306 research outputs found
Episodic starbursts in dwarf spheroidal galaxies: a simple model
Dwarf galaxies in the Local Group appear to be stripped of their gas within
270 kpc of the host galaxy. Color-magnitude diagrams of these dwarfs, however,
show clear evidence of episodic star formation (\Delta{}t ~ a few Gyr) over
cosmic time. We present a simple model to account for this behaviour. Residual
gas within the weak gravity field of the dwarf experiences dramatic variations
in the gas cooling time around the eccentric orbit. This variation is due to
two main effects. The azimuthal compression along the orbit leads to an
increase in the gas cooling rate of ~([1+\epsilon]/[1-\epsilon])^2. The
Galaxy's ionizing field declines as 1/R^2 for R>R_disk although this reaches a
floor at R~150 kpc due to the extragalactic UV field ionizing intensity. We
predict that episodic SF is mostly characteristic of dwarfs on moderately
eccentric orbits (\epsilon>0.2) that do not come too close to the centre
(R>R_disk) and do not spend their entire orbit far away from the centre (R>200
kpc). Up to 40% of early infall dwarf spheroidals can be expected to have
already had at least one burst since the initial epoch of star formation, and
10% of these dwarf spheriodals experiencing a second burst. Such a model can
explain the timing of bursts in the Carina dwarf spheroidal and restrict the
orbit of the Fornax dwarf spheroidal. However, this model fails to explain why
some dwarfs, such as Ursa Minor, experience no burst post-infall.Comment: 8 pages, 8 figures. ApJ accepte
Pacific Southwest United States Holocene Droughts and Pluvials Inferred From Sediment ÎŽ18O (calcite) and Grain Size Data (Lake Elsinore, California)
Records of past climate can inform us on the natural range and mechanisms of climate change. In the arid Pacific southwestern United States (PSW), which includes southern California, there exist a variety of Holocene records that can be used to infer past winter conditions (moisture and/or temperature). Holocene records of summer climate, however, are rare from the PSW. In the future, climate changes due to anthropogenic forcing are expected to increase the severity of drought in the already water stressed PSW. Hot droughts are of considerable concern as summer temperatures rise. As a result, understanding how summer conditions changed in the past is critical to understanding future predictions under varied climate forcings. Here, we present a c. 10.9 kcal BP ÎŽ18O(calcite) record from Lake Elsinore, California, interpreted to reflect ÎŽ18O(lake water) values as controlled by over-water evaporation from summer-to-early fall. Our results reveal three millennial scale intervals: (1) the highly evaporative Early Holocene (10.55â6.65 kcal BP), (2) the less evaporative Mid-Holocene (6.65â2.65 kcal BP); and (3) the evaporative Late Holocene (2.65â0.55 kcal BP). These results are coupled with an inferred winter precipitation runoff (sand content) record from Kirby et al. (2010). Using these data together, we estimate the duration and severity of centennial-scale Holocene droughts and pluvials (e.g., high ÎŽ18O(calcite) values plus low sand content = drought and vice versa). Furthermore, the coupled ÎŽ18O(calcite) and sand data provide a generalized Holocene lake level history. The most severe, long-lasting droughts (i.e., maximum summer-to-early fall evaporation and minimum winter precipitation runoff) occur in the Early Holocene. Fewer, less severe, and shorter duration droughts occurred during the Mid-Holocene as pluvials became more common. Droughts return with less severity and duration in the Late Holocene. Notably, the Little Ice Age is characterized as the wettest period during the Late Holocene
The Smith Cloud: high-velocity accretion and dark-matter confinement
The Smith Cloud is a massive system of metal-poor neutral and ionized gas
M_gas >= 2x10^6 M_sun) that is presently moving at high velocity (V_GSR ~300 km
s^-1) with respect to the Galaxy at a distance of 12 kpc from the Sun. The
kinematics of the cloud's cometary tail indicates that the gas is in the
process of accretion onto the Galaxy, as first discussed by Lockman et al.
