2,698 research outputs found
Trans-Neptunian Objects with Hubble Space Telescope ACS/WFC
We introduce a novel search technique that can identify trans-neptunian
objects in three to five exposures of a pointing within a single Hubble Space
Telescope orbit. The process is fast enough to allow the discovery of
candidates soon after the data are available. This allows sufficient time to
schedule follow up observations with HST within a month. We report the
discovery of 14 slow-moving objects found within 5\circ of the ecliptic in
archival data taken with the Wide Field Channel of the Advanced Camera for
Surveys. The luminosity function of these objects is consistent with previous
ground-based and space-based results. We show evidence that the size
distribution of both high and low inclination populations is similar for
objects smaller than 100 km, as expected from collisional evolution models,
while their size distribution differ for brighter objects. We suggest the two
populations formed in different parts of the protoplanetary disk and after
being dynamically mixed have collisionally evolved together. Among the objects
discovered there is an equal mass binary with an angular separation ~ 0."53.Comment: 16 page, 10 figures, accepted by Ap
The evolution and star formation of dwarf galaxies in the Fornax Cluster
We present the results of a spectroscopic survey of 675 bright (16.5<Bj<18)
galaxies in a 6 degree field centred on the Fornax cluster with the FLAIR-II
spectrograph on the UK Schmidt Telescope. We measured redshifts for 516
galaxies of which 108 were members of the Fornax Cluster. Nine of these are new
cluster members previously misidentified as background galaxies. The cluster
dynamics show that the dwarf galaxies are still falling into the cluster
whereas the giants are virialised. Our spectral data reveal a higher rate of
star formation among the dwarf galaxies than suggested by morphological
classification: 35 per cent have H-alpha emission indicative of star formation
but only 19 per cent were morphologically classified as late-types. The
distribution of scale sizes is consistent with evolutionary processes which
transform late-type dwarfs to early-type dwarfs. The fraction of dwarfs with
active star formation drops rapidly towards the cluster centre. The
star-forming dwarfs are concentrated in the outer regions of the cluster, the
most extreme in an infalling subcluster. We estimate gas depletion time scales
for 5 dwarfs with detected HI emission: these are long (of order 10 Gyr),
indicating that active gas removal must be involved if they are transformed
into gas-poor dwarfs as they fall further into the cluster. In agreement with
our previous results, we find no compact dwarf elliptical (M32-like) galaxies
in the Fornax Cluster.Comment: To appear in Monthly Notices of the Royal Astronomical Societ
A robust and parsimonious regional disaggregation method for deriving hourly rainfall intensities for the UK
International audienceA regional rainfall disaggregation method from daily to hourly intensities is presented for the entire UK, which was developed for use with regionalised hydrological and water quality models. The approach is based on the inter-dependence of the hourly rainfall intensities during a rainfall event. The analysis of 23 229 days with at least 15 mm of precipitation from 238 weather stations throughout the UK allowed regional parameters for climatically homogeneous regions of the UK to be derived for each season. The method reproduces well the main statistical characteristics of the data (mean, minimum and maximum intensity and standard deviation). The method is fully operational, computationally efficient and can be applied to any location throughout the UK
Development and application of a catchment scale pesticide fate and transport model for use in drinking water risk assessment
This paper describes the development and application of IMPT (Integrated Model for Pesticide Transport), a parameter-efficient tool for predicting diffuse-source pesticide concentrations in surface waters used for drinking water supply. The model was applied to a small UK headwater catchment with high frequency (8 h) pesticide monitoring data and to five larger catchments (479–1653 km2) with sampling approximately every 14 days. Model performance was good for predictions of both flow (Nash Sutcliffe Efficiency generally > 0.59 and PBIAS < 10%) and pesticide concentrations, although low sampling frequency in the larger catchments is likely to mask the true episodic nature of exposure. The computational efficiency of the model, along with the fact that most of its parameters can be derived from existing national soil property data mean that it can be used to rapidly predict pesticide exposure in multiple surface water resources to support operational and strategic risk assessments
Response of Drought Tolerant and Conventional Corn to Limited Irrigation
With declining water levels in the Ogallala aquifer, many wells cannot supply peak irrigation water needs for corn. Emerging drought-tolerant (DT) corn hybrids could help farmers maintain yield with limited capacity wells. A knowledge gap exists comparing transgenic DT and conventional corn hybrids in yield response to water level. The purpose of this study was to compare yield, yield components, water productivity, and irrigation water use efficiency response of DT corn with cspB (DKC 6267 DGVT- 2PRO) transgene trait and conventional corn hybrid (DKC 62-98 VT2PRO) with similar maturity to full and limited irrigation. Preliminary results from the 2014 growing season indicate the effect of irrigation level on corn yield was significant (P-value \u3c0.001). The effect of the cspB transgene trait in the DT hybrid did not affect yield (P-value=0.32), and there was no effect of the interaction between irrigation level and corn hybrid on yield (P-value=0.82). The effect of irrigation and hybrid on 100 kernel weight was significant, with P-value\u3c0.001 and P-value\u3c0.001 respectively. The 100 kernel weight is a measure of kernel size, and was higher for the conventional hybrid compared to the DT hybrid
The effect of wave-particle interactions on low energy cutoffs in solar flare electron spectra
Solar flare hard X-ray spectra from RHESSI are normally interpreted in terms
of purely collisional electron beam propagation, ignoring spatial evolution and
collective effects. In this paper we present self-consistent numerical
simulations of the spatial and temporal evolution of an electron beam subject
to collisional transport and beam-driven Langmuir wave turbulence. These
wave-particle interactions represent the background plasma's response to the
electron beam propagating from the corona to chromosphere and occur on a far
faster timescale than coulomb collisions. From these simulations we derive the
mean electron flux spectrum, comparable to such spectra recovered from high
resolution hard X-rays observations of solar flares with RHESSI. We find that a
negative spectral index (i.e. a spectrum that increases with energy), or local
minima when including the expected thermal spectral component at low energies,
occurs in the standard thick-target model, when coulomb collisions are only
considered. The inclusion of wave-particle interactions does not produce a
local minimum, maintaining a positive spectral index. These simulations are a
step towards a more complete treatment of electron transport in solar flares
and suggest that a flat spectrum (spectral index of 0 to 1) down to thermal
energies maybe a better approximation instead of a sharp cut-off in the
injected electron spectrum.Comment: 6 pages, 5 figures, accepted by ApJ
The Short Rotation Period of Nereid
We determine the period, p = 11.52 \pm 0.14 h, and a light curve peak-to-peak
amplitude, a = 0.029 \pm 0.003 magnitudes, of the Neptunian irregular satellite
Nereid. If the light curve variation is due to albedo variations across the
surface, rather than solely to the shape of Nereid variations, the rotation
period would be a factor of two shorter. In either case, such a rotation period
and light curve amplitude, together with Nereid's orbital period, p=360.14
days, imply that Nereid is almost certainly in a regular rotation state, rather
than the chaotic rotation state suggested by Schaefer and Schaefer (1988,2000)
and Dobrovolskis (1995).
Assuming that Nereid is perfectly spherical, the albedo variation is 3%
across the observed surface. Assuming a uniform geometric albedo, the observed
cross sectional area varies by 3%. We caution that the lightcurve found in this
paper only sets limits on the combination of albedo and physical irregularity
and that we cannot determine the orientation of Nereid's spin axis from our
data.Comment: Accepted by ApJ Letters, 11 pages (incl. 1 figure
- …