Biosecurity: A 21st Century Challenge

Based on a review of key reports and experts' opinions, summarizes the debate over "dual-use" technologies and the various approaches to controlling biosecurity risk. Outlines proposed preventive measures and steps to build response capacity

Externally-polluted white dwarfs with dust disks

We report Spitzer Space Telescope photometry of eleven externally-polluted white dwarfs. Of the nine stars for which we have IRAC photometry, we find that GD 40, GD 133 and PG 1015+161 each has an infrared excess that can be understood as arising from a flat, opaque, dusty disk. GD 56 also has an infrared excess characteristic of circumstellar dust, but a flat-disk model cannot reproduce the data unless there are grains as warm as 1700 K and perhaps not even then. Our data support the previous suggestion that the metals in the atmosphere of GD 40 are the result of accretion of a tidally-disrupted asteroid with a chondritic composition.Comment: ApJ, in pres

The Meeting of Two Cultures: Public Broadcasting on the Threshold of the Digital Age

Provides a summary of discussions held in November 2007 on "Public Broadcasting: The Digital Challenge" among representatives of foundations, public broadcasting corporations and academia. Includes essays on visions for the future of public media

Covariant Symplectic Structure and Conserved Charges of New Massive Gravity

We show that the symplectic current obtained from the boundary term, which arises in the first variation of a local diffeomorphism invariant action, is covariantly conserved for any gravity theory described by that action. Therefore, a Poincare invariant 2-form can be constructed on the phase space, which is shown to be closed without reference to a specific theory. Finally, we show that one can obtain a charge expression for gravity theories in various dimensions, which plays the role of the Abbott-Deser-Tekin (ADT) charge for spacetimes with non-constant curvature backgrounds, by using the diffeomorphism invariance of the symplectic 2-form. As an example, we calculate the conserved charges of some solutions of New Massive Gravity (NMG) and compare the results with the previous works.Comment: 18 pages, No figures, RevTEX4.1; ver 2: minor corrections, version accepted for publication in Physical Review

Sirius B Imaged in the Mid-Infrared: No Evidence for a Remnant Planetary System

Evidence is building that remnants of solar systems might orbit a large percentage of white dwarfs, as the polluted atmospheres of DAZ and DBZ white dwarfs indicate the very recent accretion of metal-rich material. (Zuckerman et al. 2010). Some of these polluted white dwarfs are found to have large mid-infrared excesses from close-in debris disks that are thought to be reservoirs for the metal accretion. These systems are coined DAZd white dwarfs (von Hippel et al. 2007). Here we investigate the claims of Bonnet-Bidaud & Pantin (2008) that Sirius B, the nearest white dwarf to the Sun, might have an infrared excess from a dusty debris disk. Sirius B's companion, Sirius A is commonly observed as a mid-infrared photometric standard in the Southern hemisphere. We combine several years of Gemini/T-ReCS photometric standard observations to produce deep mid-infrared imaging in five ~10 micron filters (broad N + 4 narrowband), which reveal the presence of Sirius B. Our photometry is consistent with the expected photospheric emission such that we constrain any mid-infrared excess to <10% of the photosphere. Thus we conclude that Sirius B does not have a large dusty disk, as seen in DAZd white dwarfs.Comment: 13 pages, 3 figures, accepted to Ap

