Scandium: A key element for understanding Am stars

{\rm Context.} Atomic diffusion is believed to cause the abundance anomalies observed in AmFm stars. However, the detailed process has still not been well-established. For instance, two possible scenarios for the diffusion theory are presently envisaged. They differ mainly by the depth from which the abundance anomalies emanate. The first scenario predicts that the abundances are modified in the superficial regions of the star, just below the hydrogen convection zone. The second scenario predicts that a much deeper extension of the mixing zone exists due to the convection caused by Fe accumulation in regions below the hydrogen convection zone. {\rm Aims.} We calculate much more accurate radiative accelerations of Sc than previously, to better understand the observed abundance anomalies of this element. We believe that it is a key element to use as a diagnostic tool for understanding AmFm stars. {\rmMethods.} The method employed to obtain these radiative accelerations is based on an interpolation from the parameters of the so-called SVP parametric method. {\rm Results.} The radiative accelerations, shown here in a typical Am stellar model, are discussed in light of the observed anomalies of Ca and Sc. Our results suggest that the deeper mixing scenario is not entirely satisfactory: the mixing zone should be deeper than what is predicted by recent models to account for observed Sc underabundances. Our results seem more compatible with the scenario where the abundances anomalies are created in the superficial regions. However, only detailed evolutionary modelling with mass loss and diffusion of all important species, including Ca and Sc, with accurate radiative accelerations, will be able to give more insight into where the source of these anomalies occur in AmFm stars.Comment: 6 pages, 3 figures, accepted for publication in A&

Stratification of the elements in the atmospheres of blue horizontal-branch stars

Blue horizontal-branch (BHB) stars with $T_{\rm eff}$ approximately larger than 11500 K show several observational anomalies. In globular clusters, they exhibit low rotational velocities, abundance anomalies (as compared to cluster abundances), photometric jumps and gaps and spectroscopic gravities lower than predicted by canonical models. It is commonly believed that the low rotational velocities of these stars permit atomic diffusion to be efficient in their atmosphere thereby causing the observed anomalies. Recent detections of vertical stratification of iron (and some other chemical elements) in several BHB stars concur with this framework. In this paper, improved model atmospheres that include the vertical stratification of the elements are applied to BHB stars to verify if they can explain their observational anomalies. The results from theoretical model atmospheres are consistent with the photometric jumps and gaps observed for BHB stars in globular clusters. It is found that iron stratification in the theoretical models and that obtained from observations have similar tendancies. Our results also show that the spectroscopic gravities obtained while using chemically homogeneous model atmospheres to fit observations are underestimated. These results significantly strengthen the belief that atomic diffusion is responsible for these BHB-star anomalies.Comment: 6 pages, 6 figure

Modelling element distributions in the atmospheres of magnetic Ap stars

In recent papers convincing evidence has been presented for chemical stratification in Ap star atmospheres, and surface abundance maps have been shown to correlate with the magnetic field direction. Radiatively driven diffusion in magnetic fields is among the processes responsible for these inhomogeneities. Here we explore the hypothesis that equilibrium stratifications can, in a number of cases, explain the observed abundance maps and vertical distributions of the various elements. The investigation of equilibrium stratifications in stellar atmospheres with temperatures from 8500K to 12000K and fields up to 10 kG reveals considerable variations in the vertical distribution of the 5 elements studied (Mg, Si, Ca, Ti, Fe), often with zones of large over- or under-abundances and with indications of other competing processes (such as mass loss). Horizontal magnetic fields can be very efficient in helping the accumulation of elements in higher layers. A comparison between our calculations and the vertical abundance profiles and surface maps derived by magnetic Doppler imaging reveals that equilibrium stratifications are in a number of cases consistent with the main trends inferred from observed spectra. However, it is not clear whether such equilibrium solutions will ever be reached during the evolution of an Ap star.Comment: 7 pages, 6 figures, the paper will be published in Astronomy & Astrophysics, on November 200

Modelling of the scandium abundance evolution in AmFm stars

Scandium is a key element of the Am star phenomenon since its surface under-abundance is one of the criteria that characterise such stars. Thanks to the availability of a sufficiently complete set of theoretical atomic data for this element, reliable radiative accelerations for Sc can now be computed, which allows its behaviour under the action of atomic diffusion to be modelled. We explore the required conditions, in terms of mixing processes or mass loss, for our models to reproduce the observed surface abundances of Sc in Am stars. The models are computed with the Toulouse-Geneva evolution code, which uses the parametric single-valued parameter method for the calculation of radiative accelerations. Fingering mixing is included, using a prescription that comes from 3D hydrodynamical simulations. Other parameter-dependent turbulent mixing processes are also considered. A global mass loss is also implemented. When no mass loss is considered, the observed abundances of Sc are rather in favour of the models whose superficial layers are fully mixed down to the iron accumulation zone, although other mixing prescriptions are also able to reproduce the observations for the most massive model presented here ($2.0 M_\odot$). The models including mass loss with rates in the range of $[10^{-13};10^{-14}] M_\odot$/yr are compatible with some of the observations, while other observations suggest that the mass-loss rate could be lower. The constraints brought by the modelling of Sc are consistent with those derived using other chemical elements.Comment: 9 pages, 7 figure

