Spectroscopic properties of a two-dimensional time-dependent Cepheid model I. Description and validation of the model

Standard spectroscopic analyses of Cepheid variables are based on hydrostatic one-dimensional model atmospheres, with convection treated using various formulations of mixing-length theory. This paper aims to carry out an investigation of the validity of the quasi-static approximation in the context of pulsating stars. We check the adequacy of a two-dimensional time-dependent model of a Cepheid-like variable with focus on its spectroscopic properties. With the radiation-hydrodynamics code CO5BOLD, we construct a two-dimensional time-dependent envelope model of a Cepheid with $T_\mathrm{eff}= 5600$ K, $\log g=2.0$, solar metallicity, and a 2.8-day pulsation period. Subsequently, we perform extensive spectral syntheses of a set of artificial iron lines in local thermodynamic equilibrium. The set of lines allows us to systematically study effects of line strength, ionization stage, and excitation potential. We evaluate the microturbulent velocity, line asymmetry, projection factor, and Doppler shifts. The mean Doppler shift is non-zero and negative, -1 km/s, after averaging over several full periods and lines. This residual line-of-sight velocity (related to the "K-term") is primarily caused by horizontal inhomogeneities, and consequently we interpret it as the familiar convective blueshift ubiquitously present in non-pulsating late-type stars. Limited statistics prevent firm conclusions on the line asymmetries. Our two-dimensional model provides a reasonably accurate representation of the spectroscopic properties of a short-period Cepheid-like variable star. Some properties are primarily controlled by convective inhomogeneities rather than by the Cepheid-defining pulsations

Spectroscopic properties of a two-dimensional time-dependent Cepheid model II. Determination of stellar parameters and abundances

Standard spectroscopic analyses of variable stars are based on hydrostatic one-dimensional model atmospheres. This quasi-static approach has theoretically not been validated. We aim at investigating the validity of the quasi-static approximation for Cepheid variables. We focus on the spectroscopic determination of the effective temperature $T_\mathrm{eff}$, surface gravity $\log \,g$, microturbulent velocity $\xi_\mathrm{t}$, and a generic metal abundance $\log\,A$ -- here taken as iron. We calculate a grid of 1D hydrostatic plane-parallel models covering the ranges in effective temperature and gravity encountered during the evolution of a two-dimensional time-dependent envelope model of a Cepheid computed with the radiation-hydrodynamics code CO5BOLD. We perform 1D spectral syntheses for artificial iron lines in local thermodynamic equilibrium varying the microturbulent velocity and abundance. We fit the resulting equivalent widths to corresponding values obtained from our dynamical model. For the four-parametric case, the stellar parameters are typically underestimated exhibiting a bias in the iron abundance of \approx-0.2\,\mbox{dex}. To avoid biases of this kind it is favourable to restrict the spectroscopic analysis to photometric phases $\phi_\mathrm{ph}\approx0.3\ldots 0.65$ using additional information to fix effective temperature and surface gravity. Hydrostatic 1D model atmospheres can provide unbiased estimates of stellar parameters and abundances of Cepheid variables for particular phases of their pulsations. We identified convective inhomogeneities as the main driver behind potential biases. For obtaining a complete view on the effects when determining stellar parameters with 1D models, multi-dimensional Cepheid atmosphere models are necessary for variables of longer period than investigated here.Comment: accepted for publication in Astronomy & Astrophysic

Uncertainties of Synthetic Integrated Colors as Age Indicators

We investigate the uncertainties in the synthetic integrated colors of simple stellar populations. Three types of uncertainties are from the stellar models, the population synthesis techniques, and from the spectral libraries. Despite some skepticism, synthetic colors appear to be reliable age indicators when used for select age ranges. Rest-frame optical colors are good age indicators at ages 2 -- 7Gyr. At ages sufficiently large to produce hot HB stars, the UV-to-optical colors provide an alternative means for measuring ages. This UV technique may break the age-metallicity degeneracy because it separates old populations from young ones even in the lack of metallicity information. One can use such techniques on extragalactic globular clusters and perhaps even for high redshift galaxies that are passively evolving to study galaxy evolution history.Comment: 38 pages, 21 figures, LaTex, 2003, ApJ, 582 (Jan 1), in pres

Can we trust elemental abundances derived in late-type giants with the classical 1D stellar atmosphere models?

