59,230 research outputs found
Kepler and the Kuiper Belt
The proposed field-of-view of the Kepler mission is at an ecliptic latitude
of ~55 degrees, where the surface density of scattered Kuiper Belt Objects
(KBOs) is a few percent that in the ecliptic plane. The rate of occultations of
Kepler target stars by scattered KBOs with radii r>10km is ~10^-6 to 10^-4 per
star per year, where the uncertainty reflects the current ignorance of the
thickness of the scattered KBO disk and the faint-end slope of their magnitude
distribution. These occultation events will last only ~0.1% of the planned
t_exp=15 minute integration time, and thus will appear as single data points
that deviate by tiny amounts. However, given the target photometric accuracy of
Kepler, these deviations will nevertheless be highly significant, with typical
signal-to-noise ratios of ~10. I estimate that 1-20 of the 10^5 main-sequence
stars in Kepler's field-of-view will exhibit detectable occultations during its
four-year mission. For unresolved events, the signal-to-noise of individual
occultations scales as t_exp^{-1/2}, and the minimum detectable radius could be
decreased by an order of magnitude to ~1 km by searching the individual
3-second readouts for occultations. I propose a number of methods by which
occultation events may be differentiated from systematic effects. Kepler should
measure or significantly constrain the frequency of highly-inclined, ~10
km-sized KBOs.Comment: 5 pages, 1 figure. No changes. Accepted to ApJ, to appear in the
August 1, 2004 issue (v610
Mixed valency in cerium oxide crystallographic phases: Determination of valence of the different cerium sites by the bond valence method
We have applied the bond valence method to cerium oxides to determine the
oxidation states of the Ce ion at the various site symmetries of the crystals.
The crystals studied include cerium dioxide and the two sesquioxides along with
some selected intermediate phases which are crystallographically well
characterized. Our results indicate that cerium dioxide has a mixed-valence
ground state with an f-electron population on the Ce site of 0.27 while both
the A- and C-sesquioxides have a nearly pure f^1 configuration. The Ce sites in
most of the intermediate oxides have non-integral valences. Furthermore, many
of these valences are different from the values predicted from a naive
consideration of the stoichiometric valence of the compound
Anomalous diffusion in quantum Brownian motion with colored noise
Anomalous diffusion is discussed in the context of quantum Brownian motion
with colored noise. It is shown that earlier results follow simply and directly
from the fluctuation-dissipation theorem. The limits on the long-time
dependence of anomalous diffusion are shown to be a consequence of the second
law of thermodynamics. The special case of an electron interacting with the
radiation field is discussed in detail. We apply our results to wave-packet
spreading
Pressure of thermal excitations in superfluid helium
We find the pressure, due to the thermal excitations of superfluid helium, at
the interface with a solid. The separate contributions of phonons, rotons
and rotons are derived. The pressure due to rotons is shown to be
negative and partially compensates the positive contribution of rotons,
so the total roton pressure is positive but several times less than the
separate and roton contributions. The pressure of the quasiparticle
gas is shown to account for the fountain effect in . An experiment is
proposed to observe the negative pressure due to rotons.Comment: 14 pages, 4 figure
Complex Instantons and Charged Rotating Black Hole Pair Creation
We consider the general process of pair-creation of charged rotating black
holes. We find that instantons which describe this process are necessarily
complex due to regularity requirements. However their associated probabilities
are real, and fully consistent with the interpretation that the entropy of a
charged rotating black hole is the logarithm of the number of its quantum
states.Comment: 11 pages, 1 figure, Latex, text shortened with only minor changes in
content, accepted for Phys Rev Letter
Biological Records Centre Annual Report 2005-2006
The period covered by this report is the first year of a new six-year partnership between CEH and JNCC. For this period, there is increased emphasis on targeted survey, on analysis and interpretation and on communications and outreach. These activities were always part of BRCâs work, but they have been given greater prominence as a result of rapid developments in information technology. Data are increasingly reaching BRC in electronic form, so that the effort of data entry and collation is reduced.
The data, collected by many volunteers and then collated and analysed at BRC, document the changing status and distribution of plants and animals in Britain. Distribution maps are published in atlases and are available via the internet through the NBN Gateway. The effects of change or loss of habitats, the influence of climate change and the consequences of changing water quality are all examples of the environmental factors that affect our biodiversity and which BRC aims to document and understand. The results are vital for developing environmental policies, to support conservation, and for fundamental ecological research.
