Modelling convection in A star atmospheres. Bisectors and lineshapes of HD108642

We present a code, VeDyn, for modelling envelopes and atmospheres of A to F stars focusing on accurate treatment of convective processes. VeDyn implements the highly sophisticated convection model of Canuto and Dubovikov (1998) but is fast and handy enough to be used in practical astrophysical applications. We developed the HME envelope solver for this convection model furtheron to consistently model the envelope together with the stellar atmosphere. The synthesis code SynthV was extended to account for the resulting velocity structure. Finally, we tested our approach on atomic line bisectors. It is shown that our synthetic line bisectors of HD108642 bend towards the blue and are of a magnitude comparable to the observed ones. Even though this approach of modelling convection requires the solution of a coupled system of nonlinear differential equations, it is fast enough to be applicable to many of the investigation techniques relying on model atmospheres.Comment: 3 pages, 3 figure

Abundance analysis of 5 early-type stars in the young open cluster IC2391

It is unclear whether chemically peculiar stars of the upper main sequence represent a class completely distinct from normal A-type stars, or whether there exists a continuous transition from the normal to the most peculiar late F- to early B-type stars. A systematic abundance analysis of open cluster early-type stars would help to relate the observed differences of the chemical abundances of the photospheres to other stellar characteristics, without being concerned by possible different original chemical composition. Furthermore, if a continuous transition region from the very peculiar to the so called normal A-F stars exists, it should be possible to detect objects with mild peculiarities. As a first step of a larger project, an abundance analysis of 5 F-A type stars in the young cluster IC2391 was performed using high resolution spectra obtained with the UVES instrument of the ESO VLT. Our targets seem to follow a general abundance pattern: close to solar abundance of the light elements and iron peak elements, heavy elements are slightly overabundant with respect to the sun, similar to what was found in previous studies of normal field A-type stars of the galactic plane. We detected a weakly chemically peculiar star, HD74044. Its element pattern contains characteristics of CP1 as well as CP2 stars, enhanced abundances of iron peak elements and also higher abundances of Sc, Y, Ba and Ce. We did not detect a magnetic field in this star (detection limit was 2kG). We also studied the star SHJM2, proposed as a pre-main sequence object in previous works. Using spectroscopy we found a high surface gravity, which suggests that the star is very close to the ZAMS.Comment: 7 pages + online materia

On the

Context. It is still a matter of debate whether the group of λ Bootis stars have been homogeneously defined. A widely discussed working hypothesis postulates that two apparent, solar-abundant stars of an undetected spectroscopic binary system mimic a single metal-weak spectrum thus preventing any reliable analysis of the group characteristics. Aims. We investigate whether the proposed spectroscopic binary model can explain the observed abundance pattern as well photometric metallicity indices and what the percentage of undetected spectroscopic binary systems is. Methods. We used the newest available stellar atmospheres to synthesize 105 hypothetical binary systems in the relevant astrophysical parameter range. These models were used to derive photometric indices. As a test, values were generated for single stellar atmospheres, Vega and two typical λ Bootis stars, HD 107233 and HD 204041. Results. The synthesized indices fit the standard lines and the observations of the three stars excellently. For about 90% of the group members, the spectroscopic binary hypothesis cannot explain the observations. Conclusions. A careful preselection of λ Bootis stars results in a homogeneous group of objects that can be used to investigate the group characteristics

Magnetic field measurements of Ap stars

HD 18610 was included in our project on abundance determination of rapidly oscillating (roAp) and non–oscillating (noAp) cool chemically peculiar stars. Already a preliminary investigation of the raw spectra indicated the presence of an intense magnetic field which was confirmed by a more detailed analysis.
To determine the magnetic field strength we primarily used the Fe i

On the detection of chemically peculiar stars using

We have summarized all $\Delta a$ measurements for galactic field stars (1474 objects) from the literature published over more than two decades. These measurements were, for the first time, compiled and homogeneously analyzed. The $\Delta a$ intermediate band photometric system samples the depth of the 5200 Å  flux depression by comparing the flux at the center with the adjacent regions with bandwidths of 110 Å  to 230 Å. Because it was slightly modified over the last three decades, we checked for systematic trends for the different measurements but found no correlations whatsoever. The $\Delta a$ photometric system is most suitable to detecting magnetic chemically peculiar (CP) stars with high efficiency, but is also capable of detecting a small percentage of non-magnetic CP objects. Furthermore, the groups of (metal-weak) λ Bootis, as well as classical Be/shell stars, can be successfully investigated. In addition, we also analyzed the behaviour of supergiants (luminosity class I and II). On the basis of apparent normal type objects, the correlation of the 3σ significance limit and the percentage of positive detection for all groups was derived. We compared the capability of the $\Delta a$ photometric system with the $\Delta (V1 - G)$ and Z indices of the Geneva 7-color system to detect peculiar objects. Both photometric systems show the same efficiency for the detection of CP and λ Bootis stars, while the indices in the Geneva system are even more efficient at detecting Be/shell objects. On the basis of this statistical analysis it is possible to derive the incidence of CP stars in galactic open cluster and extragalactic systems including the former unknown bias of undetected objects. This is especially important in order to make a sound statistical analysis of the correlation between the occurrence of these objects and astrophysical parameters such as the age, metallicity, and strength of global, as well as local, magnetic fields

High-resolution spectroscopy of the

Aims.We carried out a detailed spectroscopic study of the δ Sct star 44 Tau to investigate its element abundances and stellar parameters as well as to provide an identification of its non-radial oscillation modes. Methods.We acquired high-resolution high-signal-to-noise ratio time-series spectroscopy of 44 Tau from two observing sites in Europe during a time span of 30 days. We determined atmospheric element abundances and stellar properties using spectroscopic methods. Due to the low value of $v\sin{i}$ the mode identification was carried out by applying the moment method. Results.A very low value of the projected rotational velocity of $v\sin{i}$ = 2 ± 1 km s-1 was determined. We detected spectroscopically 12 independent frequencies between 6.8 and 11.6 d-1 and four combination frequencies by analyzing the line moments as well as the pixel-intensities across the profiles of carefully selected Fe absorption lines. The mode identification applying the moment method yielded six axisymmetric pulsation modes. Two frequencies are low-degree prograde pulsation modes. A rather low value of log g = 3.6 was determined implying an evolved evolutionary status of 44 Tau. The derived atmospheric element abundances are close to solar. Conclusions.Our analysis revealed that 44 Tau is an exceptionally slowly rotating δ Scuti star pulsating with a rich spectrum of axisymmetric pulsation modes. This simplifies the seismic modeling since rotational perturbations of higher order can be neglected