1,030 research outputs found
Effect of branch position and light availability on shoot growth of understory sugar maple and yellow birch saplings
Phenotypic plasticity enables tree saplings to change their morphology according to their environment to grow toward a better light micro-habitat. Therefore, processes of crown development could be expected to vary as a function of light. The objectives of this study were to (i) evaluate the effects of position and light availability on shoot growth within the crowns of understory saplings of sugar maple (Acer saccharum Marsh.) and yellow birch (Betula alleghaniensis Britton.); (ii) develop a new vigour index for shoots; and (iii) evaluate the possible factors relating to branch mortality in the crown of sugar maple saplings. The results revealed that there is a clear branch position effect on shoot growth in the crown for yellow birch saplings and that it is partly related to the presence of two types of shoots. Dead branches were located at the bottom of the crown of sugar maple saplings; they were smaller in size, had wider angles and had lower indexes of vigour than live branches found nearby. Preliminary results obtained on the vigour index indicate that it is a potentially useful tool for predicting the growth and vigour status of a branch
Theoretical study of dislocation nucleation from simple surface defects in semiconductors
Large-scale atomistic calculations, using empirical potentials for modeling
semiconductors, have been performed on a stressed system with linear surface
defects like steps. Although the elastic limits of systems with surface defects
remain close to the theoretical strength, the results show that these defects
weaken the atomic structure, initializing plastic deformations, in particular
dislocations. The character of the dislocation nucleated can be predicted
considering both the resolved shear stress related to the applied stress
orientation and the Peierls stress. At low temperature, only glide events in
the shuffle set planes are observed. Then they progressively disappear and are
replaced by amorphization/melting zones at a temperature higher than 900 K
Optical variability of the accretion disk around the intermediate mass black hole ESO 243-49 HLX-1 during the 2012 outburst
We present dedicated quasi-simultaneous X-ray (Swift) and optical (Very Large
Telescope (VLT), V- and R-band) observations of the intermediate mass black
hole candidate ESO 243-49 HLX-1 before and during the 2012 outburst. We show
that the V-band magnitudes vary with time, thus proving that a portion of the
observed emission originates in the accretion disk. Using the first quiescent
optical observations of HLX-1, we show that the stellar population surrounding
HLX-1 is fainter than V~25.1 and R~24.2. We show that the optical emission may
increase before the X-ray emission consistent with the scenario proposed by
Lasota et al. (2011) in which the regular outbursts could be related to the
passage at periastron of a star circling the intermediate mass black hole in an
eccentric orbit, which triggers mass transfer into a quasi-permanent accretion
disk around the black hole. Further, if there is indeed a delay in the X-ray
emission we estimate the mass-transfer delivery radius to be ~1e11 cm.Comment: 9 pages, 2 figures, accepted for publication in ApJ
Modelling the spectral response of the Swift-XRT CCD camera: Experience learnt from in-flight calibration
(Abbreviated) We show that the XRT spectral response calibration was
complicated by various energy offsets in photon counting (PC) and windowed
timing (WT) modes related to the way the CCD is operated in orbit (variation in
temperature during observations, contamination by optical light from the sunlit
Earth and increase in charge transfer inefficiency). We describe how these
effects can be corrected for in the ground processing software. We show that
the low-energy response, the redistribution in spectra of absorbed sources, and
the modelling of the line profile have been significantly improved since launch
by introducing empirical corrections in our code when it was not possible to
use a physical description. We note that the increase in CTI became noticeable
in June 2006 (i.e. 14 months after launch), but the evidence of a more serious
degradation in spectroscopic performance (line broadening and change in the
low-energy response) due to large charge traps (i.e. faults in the Si crystal)
became more significant after March 2007. We describe efforts to handle such
changes in the spectral response. Finally, we show that the commanded increase
in the substrate voltage from 0 to 6V on 2007 August 30 reduced the dark
current, enabling the collection of useful science data at higher CCD
temperature (up to -50C). We also briefly describe the plan to recalibrate the
XRT response files at this new voltage.Comment: 27 pages, 29 figures (many in colour), accepted for publication in
A&
Comparison between classical potentials and ab initio for silicon under large shear
The homogeneous shear of the {111} planes along the direction of bulk
silicon has been investigated using ab initio techniques, to better understand
the strain properties of both shuffle and glide set planes. Similar
calculations have been done with three empirical potentials, Stillinger-Weber,
Tersoff and EDIP, in order to find the one giving the best results under large
shear strains. The generalized stacking fault energies have also been
calculated with these potentials to complement this study. It turns out that
the Stillinger-Weber potential better reproduces the ab initio results, for the
smoothness and the amplitude of the energy variation as well as the
localization of shear in the shuffle set
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