The Solar Photospheric Nitrogen Abundance: Determination with 3D and 1D Model Atmospheres

We present a new determination of the solar nitrogen abundance making use of 3D hydrodynamical modelling of the solar photosphere, which is more physically motivated than traditional static 1D models. We selected suitable atomic spectral lines, relying on equivalent width measurements already existing in the literature. For atmospheric modelling we used the co 5 bold 3D radiation hydrodynamics code. We investigated the influence of both deviations from local thermodynamic equilibrium (non-LTE effects) and photospheric inhomogeneities (granulation effects) on the resulting abundance. We also compared several atlases of solar flux and centre-disc intensity presently available. As a result of our analysis, the photospheric solar nitrogen abundance is A(N) = 7.86 +/- 0.12.Comment: 6 pages, 4 figure

Chemical composition of a sample of bright solar-metallicity stars

We present a detailed analysis of seven young stars observed with the spectrograph SOPHIE at the Observatoire de Haute-Provence for which the chemical composition was incomplete or absent in the literature. For five stars, we derived the stellar parameters and chemical compositions using our automatic pipeline optimized for F, G, and K stars, while for the other two stars with high rotational velocity, we derived the stellar parameters by using other information (parallax), and performed a line-by-line analysis. Chromospheric emission-line fluxes from CaII are obtained for all targets. The stellar parameters we derive are generally in good agreement with what is available in the literature. We provide a chemical analysis of two of the stars for the first time. The star HIP 80124 shows a strong Li feature at 670.8 nm implying a high lithium abundance. Its chemical pattern is not consistent with it being a solar sibling, as has been suggested.Comment: To be published on A

Sulphur abundances from the SI near-infrared triplet at 1045 nm

context : Unlike silicon and calcium, sulphur is an alpha-element which does not form dust. Some of the available observations of the evolution of sulphur with metallicity indicate an increased scatter of sulphur to iron ratios at low metallicities or even a bimodal distribution, with some stars showing constant S/Fe at all metallicities and others showing an increasing S/Fe ratio with decreasing metallicity. In metal-poor stars SI lines of Multiplet 1 at 920 nm are not yet too weak to permit to measure the sulphur abundance A(S), however in ground-based observations they are severely affected by telluric lines. aims : We investigate the possibility to measure sulphur abundances from SI Mult. 3 at 1045 nm lines, which lie in the near infra-red. These are slightly weaker than those of Mult. 1, but lie in a range not affected by telluric lines. method We investigate the lines of Mult.3 in the Sun (G2V), Procyon (F5V), HD 33256 (F5V), HD 25069 (G9V) and epsilon Eri (HD 22049, K2V). For the Sun and Procyon the analysis has been performed with CO5BOLD 3D hydrodynamical model atmospheres, for the other three stars, for which hydrodynamical simulations are not available, the analysis has been performed using 1D model atmospheres. results For our sample of stars we find a global agreement of A(S) from lines of different multiplets. conclusions : Our results suggest that the infrared lines of Mult. 3 are a viable indicator of the sulphur abundance which, because of the intrinsic strength of this multiplet, should be suitable to study the trend of [S/Fe] at low metallicities

Computational modelling and experimental tank testing of the multi float WaveSub under regular wave forcing

A submerged wave device generates energy from the relative motion of floating bodies. In WaveSub, three floats are joined to a reactor; each connected to a spring and generator. Electricity generated damps the orbital movements of the floats. The forces are non-linear and each float interacts with the others. Tuning to the wave climate is achieved by changing the line lengths, so there is a need to understand the performance trade-offs for a large number of configurations. This requires an efficient, large displacement, multidirectional, multi-body numerical scheme. Results from a 1/25 scale wave basin experiment are described. Here, we show that a time domain linear potential flow formulation (Nemoh, WEC-Sim) can match the tank testing provided that suitably tuned drag coefficients are employed. Inviscid linear potential models can match some wave device experiments; however, additional viscous terms generally provide better accuracy. Scale experiments are also prone to mechanical friction, and we estimate friction terms to improve the correlation further. The resulting error in mean power between numerical and physical models is approximately 10%. Predicted device movement shows a good match. Overall, drag terms in time domain wave energy modelling will improve simulation accuracy in wave renewable energy device design

Computational modelling and experimental tank testing of the multi float WaveSub under regular wave forcing

A submerged wave device generates energy from the relative motion of floating bodies. In 1 WaveSub, three floats are joined to a reactor; each connected to a spring and generator. Electricity generated 2 damps the orbital movements of the floats. The forces are non-linear and each float interacts with the others. 3 Tuning to the wave climate is achieved by changing the line lengths so there is a need to understand the 4 performance trade-offs for a large number of configurations. This requires an efficient, large displacement, 5 multidirectional, multi-body numerical scheme. Results from a 1/25 scale wave basin experiment are described. 6 Here we show that a time domain linear potential flow formulation (Nemoh, WEC-Sim) can match the tank 7 testing provided that suitably tuned drag coefficients are employed. Inviscid linear potential models can match 8 some wave device experiments, however, additional viscous terms generally provide better accuracy. Scale 9 experiments are also prone to mechanical friction and we estimate friction terms to improve the correlation 10 further. The resulting error in mean power between numerical and physical models is approximately 10%. 11 Predicted device movement shows a good match. Overall, drag terms in time domain wave energy modelling 12 will improve simulation accuracy in wave renewable energy device design

Spectroscopy of Blue Stragglers and Turnoff Stars in M67 (NGC 2682)

