1,745 research outputs found
Sub-millimeter images of a dusty Kuiper belt around eta Corvi
We present sub-millimeter and mid-infrared images of the circumstellar disk
around the nearby F2V star eta Corvi. The disk is resolved at 850um with a size
of ~100AU. At 450um the emission is found to be extended at all position
angles, with significant elongation along a position angle of 130+-10deg; at
the highest resolution (9.3") this emission is resolved into two peaks which
are to within the uncertainties offset symmetrically from the star at 100AU
projected separation. Modeling the appearance of emission from a narrow ring in
the sub-mm images shows the observed structure cannot be caused by an edge-on
or face-on axisymmetric ring; the observations are consistent with a ring of
radius 150+-20AU seen at 45+-25deg inclination. More face-on orientations are
possible if the dust distribution includes two clumps similar to Vega; we show
how such a clumpy structure could arise from the migration over 25Myr of a
Neptune mass planet from 80-105AU. The inner 100AU of the system appears
relatively empty of sub-mm emitting dust, indicating that this region may have
been cleared by the formation of planets, but the disk emission spectrum shows
that IRAS detected an additional hot component with a characteristic
temperature of 370+-60K (implying a distance of 1-2AU). At 11.9um we found the
emission to be unresolved with no background sources which could be
contaminating the fluxes measured by IRAS. The age of this star is estimated to
be ~1Gyr. It is very unusual for such an old main sequence star to exhibit
significant mid-IR emission. The proximity of this source makes it a perfect
candidate for further study from optical to mm wavelengths to determine the
distribution of its dust.Comment: 22 pages, 4 figures. Scheduled for publication in ApJ 10 February
2005 issu
Effect of quantum nuclear motion on hydrogen bonding
This work considers how the properties of hydrogen bonded complexes,
D-H....A, are modified by the quantum motion of the shared proton. Using a
simple two-diabatic state model Hamiltonian, the analysis of the symmetric
case, where the donor (D) and acceptor (A) have the same proton affinity, is
carried out. For quantitative comparisons, a parametrization specific to the
O-H....O complexes is used. The vibrational energy levels of the
one-dimensional ground state adiabatic potential of the model are used to make
quantitative comparisons with a vast body of condensed phase data, spanning a
donor-acceptor separation (R) range of about 2.4-3.0 A, i.e., from strong to
weak bonds. The position of the proton and its longitudinal vibrational
frequency, along with the isotope effects in both are discussed. An analysis of
the secondary geometric isotope effects, using a simple extension of the
two-state model, yields an improved agreement of the predicted variation with R
of frequency isotope effects. The role of the bending modes in also considered:
their quantum effects compete with those of the stretching mode for certain
ranges of H-bond strengths. In spite of the economy in the parametrization of
the model used, it offers key insights into the defining features of H-bonds,
and semi-quantitatively captures several experimental trends.Comment: 12 pages, 8 figures. Notation clarified. Revised figure including the
effect of bending vibrations on secondary geometric isotope effect. Final
version, accepted for publication in Journal of Chemical Physic
An automated graphics tool for comparative genomics: the Coulson plot generator.
