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

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

3D-2D crossover in the naturally layered superconductor (LaSe)1.14(NbSe2)

The temperature and angular dependencies of the resistive upper critical magnetic field $B_{c2}$ reveal a dimensional crossover of the superconducting state in the highly anisotropic misfit-layer single crystal of (LaSe)$_{1.14}$(NbSe$_2$) with the critical temperature $T_c$ of 1.23 K. The temperature dependence of the upper critical field $B_{c2\parallel ab}(T)$ for a field orientation along the conducting $(ab)$-planes displays a characteristic upturn at 1.1 K and below this temperature the angular dependence of $B_{c2}$ has a cusp around the parallel field orientation. Both these typical features are observed for the first time in a naturally crystalline layered system.Comment: 7 pages incl. 3 figure

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

Molecular and mass spectroscopic analysis of isotopically labeled organic residues

Experimental studies aimed at understanding the evolution of complex organic molecules on interstellar grains were performed. The photolysis of frozen gas mixtures of various compositions containing H2O, CO, NH3, and CH4 was studied. These species were chosen because of their astrophysical importance as deducted from observational as well as theoretical studies of ice mantles on interstellar grains. These ultraviolet photolyzed ices were warmed up in order to produce refractory organic molecules like the ones formed in molecular clouds when the icy mantles are being irradiated and warmed up either by a nearby stellar source or impulsive heating. The laboratory studies give estimates of the efficiency of production of such organic material under interstellar conditions. It is shown that the gradual carbonization of organic mantles in the diffuse cloud phase leads to higher and higher visual absorptivity - yellow residues become brown in the laboratory. The obtained results can be applied to explaining the organic components of comets and their relevance to the origin of life

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

Non-linear Evolution of f(R) Cosmologies III: Halo Statistics

The statistical properties of dark matter halos, the building blocks of cosmological observables associated with structure in the universe, offer many opportunities to test models for cosmic acceleration, especially those that seek to modify gravitational forces. We study the abundance, bias and profiles of halos in cosmological simulations for one such model: the modified action f(R) theory. In the large field regime that is accessible to current observations, enhanced gravitational forces raise the abundance of rare massive halos and decrease their bias but leave their (lensing) mass profiles largely unchanged. This regime is well described by scaling relations based on a modification of spherical collapse calculations. In the small field regime, enhanced forces are suppressed inside halos and the effects on halo properties are substantially reduced for the most massive halos. Nonetheless, the scaling relations still retain limited applicability for the purpose of establishing conservative upper limits on the modification to gravity.Comment: 12 pages, 10 figures; v2: revised version accepted by Phys. Rev.