Photometric determination of rotation axis inclination, rotation rate, and mass of rapidly rotating intermediate-mass stars

Intermediate-mass stars are often fast rotators, and hence are centrifugally flattened and affected by gravity darkening. To analyse this kind of stars properly, one must turn to 2D models to compute the visible radiative flux and to take the geometrical effect of the star inclination into account. Assuming a given stellar age and chemical composition, we aim to derive the mass and rotation rates of main sequence fast rotating stars, along with their inclination, from photometric quantities. We chose three observables that vary with mass, rotation, and inclination: the infrared flux method temperature T_IRFM, the Str\"omgren c1 index, and a second index c2 built in the same way, but sensitive to the UV side of the Balmer jump. These observables are computed from synthetic spectra produced with the PHOENIX code and rely on a 2D stellar structure from the ESTER code. These quantities are computed for a grid of models in the range 2 to 7~M_Sun, and rotation rates from 30% to 80% of the critical rate. Then, for any triplet (T_IRFM, c1, c2), we try to retrieve the mass, rotation rate, and inclination using a Levenberg-Marquardt scheme, after a selection step to find the most suitable starting models. Hare-and-hound tests showed that our algorithm can recover the mass, rotation rate, and inclination with a good accuracy. The difference between input and retrieved parameters is negligible for models lying on the grid and is less than a few percent otherwise. An application to the real case of Vega showed that the u filter is located in a spectral region where the modelled and observed spectra are discrepant, and led us to define a new filter. Using this new filter and subsequent index, the Vega parameters are also retrieved with satisfactory accuracy. This work opens the possibility to determine the fundamental parameters of rapidly rotating early-type stars from photometric space observations.Comment: 12 pages, 18 figure

Teaching Hong Kong Chinese students to read and write about English literature : a proposal for curriculum renewal.

SIGLEAvailable from British Library Document Supply Centre-DSC:DX200776 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Dynamics of Electron Transport in Cytochrome P450 Systems Studied at Sub-Zero Temperature

Experimentation in fluid mixed solvents (1 : 1 v/v phosphate buffer ethylene glycol) at sub-zero temperatures has permitted us to record the two univalent reductions of the bacterial cytochrome P450 by the natural electron donor putidaredoxin, without recycling or alternative pathway reactions. Dynamic evidence shows the formation of putidaredoxincytochrome complexes prior to electron transfer. The complex formation is rate limiting in the first reduction and in our experimental conditions. The kinetics of binding between the two oxidized proteins has also been recorded in the same medium under various conditions of concentration, temperature and ionic strength. At very low ionic strength, the rate is limited by electrostatic repulsion between the two negatively charge proteins; above I = 0.03 this effect appears negligible and the affinity seems to be governed by hydrophobic interaction free energy

Dynamics of Electron Transport in Cytochrome P450 Systems Studied at Sub-Zero Temperature

Experimentation in fluid mixed solvents (1 : 1 v/v phosphate buffer ethylene glycol) at sub-zero temperatures has permitted us to record the two univalent reductions of the bacterial cytochrome P450 by the natural electron donor putidaredoxin, without recycling or alternative pathway reactions. Dynamic evidence shows the formation of putidaredoxincytochrome complexes prior to electron transfer. The complex formation is rate limiting in the first reduction and in our experimental conditions. The kinetics of binding between the two oxidized proteins has also been recorded in the same medium under various conditions of concentration, temperature and ionic strength. At very low ionic strength, the rate is limited by electrostatic repulsion between the two negatively charge proteins; above I = 0.03 this effect appears negligible and the affinity seems to be governed by hydrophobic interaction free energy

AmFm and lithium gap stars: Stellar evolution models with mass loss

A thorough study of the effects of mass loss on internal and surface abundances of A and F stars is carried out in order to constrain mass loss rates for these stars, as well as further elucidate some of the processes which compete with atomic diffusion. Self-consistent stellar evolution models of 1.3 to 2.5 M_sun stars including atomic diffusion and radiative accelerations for all species within the OPAL opacity database were computed with mass loss and compared to observations as well as previous calculations with turbulent mixing. Models with unseparated mass loss rates between 5 x 10^-14 and 10^-13 M_sun/yr reproduce observations for many cluster AmFm stars as well as Sirius A and o Leonis. These models also explain cool Fm stars, but not the Hyades lithium gap. Like turbulent mixing, these mass loss rates reduce surface abundance anomalies; however, their effects are very different with respect to internal abundances. For most of the main sequence lifetime of an A or F star, surface abundances in the presence of such mass loss depend on separation which takes place between log(Delta M/M_star)= -6 and -5. The current observational constraints do not allow us to conclude that mass loss is to be preferred over turbulent mixing (induced by rotation or otherwise) in order to explain the AmFm phenomenon. Internal concentration variations which could be detectable through asteroseismic tests should provide further information. If atomic diffusion coupled with mass loss are to explain the Hyades Li gap, the wind would need to be separated.Comment: 27 pages, 25 figures, accepted for publication in A&