
- …