Fe XI emission lines in a high resolution extreme ultraviolet spectrum obtained by SERTS

New calculations of radiative rates and electron impact excitation cross sections for Fe XI are used to derive emission line intensity ratios involving 3s^23p^4 - 3s^23p^33d transitions in the 180-223 A wavelength range. These ratios are subsequently compared with observations of a solar active region, obtained during the 1995 flight Solar EUV Research Telescope and Spectrograph (SERTS). The version of SERTS flown in 1995 incorporated a multilayer grating that enhanced the instrumental sensitivity for features in the 170 - 225 A wavelength range, observed in second-order between 340 and 450 A. This enhancement led to the detection of many emission lines not seen on previous SERTS flights, which were measured with the highest spectral resolution (0.03 A) ever achieved for spatially resolved active region spectra in this wavelength range. However, even at this high spectral resolution, several of the Fe XI lines are found to be blended, although the sources of the blends are identified in the majority of cases. The most useful Fe XI electron density diagnostic line intensity ratio is I(184.80 A)/I(188.21 A). This ratio involves lines close in wavelength and free from blends, and which varies by a factor of 11.7 between N_e = 10^9 and 10^11 cm^-3, yet shows little temperature sensitivity. An unknown line in the SERTS spectrum at 189.00 A is found to be due to Fe XI, the first time (to our knowledge) this feature has been identified in the solar spectrum. Similarly, there are new identifications of the Fe XI 192.88, 198.56 and 202.42 A features, although the latter two are blended with S VIII/Fe XII and Fe XIII, respectively.Comment: 21 pages, 9 gigures, accepted for publication in the Astrophysical Journa

The VLT-FLAMES Tarantula Survey X: Evidence for a bimodal distribution of rotational velocities for the single early B-type stars

Aims: Projected rotational velocities (\vsini) have been estimated for 334 targets in the VLT-FLAMES Tarantula survey that do not manifest significant radial velocity variations and are not supergiants. They have spectral types from approximately O9.5 to B3. The estimates have been analysed to infer the underlying rotational velocity distribution, which is critical for understanding the evolution of massive stars. Methods: Projected rotational velocities were deduced from the Fourier transforms of spectral lines, with upper limits also being obtained from profile fitting. For the narrower lined stars, metal and non-diffuse helium lines were adopted, and for the broader lined stars, both non-diffuse and diffuse helium lines; the estimates obtained using the different sets of lines are in good agreement. The uncertainty in the mean estimates is typically 4% for most targets. The iterative deconvolution procedure of Lucy has been used to deduce the probability density distribution of the rotational velocities. Results: Projected rotational velocities range up to approximately 450 \kms and show a bi-modal structure. This is also present in the inferred rotational velocity distribution with 25% of the sample having $0\leq$\ve$\leq$100\,\kms and the high velocity component having \ve$\sim 250$\,\kms. There is no evidence from the spatial and radial velocity distributions of the two components that they represent either field and cluster populations or different episodes of star formation. Be-type stars have also been identified. Conclusions: The bi-modal rotational velocity distribution in our sample resembles that found for late-B and early-A type stars. While magnetic braking appears to be a possible mechanism for producing the low-velocity component, we can not rule out alternative explanations.Comment: to be publisged in A&

Mass and Angular Momentum Transfer in the Massive Algol Binary RY Persei

We present an investigation of H-alpha emission line variations observed in the massive Algol binary, RY Per. We give new radial velocity data for the secondary based upon our optical spectra and for the primary based upon high dispersion UV spectra. We present revised orbital elements and an estimate of the primary's projected rotational velocity (which indicates that the primary is rotating 7 times faster than synchronous). We use a Doppler tomography algorithm to reconstruct the individual primary and secondary spectra in the region of H-alpha, and we subtract the latter from each of our observations to obtain profiles of the primary and its disk alone. Our H-alpha observations of RY Per show that the mass gaining primary is surrounded by a persistent but time variable accretion disk. The profile that is observed outside-of-eclipse has weak, double-peaked emission flanking a deep central absorption, and we find that these properties can be reproduced by a disk model that includes the absorption of photospheric light by the band of the disk seen in projection against the face of the star. We developed a new method to reconstruct the disk surface density distribution from the ensemble of H-alpha profiles observed around the orbit, and this method accounts for the effects of disk occultation by the stellar components, the obscuration of the primary by the disk, and flux contributions from optically thick disk elements. The resulting surface density distribution is elongated along the axis joining the stars, in the same way as seen in hydrodynamical simulations of gas flows that strike the mass gainer near trailing edge of the star. This type of gas stream configuration is optimal for the transfer of angular momentum, and we show that rapid rotation is found in other Algols that have passed through a similar stage.Comment: 39 pages, 12 figures, ApJ in press, 2004 June 20 issu

