Radiation-hydrodynamics simulations of surface convection in low-mass stars: connections to stellar structure and asteroseismology

Radiation-hydrodynamical simulations of surface convection in low-mass stars can be exploited to derive estimates of i) the efficiency of the convective energy transport in the stellar surface layers; ii) the convection-related photometric micro-variability. We comment on the universality of the mixing-length parameter, and point out potential pitfalls in the process of its calibration which may be in part responsible for the contradictory findings about its variability across the Hertzsprung-Russell digramme. We further comment on the modelling of the photometric micro-variability in HD49933 - one of the first main COROT targets.Comment: 6 pages, 5 figures, Proceedings paper of IAU Symposium 25

The photospheric solar oxygen project: II. Non-concordance of the oxygen abundance derived from two forbidden lines

In the Sun, the two forbidden [OI] lines at 630 and 636 nm were previously found to provide discrepant oxygen abundances. aims: We investigate whether this discrepancy is peculiar to the Sun or whether it is also observed in other stars. method: We make use of high-resolution, high signal-to-noise ratio spectra of four dwarf to turn-off stars, five giant stars, and one sub-giant star observed with THEMIS, HARPS, and UVES to investigate the coherence of the two lines. results: The two lines provide oxygen abundances that are consistent, within observational errors, in all the giant stars examined by us. On the other hand, for the two dwarf stars for which a measurement was possible, for Procyon, and for the sub-giant star Capella, the 636 nm line provides systematically higher oxygen abundances, as already seen for the Sun. conclusions: The only two possible reasons for the discrepancy are a serious error in the oscillator strength of the NiI line blending the 630 nm line or the presence of an unknown blend in the 636 nm line, which makes the feature stronger. The CN lines blending the 636 nm line cannot be responsible for the discrepancy. The CaI autoionisation line, on the red wing of which the 636 nm line is formed, is not well modelled by our synthetic spectra. However, a better reproduction of this line would result in even higher abundances from the 636 nm, thus increasing the discrepancy.Comment: A&A accepte

An investigation of the formation and line properties of MgH in 3D hydrodynamical model stellar atmospheres

Studies of the isotopic composition of magnesium in cool stars have so far relied upon the use of one-dimensional (1D) model atmospheres. Since the isotopic ratios derived are based on asymmetries of optical MgH lines, it is important to test the impact from other effects affecting line asymmetries, like stellar convection. Here, we present a theoretical investigation of the effects of including self-consistent modeling of convection. Using spectral syntheses based on 3D hydrodynamical CO$^5$BOLD models of dwarfs (4000K$\lesssim T_\mathrm{eff}\lesssim5160K$, $4.0\leq$log(g)$\leq4.5$, $-3.0\leq[\mathrm{Fe/H}]\leq-1.0$) and giants ($T_\mathrm{eff}\sim4000$K, log(g)$=1.5$, $-3.0\leq[\mathrm{Fe/H}]\leq-1.0$), we perform a detailed analysis comparing 3D and 1D syntheses. We describe the impact on the formation and behavior of MgH lines from using 3D models, and perform a qualitative assessment of the systematics introduced by the use of 1D syntheses. Using 3D model atmospheres significantly affect the strength of the MgH lines, especially in dwarfs, with 1D syntheses requiring an abundance correction of up to +0.69 dex largest for our 5000K models. The corrections are correlated with $T_\mathrm{eff}$ and are also affected by the metallicity. The shape of the strong $^{24}$MgH component in the 3D syntheses is poorly reproduced in 1D. This results in 1D syntheses underestimating $^{25}$MgH by up to $\sim5$ percentage points and overestimating $^{24}$MgH by a similar amount for dwarfs. This discrepancy increases with decreasing metallicity. $^{26}$MgH is recovered relatively well, with the largest difference being $\sim2$ percentage points. The use of 3D for giants has less impact, due to smaller differences in the atmospheric structure and a better reproduction of the line shape in 1D.Comment: 20 pages, 15 figures, accepted for publication in Ap

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

The solar photospheric abundance of phosphorus: results from co5bold 3D model atmospheres

aims: We determine the solar abundance of phosphorus using co5bold 3D hydrodynamic model atmospheres. method: High resolution, high signal-to-noise solar spectra of the PI lines of Multiplet 1 at 1051-1068 nm are compared to line formation computations performed on a co5bold solar model atmosphere. results: We find A(P)=5.46+- 0.04, in good agreement with previous analysis based on 1D model atmospheres, due to the fact that the PI lines of Mult. 1 are little affected by 3D effects. We cannot confirm an earlier claim by other authors of a downward revision of the solar P abundance by 0.1 dex employing a 3D model atmosphere. Concerning other stars, we found modest (<0.1 dex) 3D abundance corrections for P among four F dwarf model atmospheres of different metallicity, being largest at lowest metallicity. conclusions: We conclude that 3D abundance corrections are generally rather small for the PI lines studied in this work. They are marginally relevant for metal-poor stars, but may be neglected in the Sun

Condensate formation in a dark state of a driven atom-cavity system

We demonstrate condensate formation in a dark state in an ultracold quantum gas coupled to a high-finesse cavity and pumped by a shaken optical lattice. We show experimentally and theoretically that the atoms in the dark state display a strong suppression of the coupling to the cavity. On the theory side, this is supported by solving the dynamics of a minimal three-level model and of the full atom-cavity system. The symmetry of the condensate wave function is anti-symmetric with respect to the potential minima of the pump lattice, and displays a staggered sign along the cavity direction. This symmetry decouples the dark state from the cavity, and is preserved when the pump intensity is switched off

NLTE determination of the calcium abundance and 3D corrections in extremely metal-poor stars

Accepted in Astronomy and Astrophysics(Abridged) Extremely metal-poor stars contain the fossil records of the chemical composition of the early Galaxy. The NLTE profiles of the calcium lines were computed in a sample of 53 extremely metal-poor stars with a modified version of the program MULTI. With our new model atom we are able to reconcile the abundance of Ca deduced from the Ca I and Ca II lines in Procyon. -We find that [Ca/Fe] = 0.50 ± 0.09 in the early Galaxy, a value slightly higher than the previous LTE estimations. -The scatter of the ratios [X/Ca] is generally smaller than the scatter of the ratio [X/Mg] where X is a "light metal" (O, Na, Mg, Al, S, and K) with the exception of Al. These scatters cannot be explained by error of measurements, except for oxygen. Surprisingly, the scatter of [X/Fe] is always equal to, or even smaller than, the scatter around the mean value of [X/Ca]. -We note that at low metallicity, the wavelength of the Ca I resonance line is shifted relative to the (weaker) subordinate lines, a signature of the effect of convection. -The Ca abundance deduced from the Ca I resonance line (422.7 nm) is found to be systematically smaller at very low metallicity, than the abundance deduced from the subordinate lines