XMM-Newton First Observation in the Pleiades

We present the first results from a 40 ks Guaranteed Time XMM-Newton pointing in the Pleiades. We detect almost all early-mid dM members in the field and several very low mass (VLM) stars - including the brown dwarf (BD) candidate Roque 9 - and investigate the variation of X-ray activity levels, hardness ratios and flare frequency with spectral type down to the BD regime.Comment: 5 pages, 6 figures, to be published in proceedings of `Stellar Coronae in the Chandra and XMM-Newton Era

Modified hydraulic braking system limits angular deceleration to safe values

Conventional spring actuated, hydraulically released, fail-safe disk braking system is modified to control the angular deceleration of a massive antenna. The hydraulic system provides an immediate preset pressure to the spring-loaded brake shoes and holds it at this value to decelerate the antenna at the desired rate

An XMM-Newton observation of the young open cluster NGC 2547: coronal activity at 30 Myr

We report XMM-Newton observations of the young open cluster NGC 2547 which allow us to characterise coronal activity in solar-type stars at an age of 30 Myr. X-ray emission peaks among G-stars at luminosities (0.3-3keV) of Lx~10^{30.5} erg/s and declines to Lx<=10^{29.0} erg/s among M-stars. Coronal spectra show evidence for multi-temperature differential emission measures and low coronal metal abundances (Z~0.3). The G- and K-type stars follow the same relationship between X-ray activity and Rossby number established in older clusters and field stars, although most solar-type stars in NGC 2547 exhibit saturated/super-saturated X-ray activity levels. Median levels of Lx and Lx/Lbol in the solar-type stars of NGC 2547 are similar to T-Tauri stars of the Orion Nebula cluster (ONC), but an order of magnitude higher than in the older Pleiades. The spread in X-ray activity levels among solar-type stars in NGC 2547 is much smaller than in older or younger clusters. Coronal temperatures increase with Lx, Lx/Lbol and surface X-ray flux. Active solar-type stars in NGC 2547 have coronal temperatures between those in the ONC and the most active older ZAMS stars. A flaring rate (for total flare energies [0.3-3keV] >10^{34} erg) of 1 every 350^{+350}_{-120} ks was found for solar-type stars, similar to rates found in the ONC and Pleiades. Comparison with ROSAT HRI data taken 7 years previously reveals that only 10-15 percent of solar-type stars or stars with Lx>3x10^{29} erg/s exhibit X-ray variability by more than a factor of two. The similar levels of X-ray activity and rate of occurrence for large flares in NGC 2547 and the ONC demonstrate that the X-ray radiation environment around young solar-type stars remains relatively constant over their first 30 Myr (abridged).Comment: Accepted for publication in MNRAS. Electronic tables available from the autho

The nursery culture and nutrition of post-larval black tiger shrimp Penaeus monodon Fabricius

An evaluation was made of the role of secondary nursery rearing within the intensive shrimp culture industry. A series of concrete nursery tank trials were conducted rearing post-larval Penaeus monodon Fabricius for 35 days from PL,5 to PL50. Shrimp production was directly related to stocking density at up to 2,000 shrimp m'^ but growth, survival, FCE and size range were negatively density dependant. Water exchange and aeration were necessary to maintain shrimp production at high density. The use of mesh habitats and sand substrates and the effects of dietary formulation and feeding regime also influenced shrimp production. The production of shrimp nursed in net cages was comparable to that of shrimp nursed in concrete tanks at densities of 500 shrimp m'^. An economic analysis of secondary nursing in tanks suggested that it may be profitable if the value of the juvenile shrimp produced is > 2.5 times that of postlarvae. A cage on-growing trial showed that the stunting of shrimp during high density secondary nursing was not permanent, but was rapidly compensated for by increased growth rate during on-growing at lower density. A stress test was developed which enabled quantification of the stress tolerance (vigour) of post-larval shrimp. This entailed direct transfer of post-larvae into water at reduced or increased temperature and salinity for one hour. The tests facilitated discrimination between batches of post-larvae produced from a single hatchery. The stress resistance of juvenile shrimp following secondary nursing was significantly better than that of exhatchery post-larvae, suggesting that older or secondary-nursed juveniles are more likely to survive transfer and subsequent on-growing than are primary-nursed post-larvae. Low vigour shrimp are suggested to have poor growth potential, although further research into the effects of shrimp vigour on shrimp production during pond on-growing is required. A series of nutritional trials were conducted in laboratory and outdoor nursery tanks to study the optimal levels of, and relationship between, protein, lipid, carbohydrate and energy in diets for post-larval P. monodon. Lipids including cod liver oil:soybean oil (3:1), soy lecithin (3-6 %) and cholesterol (0.5 %) were necessary to optimise shrimp production. Dietary lipid levels of 8-12 % in nutrient balanced diets maximised shrimp survival and production. Carbohydrate (starch) levels of 21-38 % were optimal in nutrient balanced diets at carbohydrate:lipid ratios of 2-5:1. Both carbohydrate and lipid were shown to be able to spare protein for growth. Protein levels were decreased from 44 % to 38 % in nutrient balanced diets without compromising shrimp production. Further reductions in protein levels may be possible in nutrient balanced diets. The level of protein supporting-optimal shrimp production was shown to be directly related to the total energy and protein:energy ratio of the diet. Models are proposed for estimating the level and proportions of protein, lipid and carbohydrate which are best able to supply the nutrient and energy requirements of post-larval P. monodon. The benefits of reducing the protein level of shrimp diets are discussed in terms of minimising diet cost and feed wastag