BACKGROUND: Comparative analysis is an essential component to biology. When applied to genomics for example, analysis may require comparisons between the predicted presence and absence of genes in a group of genomes under consideration. Frequently, genes can be grouped into small categories based on functional criteria, for example membership of a multimeric complex, participation in a metabolic or signaling pathway or shared sequence features and/or paralogy. These patterns of retention and loss are highly informative for the prediction of function, and hence possible biological context, and can provide great insights into the evolutionary history of cellular functions. However, representation of such information in a standard spreadsheet is a poor visual means from which to extract patterns within a dataset. RESULTS: We devised the Coulson Plot, a new graphical representation that exploits a matrix of pie charts to display comparative genomics data. Each pie is used to describe a complex or process from a separate taxon, and is divided into sectors corresponding to the number of proteins (subunits) in a complex/process. The predicted presence or absence of proteins in each complex are delineated by occupancy of a given sector; this format is visually highly accessible and makes pattern recognition rapid and reliable. A key to the identity of each subunit, plus hierarchical naming of taxa and coloring are included. A java-based application, the Coulson plot generator (CPG) automates graphic production, with a tab or comma-delineated text file as input and generating an editable portable document format or svg file. CONCLUSIONS: CPG software may be used to rapidly convert spreadsheet data to a graphical matrix pie chart format. The representation essentially retains all of the information from the spreadsheet but presents a graphically rich format making comparisons and identification of patterns significantly clearer. While the Coulson plot format is highly useful in comparative genomics, its original purpose, the software can be used to visualize any dataset where entity occupancy is compared between different classes. AVAILABILITY: CPG software is available at sourceforge http://sourceforge.net/projects/coulson and http://dl.dropbox.com/u/6701906/Web/Sites/Labsite/CPG.html.RIGHTS : This article is licensed under the BioMed Central licence at http://www.biomedcentral.com/about/license which is similar to the 'Creative Commons Attribution Licence'. In brief you may : copy, distribute, and display the work; make derivative works; or make commercial use of the work - under the following conditions: the original author must be given credit; for any reuse or distribution, it must be made clear to others what the license terms of this work are
Field testing methodology for investigating the effect of systemic insecticides on honey bees
contribution to session IVTest methodolog
Unsuccessful dispersal affects life history characteristics of natal populations: The role of dispersal related variation in vital rates
International audienceIndividuals that disperse from one habitat to another has consequences for individual fitness, population dynamics and gene flow. The fitness benefits accrued in the new habitat are traded off against costs associated with dispersal. Most studies focus on costs at settlement and effects on settlement populations; the influence of dispersal to natal populations is assessed by monitoring change in numbers due to emigration. However, the extent to which natal populations are affected when individuals that invest in dispersal fail to disperse/emigrate is unclear. Here, we use an Integral Projection Model (IPM) to assess how developing into a disperser affects natal population structure and growth. We do so using the bulb mite (Rhizoglyphus robini) as a study system. Bulb mites, in unfavourable environments, develop into a dispersal (deutonymph) stage during ontogeny; these individuals are called dispersers with individuals not developing into this stage called non-dispersers. We varied disperser expression and parameterised IPMs to describe three simulations of successful and unsuccessful dispersal: (i) ‘no dispersal’ - dispersal stage is excluded and demographic data are from non-disperser individuals; (ii) ‘false dispersal’ - dispersal stage included and demographic data from non-disperser individuals are used; (iii) ‘true dispersal’ - dispersal stage included and demographic data are from individuals that go through the dispersal stage and from non-disperser individuals. We found that the type of dispersal simulation (no dispersal < false dispersal < true dispersal) and disperser expression increases generation time and reduces lifetime reproductive success and population growth rate. Our findings show that disperser individuals that fail to leave, can change the structure and growth of natal populations
Measuring molecular abundances in comet C/2014 Q2 (Lovejoy) using the APEX telescope
Comet composition provides critical information on the chemical and physical
processes that took place during the formation of the Solar system. We report
here on millimetre spectroscopic observations of the long-period bright comet
C/2014 Q2 (Lovejoy) using the Atacama Pathfinder Experiment (APEX) band 1
receiver between 2015 January UT 16.948 to 18.120, when the comet was at
heliocentric distance of 1.30 AU and geocentric distance of 0.53 AU. Bright
comets allow for sensitive observations of gaseous volatiles that sublimate in
their coma. These observations allowed us to detect HCN, CH3OH (multiple
transitions), H2CO and CO, and to measure precise molecular production rates.