Stellar Rotation in Young Clusters. I. Evolution of Projected Rotational Velocity Distributions

Open clusters offer us the means to study stellar properties in samples with well-defined ages and initial chemical composition. Here we present a survey of projected rotational velocities for a large sample of mainly B-type stars in young clusters to study the time evolution of the rotational properties of massive stars. The survey is based upon moderate resolution spectra made with the WIYN 3.5 m and CTIO 4 m telescopes and Hydra multi-object spectrographs, and the target stars are members of 19 young open clusters with an age range of approximately 6 to 73 Myr. We made fits of the observed lines He I 4026, 4387, 4471 and Mg II 4481 using model theoretical profiles to find projected rotational velocities for a total of 496 OB stars. We find that there are fewer slow rotators among the cluster B-type stars relative to nearby B stars in the field. We present evidence consistent with the idea that the more massive B stars (M > 9 solar masses) spin down during their main sequence phase. However, we also find that the rotational velocity distribution appears to show an increase in the numbers of rapid rotators among clusters with ages of 10 Myr and higher. These rapid rotators appear to be distributed between the zero age and terminal age main sequence locations in the Hertzsprung-Russell diagram, and thus only a minority of them can be explained as the result of a spin up at the terminal age main sequence due to core contraction. We suggest instead that some of these rapid rotators may have been spun up through mass transfer in close binary systems.Comment: 33 pages, 11 figures, accepted by Ap

DFT investigation of 3d transition metal NMR shielding tensors in diamagnetic systems using the gauge-including projector augmented-wave method

We present a density functional theory based method for calculating NMR shielding tensors for 3d transition metal nuclei using periodic boundary conditions. Calculations employ the gauge-including projector augmented-wave pseudopotentials method. The effects of ultrasoft pseudopotential and induced approximations on the second-order magnetic response are intensively examined. The reliability and the strength of the approach for 49Ti and 51V nuclei is shown by comparison with traditional quantum chemical methods, using benchmarks of finite organometallic systems. Application to infinite systems is validated through comparison to experimental data for the 51V nucleus in various vanadium oxide based compounds. The successful agreement obtained for isotropic chemical shifts contrasts with full estimation of the shielding tensor eigenvalues, revealing the limitation of pure exchange-correlation functionals compared to their exact-exchange corrected analogues.Comment: 56 page

NICMOS images of JVAS/CLASS gravitational lens systems

We present Hubble Space Telescope (HST) infrared images of four gravitational lens systems from the JVAS/CLASS gravitational lens survey and compare the new infrared HST pictures with previously published WFPC2 HST optical images and radio maps. Apart from the wealth of information that we get from the flux ratios and accurate positions and separations of the components of the lens systems that we can use as inputs for better constraints on the lens models we are able to discriminate between reddening and optical/radio microlensing as the possible cause of differences observed in the flux ratios of the components across the three wavelength bands. Substantial reddening has been known to be present in the lens system B1600+434 and has been further confirmed by the present infrared data. In the two systems B0712+472 and B1030+074 microlensing has been pinpointed down as the main cause of the flux ratio discrepancy both in the optical/infrared and in the radio, the radio possibly caused by the substructure revealed in the lensing galaxies. In B0218+357 however the results are still not conclusive. If we are actually seeing the two "true" components of the lens system then the flux ratio differences are attributed to a combination of microlensing and reddening or alternatively due to some variability in at least one of the images. Otherwise the second "true" component of B0218+357 maybe completely absorbed by a molecular cloud and the anomalous flux density ratios and large difference in separation between the optical/infrared and radio that we see can be explained by emission from either a foreground object or from part of the lensing galaxy.Comment: 10 pages, 4 figures (original higher resolution figures can be obtained at the e-mail above), to appear in MNRAS (accepted

Quantitative analysis of WC stars: Constraints on neon abundances from ISO/SWS spectroscopy