We have analyzed high-resolution spectra of relatively cool blue stragglers and main sequence turnoff stars in the old open cluster M67 (NGC 2682). We attempt to identify blue stragglers whose spectra are least contaminated by binary effects (contamination by a binary companion or absorption by circumstellar material). These ``best'' stragglers have metallicities ([Fe/H] = -0.05) and abundance ratios of the blue stragglers are not significantly different from those of the turnoff stars. Based on arguments from hydrodynamical models of stellar collisions, we assert that the current upper limits for the lithium abundances of all blue stragglers observed in M67 (by us and others) are consistent with no mixing during the formation process, assuming pre-main sequence and main sequence depletion patterns observed for M67 main sequence stars. We discuss composition signatures that could more definitively distinguish between blue straggler formation mechanisms in open cluster stars. We confirm the spectroscopic detection of a binary companion to the straggler S 1082. From our spectra, we measure a projected rotational speed of 90+/-20 km/sec for the secondary, and find that its radial velocity varies with a peak-to-peak amplitude of ~ 25 km/sec. Because the radial velocities do not vary with a period corresponding to the partial eclipses in the system, we believe this system is currently undergoing mass transfer. In addition we present evidence that S 984 is a true blue straggler (and not an unresolved pair). If this can be proven, our detection of lithium may indicate a collisional origin.Comment: 20 pages, 4 figures, to appear in October 2000 A

Echelle long-slit optical spectroscopy of evolved stars

We present echelle long-slit optical spectra of a sample of objects evolving off the AGB, most of them in the pre-planetary nebula (pPN) phase, obtained with the ESI and MIKE spectrographs at Keck-II and Magellan-I, respectively. The total wavelength range covered with ESI (MIKE) is ~3900 to 10900 A (~3600 to 7200A). In this paper, we focus our analysis mainly on the Halpha profiles. Prominent Halpha emission is detected in half of the objects, most of which show broad Halpha wings (up to ~4000 km/s). In the majority of the Halpha-emission sources, fast, post-AGB winds are revealed by P-Cygni profiles. In ~37% of the objects Halpha is observed in absorption. In almost all cases, the absorption profile is partially filled with emission, leading to complex, structured profiles that are interpreted as an indication of incipient post-AGB mass-loss. All sources in which Halpha is seen mainly in absorption have F-G type central stars, whereas sources with intense Halpha emission span a larger range of spectral types from O to G. Shocks may be an important excitation agent of the close stellar surroundings for objects with late type central stars. Sources with pure emission or P Cygni Halpha profiles have larger J-K color excess than objects with Halpha mainly in absorption, which suggests the presence of warm dust near the star in the former. The two classes of profile sources also segregate in the IRAS color-color diagram in a way that intense Halpha-emitters have dust grains with a larger range of temperatures. (abridged)Comment: 68 pages, 14 figures, accepted for publication in ApJS (abstract abridged

V2324Cyg - an F-type star with fast wind

For the first time high-resolution optical spectroscopy of the variable star V2324Cyg associated with the IR-source IRAS20572+4919 is made. More than 200 absorption features (mostly FeII, TiII, CrII, YII, BaII, and YII) are identified within the wavelength interval 4549-7880AA. The spectral type and rotation velocity of the star are found to be F0III and Vsini=69km/s, respectively. HI and NaID lines have complex PCyg-type profiles with an emission component. Neither systematic trend of radial velocity Vr with line depth Ro nor temporal variability of Vr have been found. We determined the average heliocentric radial velocity Vr=-16.8\pm 0.6km/s. The radial velocities inferred from the cores of the absorption components of the H$\beta$ and NaI wind lines vary from -140 to -225km/s (and the expansion velocities of the corresponding layers, from about 120 to 210km/s). The maximum expansion velocity is found for the blue component of the split H$\alpha$ absorption: 450km/s for December 12, 1995. The model atmospheres method is used to determine the star's parameters: Teff=7500K, log g=2.0, $\xi_t$=6.0km/s, and metallicity, which is equal to the solar value. The main peculiarity of the chemical abundances pattern is the overabundance of lithium and sodium. The results cast some doubt on the classification of V2324Cyg as a post-AGB star.Comment: 28 pages, 6 figure

The solar photospheric abundance of carbon.Analysis of atomic carbon lines with the CO5BOLD solar model

The use of hydrodynamical simulations, the selection of atomic data, and the computation of deviations from local thermodynamical equilibrium for the analysis of the solar spectra have implied a downward revision of the solar metallicity. We are in the process of using the latest simulations computed with the CO5BOLD code to reassess the solar chemical composition. We determine the solar photospheric carbon abundance by using a radiation-hydrodynamical CO5BOLD model, and compute the departures from local thermodynamical equilibrium by using the Kiel code. We measure equivalent widths of atomic CI lines on high resolution, high signal-to-noise ratio solar atlases. Deviations from local thermodynamic equilibrium are computed in 1D with the Kiel code. Our recommended value for the solar carbon abundance, relies on 98 independent measurements of observed lines and is A(C)=8.50+-0.06, the quoted error is the sum of statistical and systematic error. Combined with our recent results for the solar oxygen and nitrogen abundances this implies a solar metallicity of Z=0.0154 and Z/X=0.0211. Our analysis implies a solar carbon abundance which is about 0.1 dex higher than what was found in previous analysis based on different 3D hydrodynamical computations. The difference is partly driven by our equivalent width measurements (we measure, on average, larger equivalent widths with respect to the other work based on a 3D model), in part it is likely due to the different properties of the hydrodynamical simulations and the spectrum synthesis code. The solar metallicity we obtain from the CO5BOLD analyses is in slightly better agreement with the constraints of helioseismology than the previous 3D abundance results. (Abridged)Comment: Astronomy and Astrophysics, accepte