(2008). Here, we re-investigate the cloud's orbit by considering the
possibility that the cloud is confined by a dark matter halo. This is required
for the cloud to survive its passage through the Galactic corona. We consider
three possible models for the dark matter halo (NFW, Einasto, Burkert)
including the effects of tidal disruption and ram-pressure stripping during the
cloud's infall onto and passage through the Galactic disk. For the NFW and
Einasto dark-matter models, we are able to determine reasonable initial
conditions for the Smith Cloud, although this is only marginally possible with
the Burkert model. For all three models, the progenitor had an initial
(gas+dark matter) mass that was an order of magnitude higher than inferred
today. In agreement with Lockman et al. (2008), the cloud appears to have
punched through the disk ~70 Myr ago. For our most successful models, the
baryon to dark matter ratio is fairly constant during an orbital period but
drops by a factor of 2-5 after transiting the disk. The cloud appears to have
only marginally survived its transit, and is unlikely to retain its integrity
during the next transit ~30 Myr from now.Comment: 9 pages, 7 figures. ApJ, accepte
The Star Formation & Chemical Evolution History of the Fornax Dwarf Spheroidal Galaxy
We present deep photometry in the B,V and I filters from CTIO/MOSAIC for
about 270.000 stars in the Fornax dwarf Spheroidal galaxy, out to a radius of
r_ell\sim0.8 degrees. By combining the accurately calibrated photometry with
the spectroscopic metallicity distributions of individual Red Giant Branch
stars we obtain the detailed star formation and chemical evolution history of
Fornax. Fornax is dominated by intermediate age (1-10 Gyr) stellar populations,
but also includes ancient (10-14 Gyr), and young (<1 Gyr) stars. We show that
Fornax displays a radial age gradient, with younger, more metal-rich
populations dominating the central region. This confirms results from previous
works. Within an elliptical radius of 0.8 degrees, or 1.9 kpc from the centre,
a total mass in stars of 4.3x10^7 Msun was formed, from the earliest times
until 250 Myr ago. Using the detailed star formation history, age estimates are
determined for individual stars on the upper RGB, for which spectroscopic
abundances are available, giving an age-metallicity relation of the Fornax dSph
from individual stars. This shows that the average metallicity of Fornax went
up rapidly from [Fe/H]<-2.5 dex to [Fe/H]=-1.5 dex between 8-12 Gyr ago, after
which a more gradual enrichment resulted in a narrow, well-defined sequence
which reaches [Fe/H]\sim-0.8 dex, \sim3 Gyr ago. These ages also allow us to
measure the build-up of chemical elements as a function of time, and thus
determine detailed timescales for the evolution of individual chemical
elements. A rapid decrease in [Mg/Fe] is seen for the stars with [Fe/H]>-1.5
dex, with a clear trend in age.Comment: 18 pages, 20 figure
Timescales for the growth of sediment diapirs in subduction zones
Author Posting. © The Author(s), 2012. This article is posted here by permission of John Wiley & Sons for personal use, not for redistribution. The definitive version was published in Geophysical Journal International 190 (2012): 1361â1377, doi:10.1111/j.1365-246X.2012.05565.x.In this study, we calculate timescales for the growth of gravitational instabilities forming in the sediment layer on the downgoing slab at subduction zones. Subducted metasediments are buoyant with respect to the overlying mantle and may form diapirs that detach from the slab and rise upwards into the mantle wedge. We use a particle-in-cell, finite-difference method to calculate growth rates for instabilities forming within a buoyant, wet-quartz metasediment layer underlying a dense mantle half-space composed of wet olivine. These growth rates are used to determine where sediment diapirs initiate and detach from the slab over a range of subduction zone thermal structures. We find