BRC is funded jointly by JNCC and NERC through a partnership based on a Memorandum of Agreement (MoA). The partnership started in 1973 when the Nature Conservancy was divided to form the successor bodies Nature Conservancy Council (NCC) and Institute of Terrestrial Ecology (ITE). NCC was in turn divided further to form JNCC and three Country Agencies, while ITE was merged with other NERC units to form CEH. Through all these changes, the partnership has been maintained. A six-year memorandum of agreement ended on 31 January 2005 (Hill et al. 2005). The present report covers the first full year, 2005-6, of the new agreement for 2005-2010.
Rapid progress in information technology continues to be highly beneficial for BRC, whose data are increasingly used by the UK country conservation agencies, environmental consultants, NGOs, research workers, policy makers and volunteers. It is gratifying to know that, through our ability to display data on the National Biodiversity Network (NBN) Gateway, some of our data suppliers now have immediate access to their own data in a convenient form.
The year 2005-6 has been one of steady progress, with new datasets added to BRC, substantial additions to existing data, and improved communication with the NBN Gateway. The most high profile activity of the year has been the Harlequin Ladybird Survey, which has enabled us to observe the early stages of colonization by a mobile insect in greater detail than has been possible in any previous case
The Substellar Mass Function in sigma Orionis
We combine results from imaging searches for substellar objects in the sigma
Orionis cluster and follow-up photometric and spectroscopic observations to
derive a census of the brown dwarf population in a region of 847 arcmin^2. We
identify 64 very low-mass cluster member candidates in this region. We have
available three color (IZJ) photometry for all of them, spectra for 9 objects,
and K photometry for 27% of our sample. These data provide a well defined
sequence in the I vs I-J, I-K color magnitude diagrams, and indicate that the
cluster is affected by little reddening despite its young age (~5 Myr). Using
state-of-the-art evolutionary models, we derive a mass function from the
low-mass stars (0.2 Msol) across the complete brown dwarf domain (0.075 Msol to
0.013 Msol), and into the realm of free-floating planetary-mass objects (<0.013
Msol). We find that the mass spectrum (dN/dm ~ m^{-alpha}) increases toward
lower masses with an exponent alpha = 0.8+/-0.4. Our results suggest that
planetary-mass isolated objects could be as common as brown dwarfs; both kinds
of objects together would be as numerous as stars in the cluster. If the
distribution of stellar and substellar masses in sigma Orionis is
representative of the Galactic disk, older and much lower luminosity
free-floating planetary-mass objects with masses down to about 0.005 Msol
should be abundant in the solar vicinity, with a density similar to M-type
stars.Comment: Accepted for publication in ApJ. 19 pages, 3 figures include
Matrix Pencils and Entanglement Classification
In this paper, we study pure state entanglement in systems of dimension
. Two states are considered equivalent if they can be
reversibly converted from one to the other with a nonzero probability using
only local quantum resources and classical communication (SLOCC). We introduce
a connection between entanglement manipulations in these systems and the
well-studied theory of matrix pencils. All previous attempts to study general
SLOCC equivalence in such systems have relied on somewhat contrived techniques
which fail to reveal the elegant structure of the problem that can be seen from
the matrix pencil approach. Based on this method, we report the first
polynomial-time algorithm for deciding when two states
are SLOCC equivalent. Besides recovering the previously known 26 distinct SLOCC
equivalence classes in systems, we also determine the
hierarchy between these classes
Helicopter tail rotor thrust and main rotor wake coupling in crosswind flight
The tail rotor of a helicopter with a single main rotor configuration can experience a significant reduction in thrust when the aircraft operates in crosswind flight. Brownâs vorticity transport model has been used to simulate a main rotor and tail rotor system translating at a sideslip angle that causes the tail rotor to interact with the main rotor tip vortices as they propagate downstream at the lateral extremities of the wake. The tail rotor is shown to exhibit a distinct directionally dependent mode during which tail rotors that are configured so that the blades travel forward at the top of the disk develop less thrust than tail rotors with the reverse sense of rotation. The range of flight speeds over which this mode exists is shown to vary considerably with the vertical location of the tail rotor. At low flight speeds, the directionally dependent mode occurs because the tail rotor is immersed within not only the downwash from the main rotor but also the rotational flow associated with clusters of largely disorganized vorticity within the main rotor wake. At higher flight speeds, however, the tail rotor is immersed within a coherent supervortex that strongly influences the velocity field surrounding the tail rotor
Properties of high-z galaxies as seen through lensing clusters
We discuss the first results obtained on the study of a sample of high-z
galaxies (2 < z < 7), using the gravitational amplification effect in the core
of lensing clusters. Sources are located close to the critical lines in
clusters with well constrained mass distributions, and selected through
photometric redshifts, computed on a large wavelength domain, and lens
inversion techniques.Comment: 5 pages, 3 figures, Conference Proceedings of the "Clustering at High
Redshift" Conference, June 29 to July 2, 1999, Marseille (France
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