Rickets in very-low-birth-weight infants born at Baragwanath Hospital

Disturbed mineral and bone metabolism has been reported to occur frequently in very-low-birthweight infants fed breast-milk during the first 3 months of life. This study was designed to assess the prevalence of disturbed mineral homeostasis in a breast-milk-fed very-low-birth-weight population at Baragwanath Hospital and to determine whether the addition of a preterm infant formula to the feeds would reduce this prevalence and increase the rate of weight gain. Fifty-three neonates weighing less than 1 200 g at birth were monitored for weight gain, growth and biochemical and radiological evidence of metabolic bone disease at 2-weekly intervals during hospitalisation and for 18 weeks after discharge. The infants were randomised at 2 weeks of age to receive either breast-milk only, or a combination of breast-milk and a premature formula containing 550 mg calcium and 300 mg phosphorus. All infants received 800 IU vitamin D daily from day 14. Weight gain and growth were similar in both groups. Calcium and phosphorus intakes were higher in the mixed feeding group, but did not affect serum mineral levels. Radiological rickets was uncommon in both groups although periosteal reactions and osteopenia occurred frequently, and with similar prevalences. Vitamin D deficiency was not found to be a problem.In conclusion, overt rickets is not a major problem in very-low-birth-weight infants born at Baragwanath Hospital, although biochemical abnormalities occur frequently. Feeding with breast-milk and a premature infant formula in equal proportions (as opposed to breast-milk only) does not appear to have any effect on weight gain and growth in very-low-birth-weight infants but does partially prevent the pathological rise in alkaline phosphatase levels. It is therefore recommended that breast-feeding of very-low-birthweight infants be encouraged, provided they are monitored regularly

Simple estimation of absolute free energies for biomolecules

One reason that free energy difference calculations are notoriously difficult in molecular systems is due to insufficient conformational overlap, or similarity, between the two states or systems of interest. The degree of overlap is irrelevant, however, if the absolute free energy of each state can be computed. We present a method for calculating the absolute free energy that employs a simple construction of an exactly computable reference system which possesses high overlap with the state of interest. The approach requires only a physical ensemble of conformations generated via simulation, and an auxiliary calculation of approximately equal central-processing-unit (CPU) cost. Moreover, the calculations can converge to the correct free energy value even when the physical ensemble is incomplete or improperly distributed. As a "proof of principle," we use the approach to correctly predict free energies for test systems where the absolute values can be calculated exactly, and also to predict the conformational equilibrium for leucine dipeptide in implicit solvent.Comment: To appear in J. Chem. Phys., 10 pages, 6 figure

Swap Bribery

In voting theory, bribery is a form of manipulative behavior in which an external actor (the briber) offers to pay the voters to change their votes in order to get her preferred candidate elected. We investigate a model of bribery where the price of each vote depends on the amount of change that the voter is asked to implement. Specifically, in our model the briber can change a voter's preference list by paying for a sequence of swaps of consecutive candidates. Each swap may have a different price; the price of a bribery is the sum of the prices of all swaps that it involves. We prove complexity results for this model, which we call swap bribery, for a broad class of election systems, including variants of approval and k-approval, Borda, Copeland, and maximin.Comment: 17 page

Detailed compositional analysis of the heavily polluted DBZ white dwarf SDSS J073842.56+183509.06: A window on planet formation?

We present a new model atmosphere analysis of the most metal contaminated white dwarf known, the DBZ SDSS J073842.56+183509.06. Using new high resolution spectroscopic observations taken with Keck and Magellan, we determine precise atmospheric parameters and measure abundances of 14 elements heavier than helium. We also report new Spitzer mid-infrared photometric data that are used to better constrain the properties of the debris disk orbiting this star. Our detailed analysis, which combines data taken from 7 different observational facilities (GALEX, Gemini, Keck, Magellan, MMT, SDSS and Spitzer) clearly demonstrate that J0738+1835 is accreting large amounts of rocky terrestrial-like material that has been tidally disrupted into a debris disk. We estimate that the body responsible for the photospheric metal contamination was at least as large Ceres, but was much drier, with less than 1% of the mass contained in the form of water ice, indicating that it formed interior to the snow line around its parent star. We also find a correlation between the abundances (relative to Mg and bulk Earth) and the condensation temperature; refractory species are clearly depleted while the more volatile elements are possibly enhanced. This could be the signature of a body that formed in a lower temperature environment than where Earth formed. Alternatively, we could be witnessing the remains of a differentiated body that lost a large part of its outer layers.Comment: 16 pages, 17 figures, accepted for publication in The Astrophysical Journa