that, given a sufficient layer thickness (200â800 m, depending on slab-surface and mantle-wedge temperatures), sediment diapirs begin to grow rapidly at depths of âŒ80 km and detach from the slab within 1â3 Myr at temperatures â€900 °C and at depths roughly corresponding to the location of the slab beneath the arc. Diapir growth is most sensitive to absolute slab temperature, however it is also affected by the viscosity ratio between the sediment layer and the mantle wedge and the length-scale over which viscosity decays above the slab. These secondary affects are most pronounced in colder subduction systems with old slabs and faster subduction rates. For a broad range of subduction zone thermal conditions, we find that diapirs can efficiently transport sediments into the mantle wedge, where they would melt and be incorporated into arc magmas. Thus, we conclude that sediment diapirism is a common feature of many subduction zones, providing a potential explanation for the âsediment signatureâ in the chemistry of arc magmas.This work was supported by NSF Grant EAR-0652707
and a WHOI Deep Ocean Exploration Institute Fellowship to MB
Kinematics and chemistry of recently discovered Reticulum 2 and Horologium 1 dwarf galaxies
Photometry alone is not sufficient to unambiguously distinguish between
ultra-faint star clusters and dwarf galaxies because of their overlap in
morphological properties. Here we report on VLT/GIRAFFE spectra of candidate
member stars in two recently discovered ultra-faint satellites Reticulum 2 and
Horologium 1, obtained as part of the ongoing Gaia-ESO Survey. We identify 18
members in Reticulum 2 and 5 in Horologium 1. We find Reticulum 2 to have a
velocity dispersion of ~3.22 km/s, implying a M/L ratio of ~ 500. We have
inferred stellar parameters for all candidates and we find Reticulum 2 to have
a mean metallicity of [Fe/H] = -2.46+/-0.1, with an intrinsic dispersion of ~
0.29, and is alpha-enhanced to the level of [alpha/Fe]~0.4. We conclude that
Reticulum 2 is a dwarf galaxy. We also report on the serendipitous discovery of
four stars in a previously unknown stellar substructure near Reticulum 2 with
[Fe/H] ~ -2 and V_hel ~ 220 km/s, far from the systemic velocity of Reticulum
2. For Horologium 1 we infer a velocity dispersion of 4.9^{+2.8}_{-0.9} km/s
and a consequent M/L ratio of ~ 600, leading us to conclude that Horologium 1
is also a dwarf galaxy. Horologium 1 is slightly more metal-poor than Reticulum
2 [Fe/H] = -2.76 +/- 0.1 and is similarly alpha-enhanced: [alpha/Fe] ~ 0.3.
Despite a large error-bar, we also measure a significant spread of
metallicities of 0.17 dex which strengthen the evidence that Horologium 1 is
indeed a dwarf galaxy. The line-of-sight velocity of Reticulum 2 is offset by
some 100 km/s from the prediction of the orbital velocity of the LMC, thus
making its association with the Cloud uncertain. However, at the location of
Horologium 1, both the backward integrated LMC's orbit and the LMC's halo are
predicted to have radial velocities similar to that of the dwarf. Therefore, it
is likely that Horologium 1 is or once was a member of the Magellanic Family.Comment: submitted to ApJ, abstract abridge
Microfluidic OrganâonâaâChip Technology for Advancement of Drug Development and Toxicology
In recent years, the exploitation of phenomena surrounding microfluidics has seen an increase in popularity, as researchers have found a way to use their unique properties to create superior design alternatives. One such application is representing the properties and functions of different organs on a microscale chip for the purpose of drug testing or tissue engineering. With the introduction of âorganâonâaâchipâ systems, researchers have proposed various methods on various organâonâaâchip systems to mimic their in vivo counterparts. In this article, a systematic approach is taken to review current technologies pertaining to organâonâaâchip systems. Design processes with attention to the particular instruments, cells, and materials used are presented