Neon abundances are derived in four Galactic WC stars -- gamma Vel (WR11, WC8+O7.5III), HD156385 (WR90, WC7), HD192103 (WR135, WC8), and WR146 (WC5+O8) - using mid-infrared fine structure lines obtained with ISO/SWS. Stellar parameters for each star are derived using a non-LTE model atmospheric code (Hillier & Miller 1998) together with ultraviolet (IUE), optical (INT, AAT) and infrared (UKIRT, ISO) spectroscopy. In the case of gamma Vel, we adopt results from De Marco et al. (2000), who followed an identical approach. ISO/SWS datasets reveal the [NeIII] 15.5um line in each of our targets, while [NeII] 12.8um, [SIV] 10.5um and [SIII] 18.7um are observed solely in gamma Vel. Using a method updated from Barlow et al. (1988) to account for clumped winds, we derive Ne/He=3-4x10^-3 by number, plus S/He=6x10^-5 for gamma Vel. Neon is highly enriched, such that Ne/S in gamma Vel is eight times higher than cosmic values. However, observed Ne/He ratios are a factor of two times lower than predictions of current evolutionary models of massive stars. An imprecise mass-loss and distance were responsible for the much greater discrepancy in neon content identified by Barlow et al. Our sample of WC5--8 stars span a narrow range in T* (=55--71kK), with no trend towards higher temperature at earlier spectral type, supporting earlier results for a larger sample by Koesterke & Hamann (1995). Stellar luminosities range from 100,000 to 500,000 Lo, while 10^-5.1 < Mdot/(Mo/yr) < 10^-4.5, adopting clumped winds, in which volume filling factors are 10%. In all cases, wind performance numbers are less than 10, significantly lower than recent estimates. Carbon abundances span 0.08 < C/He < 0.25 by number, while oxygen abundances remain poorly constrained.Comment: 16 pages,7 figures accepted for MNRA

Global Patterns of Terrestrial Biological Nitrogen (N2) Fixation in Natural Ecosystems

Human activities have clearly caused dramatic alterations of the terrestrial nitrogen cycle, and analyses of the extent and effects of such changes are now common in the scientific literature. However, any attempt to evaluate N cycling processes within ecosystems, as well as anthropogenic influences on the N cycle, requires an understanding of the magnitude of inputs via biological nitrogen fixation (BNF). Although there have been many studies addressing the microbiology, physiology, and magnitude of N fixation at local scales, there are very few estimates of BNF over large scales. We utilized \u3e100 preexisting published estimates of BNF to generate biome- and global-level estimates of biological N fixation. We also used net primary productivity (NPP) and evapotranspiration (ET) estimates from the Century terrestrial ecosystem model to examine global relationships between these variables and BNF as well as to compare observed and Century-modeled BNF. Our data-based estimates showed a strong positive relationship between ecosystem ET and BNF, and our analyses suggest that while the model\u27s simple relationships for BNF predict broad scale patterns, they do not capture much of the variability or magnitude of published rates. Patterns of BNF were also similar to patterns of ecosystem NPP. Our “best estimate” of potential nitrogen fixation by natural ecosystems is ∼195 Tg N yr−1, with a range of 100–290 Tg N yr−1. Although these estimates do not account for the decrease in natural N fixation due to cultivation, this would not dramatically alter our estimate, as the greatest reductions in area have occurred in systems characterized by relatively low rates of N fixation (e.g., grasslands). Although our estimate of BNF in natural ecosystems is similar to previously published estimates of terrestrial BNF, we believe that this study provides a more documented, constrained estimate of this important flux

Physical activity attitudes, intentions and behaviour among 18-25 year olds: a mixed method study

Peer reviewedPublisher PD

The future of sovereignty in multilevel governance Europe: a constructivist reading

Multilevel governance presents a depiction of contemporary structures in EU Europe as consisting of overlapping authorities and competing competencies. By focusing on emerging non-anarchical structures in the international system, hence moving beyond the conventional hierarchy/anarchy dichotomy to distinguish domestic and international arenas, this seems a radical transformation of the familiar Westphalian system and to undermine state sovereignty. Paradoxically, however, the principle of sovereignty proves to be resilient despite its alleged empirical decline. This article argues that social constructivism can explain the paradox, by considering sovereign statehood as a process-dependent institutional fact, and by showing that multilevel governance can feed into this process