Ulrike Heiter

New grids of ATLAS9 atmospheres I: Influence of convection treatments on model structure and on observable quantities

Received ; accepted Abstract. We present several new sets of grids of model stellar atmospheres computed with modified versions of the ATLAS9 code. Each individual set consists of several grids of models with different metallicities ranging from (M/H) = 2.0 to +1.0 dex. The grids range from 4000 to 10000 K in Teff and from 2.0 to 5.0 dex in log g. The individual sets differ from each other and from previous ones essentially in the physics used for the treatment of the convective energy transport, in the higher vertical resolution of the atmospheres and in a finer grid in the (Teff, log g) plane. These improvements enable the computation of derivatives of color indices accurate enough for pulsation mode identification. In addition, we show that the chosen vertical resolution is necessary and sufficient for the purpose of stellar interior modelling.To explain the physical differences between the model grids we provide a description of the currently available modifications of ATLAS9 according to their treatment of convection. Our critical analysis of the dependence of the atmospheric structure and observable quantities on convection treatment, vertical resolution and metallicity reveals that spectroscopic and photometric observations are best represented when using an inefficient convection treatment. This conclusion holds whatever convection formulation investigated here is used, i.e. MLT(� = 0.5), CM and CGM are equivalent. We also find that changing the convection treatment can lead to a change in the effective temperature estimated from Stromgren color indices from 200 to 400 K.

New grids of ATLAS9 atmospheres - II. Limb-darkening coefficients for the Strömgren photometric system for A-F stars

Using up-to-date model atmospheres (Heiter et al. 2002) with the turbulent convection approach developed by Canuto et al. (1996, CGM), quadratic, cubic and square root limb darkening coecients (LDC) are calculated with a least square fit method for the Stromgren photometric system. This is done for a sample of solar metallicity models with eective temperatures between 6000 and 8500 K and with logg between 2.5 and 4.5. A comparison is made between these LDC and the ones computed from model atmospheres using the classical mixing length prescription with a mixing length parameter = 1:25 and= 0:5. For CGM model atmospheres, the law which reproduces better the model intensity is found to be the square root one for the u band and the cubic law for thev band. The results are more complex for the b andy bands depending on the temperature and gravity of the model. Similar conclusions are reached for MLT = 0:5 models. As expected much larger dierences are found between CGM and MLT with= 1:25. In a second part, the weighted limb-darkening integrals, b, and their derivatives with respect to temperature and gravity, are then computed using the best limb-darkening law. These integrals are known to be very important in the context of photometric mode identification of non-radial pulsating stars. The eect of convection treatment on these quantities is discussed and as expected dierences in the b coecients and derivatives computed with CGM and MLT= 0:5 are much smaller than dierences obtained between computations with CGM and MLT = 1:25.

The accretion/diffusion theory for λ Bootis stars in the light of spectroscopic data

Most of the current theories suggest the Bootis phenomenon to originate from an interaction between the stellar surface and its local environment. In this paper, we compare the abundance pattern of the Bootis stars to that of the interstellar medium and nd larger deciencies for Mg, Si, Mn and Zn than in the interstellar medium. A comparison with metal poor post-AGB stars showing evidence for circumstellar material indicates a similar physical process possibly being at work for some of the Bootis stars, but not for all of them. Despite the fact that the number of spectroscopically analysed Bootis stars has considerably increased in the past, a test of predicted eects with observations shows current abundance and temperature data to be still controversial.