Agribusiness Sheep Updates - 2004 part 2
Precision Pastures Using Species Diversity to Improve Pasture Performance Anyou Liu and Clinton Revell, Department of Agriculture, Western Australia New Annual Pasture Legumes for Sheep Graziers Phil Nichols, Angelo Loi, Brad Nutt and Darryl McClements Department of Agriculture Western Australia Pastures from Space â Can Satellite Estimates of Pasture Growth Rate be used to Increase Farm Profit? Lucy Anderton, Stephen Gherardi and Chris Oldham Department of Agriculture Western Australia Summer-active Perennial Grasses for Profitable Sheep Production Paul Sanford and John Gladman, Department of Agriculture, Western Australia Pastures From Space â Validation Of Predictions Of Pasture Growth Rates DONALD, G.E.A, EDIRISINGHE, A.A, HENRY, D.A.A, MATA, G.A, GHERARDI, S.G.B, OLDHAM, C.M.B, GITTINS, S.P.B AND SMITH, R. C. G.C ACSIRO, Livestock Industries, PMB 5, Wembley, WA, 6913. BDepartment of Agriculture Western Australia, Bentley, WA, 6983. C Department of Land Information Western Australia, Floreat, WA, 6214. Production and Management of Biserrula Pasture - Managing the Risk of Photosensitivity Dr Clinton Revell and Roy Butler, Department of Agriculture Western Australia Meat Quality of Sheep Grazed on a Saltbush-based Pasture Kelly Pearce1,2, David Masters1, David Pethick2, 1 CSIRO LIVESTOCK INDUSTRIES, WEMBLEY, WA 2 SCHOOL OF VETERINARY AND BIOMEDICAL SCIENCE, MURDOCH UNIVERSITY, MURDOCH, WA Precision Sheep Lifetime Wool â Carryover Effects on Subsequent Reproduction of the Ewe Flock Chris Oldham, Department of Agriculture Western Australia Andrew Thompson, Primary Industries Research Victoria (PIRVic), Dept of Primary Industries, Hamilton, Vic Ewe Productivity Trials - a Linked Analysis Ken Hart, Johan Greeff, Department of Agriculture Western Australia, Beth Paganoni, School of Animal Biology, Faculty of Natural and Agricultural Sciences, University of Western Australia. Grain Finishing Systems For Prime Lambs Rachel Kirby, Matt Ryan, Kira Buttler, Department of Agriculture, Western Australia The Effects of Nutrition and Genotype on the Growth and Development, Muscle Biochemistry and Consumer Response to Lamb Meat David Pethick, Department of Veterinary Science, Murdoch University, WA, Roger Heggarty and David Hopkins, New South Wales Agriculture âLifetime Woolâ - Effects of Nutrition During Pregnancy and Lactation on Mortality of Progeny to Hogget Shearing Samantha Giles, Beth Paganoni and Tom Plaisted, Department of Agriculture Western Australia, Mark Ferguson and Darren Gordon, Primary Industries Research Victoria (PIRVic), Dept of Primary Industries, Hamilton, Vic Lifetime Wool - Target Liveweights for the Ewe Flock J. Young, Farming Systems Analysis Service, Kojonup, C. Oldham, Department of Agriculture Western Australia, A. Thompson, Primary Industries Research Victoria (PIRVic), Hamilton, VIC Lifetime Wool - Effects of Nutrition During Pregnancy and Lactation on the Growth and Wool Production of their Progeny at Hogget Shearing B. Paganoni, University of Western Australia, Nedlands WA, C. Oldham, Department of Agriculture Western Australia, M. Ferguson, A. Thompson, Primary Industries Research Victoria (PIRVic), Hamilton, VIC RFID Technology â Esperance Experiences Sandra Brown, Department of Agriculture Western Australia The Role of Radio Frequency Identification (RFID) Technology in Prime Lamb Production - a Case Study. Ian McFarland, Department of Agriculture, Western Australia. John Archer, Producer, Narrogin, Western Australia Win with Twins from Merinos John Milton, Rob Davidson, Graeme Martin and David Lindsay The University of Western Australia Precision Sheep Need Precision Wool Harvesters Jonathan England, Castle Carrock Merinos, Kingston SE, South Australia Business EBVs and Indexes â Genetic Tools for your Toolbox Sandra Brown, Department of Agriculture Western Australia Green Feed Budget Paddock Calculator