Additionally, sensitive upper limits were derived on the complex molecules
acetaldehyde (CH3CHO) and formamide (NH2CHO) based on the average of the
strongest lines in the targeted spectral range to improve the signal-to-noise
ratio. Gas production rates are derived using a non-LTE molecular excitation
calculation involving collisions with H2O and radiative pumping that becomes
important in the outer coma due to solar radiation. We find a depletion of CO
in C/2014 Q2 (Lovejoy) with a production rate relative to water of 2 per cent,
and relatively low abundances of Q(HCN)/Q(H2O), 0.1 per cent, and
Q(H2CO)/Q(H2O), 0.2 per cent. In contrast the CH3OH relative abundance
Q(CH3OH)/Q(H2O), 2.2 per cent, is close to the mean value observed in other
comets. The measured production rates are consistent with values derived for
this object from other facilities at similar wavelengths taking into account
the difference in the fields of view. Based on the observed mixing ratios of
organic molecules in four bright comets including C/2014 Q2, we find some
support for atom addition reactions on cold dust being the origin of some of
the molecules.Comment: 10 pages, 7 figures, to be published in MNRA
Phonons in graphene with point defects
The phonon density of states (DOS) of graphene with different types of point
defects (carbon isotopes, substitution atoms, vacancies) is considered. Using a
solvable model which is based on the harmonic approximation and the assumption
that the elastic forces act only between nearest neighboring ions we calculate
corrections to graphene DOS dependent on type and concentration of defects. In
particular the correction due to isotopic dimers is determined. It is shown
that a relatively small concentration of defects may lead to significant and
specific changes in the DOS, especially at low frequencies, near the Van Hove
points and in the vicinity of the K-points of the Brillouin zone. In some cases
defects generate one or several narrow gaps near the critical points of the
phonon DOS as well as resonance states in the Brillouin zone regular points.
All types of defects are characterized by the appearance of one or more
additional Van Hove peaks near the (Dirac) K points and their singular
contribution may be comparable with the effect of electron-phonon interaction.
Besides, for low frequencies and near the critical points the relative change
in density of states may be many times higher than the concentration of
defects.Comment: 19 pages, 7 figure
Spectropolarimetric observations of Herbig Ae/Be Stars I: HiVIS spectropolarimetric calibration and reduction techniques
Using the HiVIS spectropolarimeter built for the Haleakala 3.7m AEOS
telescope in Hawaii, we are collecting a large number of high precision
spectropolarimetrc observations of stars. In order to precisely measure very
small polarization changes, we have performed a number of polarization
calibration techniques on the AEOS telescope and HiVIS spectrograph. We have
extended our dedicated IDL reduction package and have performed some hardware
upgrades to the instrument. We have also used the ESPaDOnS spectropolarimeter
on CFHT to verify the HiVIS results with back-to-back observations of MWC 361
and HD163296. Comparision of this and other HiVIS data with stellar
observations from the ISIS and WW spectropolarimeters in the literature further
shows the usefulness of this instrument.Comment: 35 pages, 44 figures, Accepted by PAS
CO emission from discs around isolated HAeBe and Vega-excess stars
We describe results from a survey for J=3-2 12CO emission from visible stars
with an infrared excess. The line is clearly detected in 21 objects, with
molecular gas (>10^-3 Jupiter masses) common in targets with infrared excesses
>0.01 (>56% of objects). Such high excesses indicate the presence of a disc of
opening angle >12 degrees; within this, the optically thick disc prevents CO
photodissociation. Two or three stars with associated CO have an excess <0.01,
implying a disc opening angle <1 degree. Most line profiles are double-peaked
or relatively broad. Model fits, assuming a Keplerian disc, indicate outer
radii, R_out, of ~20-300 au. As many as 5 discs have outer radii smaller than
the Solar System (50 au), and a further 4 have gas at radii <20 au. R_out is
independent of the stellar spectral type (from K through to B9), but is
correlated with total dust mass. R_out appears to decrease with time: discs
around stars of age 3-7 Myr have a mean radius of ~210 au, whereas discs of age
7-20 Myr are a factor of 3 smaller. The only bona fide debris disc with
detected CO is HD9672; this has a double peaked line profile and is the most
compact gas disc observed, with a modelled radius 17 au). A fit to HD141569
suggests the gas lies in two rings of radii 90 and 250 au, similar to the
scattered light structure. In both AB Aur and HD163296 the sizes of the
molecular and dust scattering discs are also similar, suggesting that the gas
and small dust grains are co-located.Comment: 16 pages, 5 figures MNRAS - accepte
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