XMM-Newton and the Pleiades - I. Bright coronal sources and the X-ray emission from intermediate-type stars

We present results of X-ray spectral and timing analyses of solar-like (spectral types F5-K8) and intermediate-type (B4-F4) Pleiads observed in a 40-ks XMM-Newton EPIC exposure, probing X-ray luminosities (LX) up to a factor 10 fainter than previous studies using the ROSAT PSPC. All eight solar-like members have ‘quasi-steady'LX≳ 1029erg s−1, consistent with the known rotation-activity relation and four exhibit flares. Using a hydrodynamic modelling technique, we derive loop half-lengths for the two strongest flares, on H ii 1032 and H ii 1100. Near the beginning of its flare, the light curve of H ii 1100 shows a feature with a profile suggestive of a total occultation of the flaring loop. Eclipse by a substellar companion in a close orbit is possible but would seem an extraordinarily fortuitous event; absorption by a fast-moving cloud of cool material requires NH at least two orders of magnitude greater than any solar or stellar prominence. An occultation may have been mimicked by the coincidence of two flares, though the first, with its decay time being shorter than its rise time and suggestive of , would be unusual. Spectral modelling of the quasi-steady emission shows a rising trend in coronal temperature from F and slowly rotating G stars to K stars to fast-rotating G stars, and a preference for low coronal metallicity. These features are consistent with those of nearby solar-like stars, although none of the three stars showing ‘saturated' emission bears the significant component at 2 keV seen in the saturated coronae of AB Dor and 47 Cas. Of five intermediate-type stars, two are undetected (LX < 4 × 1027erg s−1) and three show X-ray emission with a spectrum and LX consistent with origin from an active solar-like companio

X-ray emission from a brown dwarf in the Pleiades

We report on the first detection of X-ray emission from a brown dwarf in the Pleiades, the M7-type Roque 14, obtained using the EPIC detectors on XMM-Newton. This is the first X-ray detection of a brown dwarf intermediate in age between ≈12 and ≈320 Myr. The emission appears persistent, although we cannot rule out flare-like behaviour with a decay time-scale >4 ks. The time-averaged X-ray luminosity of LX≈ 3.3 ± 0.8 × 1027 erg s−1 and its ratios with the bolometric (LX/Lbol≈ 10−3.05) and Hα (LX/LHα≈ 4.0) luminosities suggest magnetic activity similar to that of active main-sequence M dwarfs, such as the M7 old-disc star VB 8, although the suspected binary nature of Roque 14 merits further attention. No emission is detected from four proposed later-type Pleiades brown dwarfs, with upper limits to LX in the range 2.1-3.8 × 1027 erg s−1 and to log (LX/Lbol) in the range −3.10 to −2.9

Absolute and convective instabilities of parallel propagating circularly polarized Alfven waves: Beat instability

Ruderman and Simpson [Phys. Plasmas 11, 4178 (2004)] studied the absolute and convective decay instabilities of parallel propagating circularly polarized Alfven waves in plasmas where the sound speed c(S) is smaller than the Alfven speed upsilon(A). We extend their analysis for the beat instability which occurs in plasmas with c(S)>upsilon(A). We assume that the dimensionless amplitude of the circularly polarized Alfven wave (pump wave), a, is small. Applying Briggs' method we study the problem analytically using expansions in power series with respect to a. It is shown that the pump wave is absolutely unstable in a reference frame moving with the velocity U with respect to the rest plasma if U-lU-r, the instability is convective. The signaling problem is studied in a reference frame where the pump wave is convectively unstable. It is shown that the spatially amplifying waves exist only when the signaling frequency is in two narrow symmetric frequency bands with the widths of the order of a(3). These results enable us to extend for the case when c(S)>upsilon(A) the conclusions, previously made for the case when c(S)<upsilon(A), that circularly polarized Alfven waves propagating in the solar wind are convectively unstable in a reference frame of any spacecraft moving with the velocity not exceeding a few tens of km/s in the solar reference frame. The characteristic scale of spatial amplification for these waves exceeds 1 a.u

The resilience of indigenous knowledge in small-scale African agriculture: key drivers

The successful use of indigenous knowledge (IK) in development practice in rural Africa over the last couple of decades has proved to be elusive and disappointing. Using empirical field data from northern Malawi, this study suggests that the two key drivers for farmers in this area are household food security and the maintenance of soil fertility. Indigenous ways of knowing underpin the agricultural system which has been developed, rather than the adoption of more modern, ‘scientific’ ways, to deliver against these drivers. Such IKs, however, are deeply embedded in the economic, social and cultural environments in which they operate