Mandy Curnow, Department of Agriculture Western Australia Minimising the Impact of Drought - Evaluating Flock Recovery Options using the ImPack Model Karina P. Wood, Ashley K. White, B. Lloyd Davies, Paul M. Carberry, NSW Department of Primary Industries (NSW DPI), Lifetime Wool - Modifying GrazFeedÂź for WA Mike Hyder, Department of Agriculture Western Australia , Mike Freer, CSIRO Plant Industry, Canberra, A.C.T. , Andrew van Burgel, and Kazue Tanaka, Department of Agriculture Western Australia Profile Calculator â A Way to Manage Fibre Diameter Throughout the Year to Maximise Returns Andrew Peterson, Department of Agriculture, Western Australia Pasture Watch - a Farmer Friendly Tool for Downloading and Analysing Pastures from Space Data Roger Wiese,Fairport Technologies International, South Perth, WA, Stephen Gherardi, BDepartment of Agriculture Western Australia, Gonzalo Mata, CCSIRO, Livestock Industries, Wembley, Western Australia, and Chris Oldham, Department of Agriculture Western Australia Sy Sheep Cropping Systems An Analysis of a Cropping System Containing Sheep in a Low Rainfall Livestock System. Evan Burt, Amanda Miller, Anne Bennett, Department of Agriculture, Western Australia Lucerne-based Pasture for the Central Wheatbelt â is it Good Economics? Felicity FluggeA, Amir AbadiA,B and Perry DollingA,B,A CRC for Plant-based Management of Dryland Salinity: BDept. of Agriculture, WA Sheep and Biserrula can Control Annual Ryegrass Dean Thomas, John Milton, Mike Ewing and David Lindsay, The University of WA, Clinton Revell, Department of Agriculture, Western Australia Sustainable Management Pasture Utilisation, Fleece Weight and Weaning Rate are Integral to the Profitability of Dohnes and SAMMs. Emma Kopke,Department of Agriculture Western Australia, John Young, Farming Systems Analysis Service Environmental Impact of Sheep Confinement Feeding Systems E A Dowling and E K Crossley, Department of Agriculture, Western Australia Smart Grazing Management for Production and Environmental Outcomes Dr Brien E (Ben) Norton, Centre for the Management of Arid Environments, Curtin University of Technology, WA Common Causes of Plant Poisoning in the Eastern Wheatbelt of Western Australia. Roy Butler, Department of Agriculture, Western Australia Selecting Sheep for Resistance to Worms and Production Trait Responses John Karlsson, Johan Greeff, Department of Agriculture, Western Australia, Geoff Pollott, Imperial College, London UK Production and Water Use of Lucerne and French Serradella in Four Soil Types, Diana Fedorenko1,4, Darryl McClements2,4 and Robert Beard3,4, 12Department of Agriculture, Western Australia; 3Farmer, Meckering; 4CRC for Plant-based Management of Dryland Salinity. Worm Burdens in Sheep at Slaughter Brown Besier, Department of Agriculture Western Australia, Una Ryan, Caroline Bath, Murdoch Universit
A call to bridge the diagnostic gap: diagnostic solutions for neonatal sepsis in low- and middle-income countries.
Low- and middle-income countries (LMICs) bear the greatest burden of neonatal mortality, with sepsis being a major contributor. Non-specificity of signs, and the absence of a definitive diagnostic present a challenge to the identification of sepsis and can lead to underdiagnosis or overdiagnosis, both of which can have harmful consequences. As early intervention can be life-saving, sepsis protocols, which commonly include empiric therapies, result in the overuse of antibiotics and the development of antimicrobial resistance. Affordable and accurate diagnostic tests that can detect neonatal sepsis at or near the point of care could contribute to reduced sepsis-related mortality in LMICs and support antimicrobial stewardship. A screening test to guide referral to hospital from primary care and an in-hospital test to guide treatment decisions, are high priorities. Considerable investment will be needed to support the development of these diagnostics
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