We compare the abundances of various chemical species as derived with 3D hydrodynamical and classical 1D stellar atmosphere codes in a late-type giant characterized by T_eff=3640K, log g = 1.0, [M/H] = 0.0. For this particular set of atmospheric parameters the 3D-1D abundance differences are generally small for neutral atoms and molecules but they may reach up to 0.3-0.4 dex in case of ions. The 3D-1D differences generally become increasingly more negative at higher excitation potentials and are typically largest in the optical wavelength range. Their sign can be both positive and negative, and depends on the excitation potential and wavelength of a given spectral line. While our results obtained with this particular late-type giant model suggest that 1D stellar atmosphere models may be safe to use with neutral atoms and molecules, care should be taken if they are exploited with ions.Comment: Poster presented at the IAU Symposium 265 "Chemical Abundances in the Universe: Connecting First Stars to Planets", Rio de Janeiro, 10-14 August 2009; 2 pages, 1 figur

Sjukdomar på juniperus, thuja, och chamaecyparis

Vintergröna barrväxter får ofta bruna vissnande barr eller skott, och någon gång dör större grenar eller t. o. m. hela plantor. Sådana symtom kan vara tecken på en lång rad olika åkommor. Vid närmare studium av de vissnande växtdelarna kan man ofta urskilja specifika symtom, som är karaktäristiska för olika sjukdomar. I detta faktablad beskrivs viktiga sjukdomar på Juniperus, Thuja och Chamaecyparis. De är konstaterade i Sverige eller förväntas finnas här. Fakta är dock hämtade från utländska källor

The intermediate-age globular cluster NGC 1783 in the Large Magellanic Cloud

We present Hubble Space Telescope ACS deep photometry of the intermediate-age globular cluster NGC 1783 in the Large Magellanic Cloud. By using this photometric dataset, we have determined the degree of ellipticity of the cluster ($\epsilon$=0.14$\pm$0.03) and the radial density profile. This profile is well reproduced by a standard King model with an extended core (r_c=24.5'') and a low concentration (c=1.16), indicating that the cluster has not experienced the collapse of the core. We also derived the cluster age, by using the Pisa Evolutionary Library (PEL) isochrones, with three different amount of overshooting (namely, $\Lambda_{os}$=0.0, 0.10 and 0.25). From the comparison of the observed Color-Magnitude Diagram (CMD) and Main Sequence (MS) Luminosity Function (LF) with the theoretical isochrones and LFs, we find that only models with the inclusion of some overshooting ($\Lambda_{os}$=0.10-0.25) are able to reproduce the observables. By using the magnitude difference $\delta V_{SGB}^{He-Cl}=0.90$ between the mean level of the He-clump and the flat region of the SGB, we derive an age $\tau$=1.4$\pm$0.2 Gyr.Comment: Accepted to publication by A

Interband mixing between two-dimensional states localized in a surface quantum well and heavy hole states of the valence band in narrow gap semiconductor

Theoretical calculations in the framework of Kane model have been carried out in order to elucidate the role of interband mixing in forming the energy spectrum of two-dimensional carriers, localized in a surface quantum well in narrow gap semiconductor. Of interest was the mixing between the 2D states and heavy hole states in the volume of semiconductor. It has been shown that the interband mixing results in two effects: the broadening of 2D energy levels and their shift, which are mostly pronounced for semiconductors with high doping level. The interband mixing has been found to influence mostly the effective mass of 2D carriers for large their concentration, whereas it slightly changes the subband distribution in a wide concentration range.Comment: 12 pages (RevTEX) and 4 PostScript-figure

Observation of a new phase transition between fully and partially polarized quantum Hall states with charge and spin gaps at ν=2/3\nu = 2/3

The average electron spin-polarization $\cal P$ of two-dimensional electron gas confined in $\rm GaAs/GaAlAs$ multiple quantum-wells was measured by nuclear magnetic resonance (NMR) near the fractional quantum Hall state with filling factor $\nu={2/3}$. Above this filling factor (${{2/3}} \leq \nu < 0.85$), a strong depolarization is observed corresponding to two spin flips per additional flux quantum. The most remarkable behavior of the polarization is observed at $\nu ={{2/3}}$, where a quantum phase transition from a partially polarized (${\cal P} \approx {{3/4}}$) to a fully polarized (${\cal P} = 1$) state can be driven by increasing the ratio between the Zeeman and the Coulomb energy above a critical value $\eta_{c} = \frac{\Delta_{Z}}{\Delta_{C}} = 0.0185$.Comment: 4 pages including 4 figure

Udvikling af kundeattraktivitet over for strategisk vigtige leverandører

God leverandørperformance er ikke en selvfølge. Leverandørperformance afhænger imidlertid af andet end leverandørens evne til at styre egne interne processer. Hvordan leverandører performer, afhænger i høj grad også af kundens attraktivitet. Langt de fleste leverandører foretager således en bevidst prioritering af kunders attraktivitet med udgangspunkt i kunders økonomiske, ressourcemæssige og sociale attraktivitet. Artiklen illustrerer med udgangspunkt i et netværksperspektiv, hvordan JAMO via en bevidst håndtering af sine leverandørrelationer kan prioritere og øge sin kundeattraktivitet, og illustrerer samtidig, hvordan JAMO i praksis kan arbejde med attraktivitetsbegrebet over for seks strategisk vigtige kinesiske leverandører. Attraktivitetsbegrebet har tidligere ikke været forsøgt anvendt i praksis og har overvejende været beskrevet teoretisk