The Journey to R4D: An institutional history of an Australian Initiative on Food Security in Africa

Internal Revie

Accurate Hydrogen Spectral Simulations with a Compact Model Atom

Many large scale numerical simulations of astrophysical plasmas must also reproduce the hydrogen ionization and the resulting emission spectrum, in some cases quite accurately. We describe a compact model hydrogen atom that can be readily incorporated into such simulations. It reproduces the recombination efficiency and line spectrum predicted by much larger calculations for a very broad range of densities and temperatures. Uncertainties in hydrogen collision data are the largest source of differences between our compact atom and predictions of more extensive calculations, and underscore the need for accurate atomic data.Comment: 18 pages, prepared in MS-Word, Postscript only, 12 Figures, also available at http://www.pa.uky.edu/~ferguson/bib/bib.html, accepted for publication in the Astrophysical Journa

Selection of Forages for the Tropics (SoFT) - A Database and Selection Tool for Identifying Forages Adapted to Local Conditions in the Tropics and Subtropics

Rising populations and incomes in developing countries are likely to double demand for livestock products by 2020 (Delgado et al. 1999). This strong demand has potential to improve profitability for farmers but will require improved animal feeding in both semi-intensive crop-livestock and more extensive livestock systems. Forages usually are the most cost-effective option to supply feed demands, particularly for ruminant-, but also for pig- and poultry- production. It is critical to select the most suitable forages for the local system and conditions. Small- and even larger-scale farmers depend heavily on advice from extension and development agencies, and from seed companies, but this advice often is limited by inexperience and the difficulty in accessing reliable information. Expert information on an extensive range of tropical forages is now readily available through the SoFT database

Radiation from early black holes - I: effects on the neutral inter-galactic medium

In the pre-reionization Universe, the regions of the inter-galactic medium (IGM) which are far from luminous sources are the last to undergo reionization. Until then, they should be scarcely affected by stellar radiation; instead, the X-ray emission from an early black hole (BH) population can have much larger influence. We investigate the effects of such emission, looking at a number of BH model populations (differing for the cosmological density evolution of BHs, the BH properties, and the spectral energy distribution of the BH emission). We find that BH radiation can easily heat the IGM to 10^3-10^4 K, while achieving partial ionization. The most interesting consequence of this heating is that BHs are expected to induce a 21-cm signal (delta T_b ~ 20-30 mK at z<~12) which should be observable with forthcoming experiments (e.g. LOFAR). We also find that at z<~10 BH emission strongly increases the critical mass separating star-forming and non-star-forming halos.Comment: 17 pages, 14 figures; accepted for publication on MNRA

Improving Predictions for Helium Emission Lines

We have combined the detailed He I recombination model of Smits with the collisional transitions of Sawey & Berrington in order to produce new accurate helium emissivities that include the effects of collisional excitation from both the 2 (3)S and 2 (1) S levels. We present a grid of emissivities for a range of temperature and densities along with analytical fits and error estimates. Fits accurate to within 1% are given for the emissivities of the brightest lines over a restricted range for estimates of primordial helium abundance. We characterize the analysis uncertainties associated with uncertainties in temperature, density, fitting functions, and input atomic data. We estimate that atomic data uncertainties alone may limit abundance estimates to an accuracy of 1.5%; systematic errors may be greater than this. This analysis uncertainty must be incorporated when attempting to make high accuracy estimates of the helium abundance. For example, in recent determinations of the primordial helium abundance, uncertainties in the input atomic data have been neglected.Comment: ApJ, accepte

The effect of dust geometry on the Lyman-alpha output of galaxies

We present the optical spectroscopic follow-up of 31 z=0.3 Lyman-alpha (Lya) emitters, previously identified by Deharveng et al. (2008). We find that 17% of the Lya emitters have line ratios that require the hard ionizing continuum produced by an AGN. The uniform dust screen geometry traditionally used in studies similar to ours is not able to simultaneously reproduce the observed high Lya/Halpha and Halpha/Hbeta line ratios. We consider different possibilities for the geometry of the dust around the emitting sources. We find that also a uniform mixture of sources and dust does not reproduce the observed line ratios. Instead, these are well reproduced by a clumpy dust screen. This more realistic treatment of the geometry results in extinction corrected (Lya/Halpha)_C values consistent with Case B recombination theory, whereas a uniform dust screen model would imply values (Lya/Halpha)_C higher than 8.7. Our analysis shows that there is no need to invoke "ad-hoc" multi phase media in which the Lya photons only scatter between the dusty clouds and eventually escape.Comment: Accepted for publication in ApJL, 5 pages emulateapj forma

Commercial chicken breeds exhibit highly divergent patterns of linkage disequilibrium

The analysis of linkage disequilibrium (LD) underpins the development of effective genotyping technologies, trait mapping and understanding of biological mechanisms such as those driving recombination and the impact of selection. We apply the Malécot-Morton model of LD to create additive LD maps that describe the high-resolution LD landscape of commercial chickens. We investigated LD in chickens (Gallus gallus) at the highest resolution to date for broiler, white egg and brown egg layer commercial lines. There is minimal concordance between breeds of fine-scale LD patterns (correlation coefficient &lt;0.21), and even between discrete broiler lines. Regions of LD breakdown, which may align with recombination hot spots, are enriched near CpG islands and transcription start sites (P&lt;2.2 × 10?16), consistent with recent evidence described in finches, but concordance in hot spot locations between commercial breeds is only marginally greater than random. As in other birds, functional elements in the chicken genome are associated with recombination but, unlike evidence from other bird species, the LD landscape is not stable in the populations studied. The development of optimal genotyping panels for genome-led selection programmes will depend on careful analysis of the LD structure of each line of interest. Further study is required to fully elucidate the mechanisms underlying highly divergent LD patterns found in commercial chickens

Late Spectral Evolution of SN 1987A: I. Temperature and Ionization

The temperature and ionization of SN 1987A is modeled between 200 and 2000 days in its nebular phase, using a time-dependent model. We include all important elements, as well as the primary composition zones in the supernova. The energy input is provided by radioactive decay of Co-56, Co-57, and Ti-44. The thermalization of the resulting gamma-rays and positrons is calculated by solving the Spencer-Fano equation. Both the ionization and the individual level populations are calculated time-dependently. Adiabatic cooling is included in the energy equation. Charge transfer is important for determining the ionization and is included with available and estimated rates. Full, multilevel atoms are used for the observationally important ions. As input models to the calculations we use explosion models for SN 1987A calculated by Woosley et al and Nomoto et al. The most important result in this paper refers to the evolution of the temperature and ionization of the various abundance zones. The metal-rich core undergoes a thermal instability, often referred to as the IR-catastrophe, at 600 - 1000 days. The hydrogen-rich zones evolve adiabatically after 500 - 800 days, while in the helium region both adiabatic cooling and line cooling are of equal importance after ~1000 days. Freeze-out of the recombination is important in the hydrogen and helium zones. Concomitant with the IR-catastrophe, the bulk of the emission shifts from optical and near-IR lines to the mid- and far-IR. After the IR-catastrophe, the cooling is mainly due to far-IR lines and adiabatic expansion. Dust cooling is likely to be important in the zones where dust forms. We find that the dust condensation temperatures occur later than ~500 days in the oxygen-rich zones, and the most favorable zone for dust condensation is the iron core.Comment: 53 pages, including 10 figures; ApJ (Main Journal); scheduled for April 1, 1998, Vol. 49