Designating a star as of A-type is a result of spectral classification. After separating the peculiar stars from those deemed to be normal using the results of a century of stellar astrophysical wisdom, I define the physical properties of the “normal” stars. The hotter A stars have atmospheres almost in radiative equilibrium. In the A stars convective motions can be found which increase in strength as the temperature decreases.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The current status of NLTE model atmosphere calculations of A type stars is reviewed. During the last decade research has concentrated on solving the restricted NLTE line formation problem for trace elements assuming LTE model atmospheres. There is a general lack of calculated NLTE line blanketed model atmospheres for A type stars, despite the availability of powerful methods and computer codes that are able to solve this task. Some directions for future model atmosphere research are suggested.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We present new numerical models of line-driven stellar winds of A supergiants. Statistical equilibrium (NLTE) equations of the most abundant elements are solved, and properly obtained occupation numbers are used to calculate consistent radiative force and radiative heating terms. Wind density, velocity, and temperature are calculated as solutions of the model's hydrodynamical equations. Our models allow for the calculation of the wind mass-loss rate and terminal velocity.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Model atmospheres of A and B stars are computed taking into account magnetic line blanketing. These calculations are based on the new stellar model atmosphere code LLMODELS which implements a direct treatment of the line opacities and ensures an accurate and detailed description of the line absorption. The anomalous Zeeman effect was calculated for field strengths between 1 and 40kG and a field vector perpendicular to the line of sight. The magnetically enhanced line blanketing changes the atmospheric structure and leads to a redistribution of energy in the stellar spectrum. The most noticeable feature in the optical region is the appearance of the $\lambda$5200 broad, continuum feature. However, this effect is prominent only in cool A stars and disappears for higher effective temperatures. The presence of a magnetic field produces an opposite variation of the flux distribution in the optical and the UV regions. A deficiency of the UV flux is found for the whole range of considered effective temperatures, whereas the “null wavelength region” where the flux remains unchanged shifts towards the bluer wavelengths for higher temperatures.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Ultra-high signal-to-noise, high dispersion spectroscopy over the wavelength range \mbox{$\lambda 4487 - 4553$} shows Vega to be a rapidly rotating star $(V_{\rm eq}\sim 160\,{\rm km\,s^{-1}})$ seen almost pole-on. These data, analyzed anew, are combined with analyses of the hydrogen lines (${\rm H}\gamma, {\rm H}\beta$ and ${\rm H}\alpha$) and the latest absolute continuum flux for Vega to yield the following results: $V \sin i=21.9\pm 0.1\,{\rm km\,s^{-1}}$, polar $T_{\rm eff}=9680\pm 10\,{\rm K}$, polar $\log g=4.00\pm 0.02\,{\rm dex}$, $V_{\rm eq}=160\pm 10\,{\rm km\,s^{-1}}$, $\xi_{\rm T}=1.08\pm 0.02\,{\rm km\, s^{-1}}$ and $i=7.9\pm 0.5^{\circ}$. The variations in $T_{\rm eff}$ and $\log g$ over the photosphere total 350 K and 0.06 dex, respectively. The mean $T_{\rm eff}=9510\pm 10\,{\rm K}$ and mean $\log g=3.97\pm 0.02\,{\rm dex}$ agree with the spherical model values derived here and by others.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

COWLEY: For O. Kochukov: This concerns the λ5200 feature which you could reproduce in the cooler Ap stars, but not in the hotter ones: 1) Do you know which atoms or ions cause the depression in the cooler stars?

2) In the hotter stars, can you suggest a cause for the observed depressions in these objects? To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We discuss the internal structure of stars in the mass range 1.5 to 4 $M_\odot$ from the PMS to the subgiant phase with a particular emphasis on the convective core and the convective superficial layers. Different physical aspects are considered such as overshooting, treatment of convection, microscopic diffusion and rotation. Their influence on the internal structure and on the photospheric chemical abundances is briefly described.

The role of binarity in determining the observed properties and as a tool to constrain the internal structure is also introduced and the current limits of theories of orbital evolution and of available binary datasets are discussed.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

In this review, I will briefly discuss the hypotheses made in the treatment of modern rotating stellar models and review the expected efficiency of mixing along the HR diagram. The role of mixing in the localization of abundance anomalies will also be discussed. Finally, I will show how mass loss and gravitational settling of helium may influence the evolution of rotating stars, and how A stars can play a unique role in constraining our models.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Young A-type stars in the pre–main-sequence (PMS) evolutionary phase are particularly interesting objects since they cover the mass range ($\sim$1.5-4 $M_\odot$) which is most sensitive to the internal conditions inherited from the protostellar phase. In particular, they undergo a process of thermal relaxation from which they emerge as fully radiative objects contracting towards the Main Sequence. A-type stars also show intense surface activity (including winds, accretion, pulsations) whose origin is still not completely understood, and infrared excesses related to the presence of circumstellar disks and envelopes. Disks display significant evolution in the dust properties, likely signalling the occurrence of protoplanetary growth. Finally, A-type stars are generally found in multiple systems and small aggregates with lower mass companions.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Double-lined-eclipsing binaries are the essential systems for the measurement of stellar masses and radii. About 50-60 systems have components (mostly A-stars) for which these values are known with an uncertainty less than 1-2%. Therefore, these systems are very suitable to improve our understanding of stellar structure and evolution. In this paper, special attention is given to the assessment of the role of internal rotation of the early-type stars in selected double-lined binaries (i.e., EK Cep, PV Cas, and $\theta^2$ Tau): it is shown that adoption of rapidly rotating cores for such stars permits the models to be in very good agreement with the observational results including the apsidal advance rates.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Putting their chemically peculiarities aside, Ap and Am stars have many common properties. The discriminating property between them could be the rotation of their interiors beneath their slowly rotating surface. In the PV Cas (HD 240208) binary system whose light curve shows Ap-like variation, the agreement between the theoretical and observed apsidal advance rate is satisfied only with differentially rotating models of the component stars in which rapid rotation is extended almost to the surface. Thus, it seems that there is a steep rotation rate gradient near the surface of the Ap stars. The conclusion we reach in the analysis of PV Cas system leads us to introduce the effective mass as a new conceptual tool in stellar astrophysics: $M_{\rm eff}= M_\star (1-\bar{\Lambda})^{1.75}$, where $M_\star$ and $\bar{\Lambda}$ are mass of the star and average of ratio of centrifugal to the gravitational acceleration throughout the model, respectively. We find that the effective mass of PV Cas A whose true mass is $2.82\pm 0.05\,M_\odot$ is $2.6\,M_\odot$ for both solar and metal rich compositions.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We reanalyzed high quality spectra of 158 stars of spectral types A0–F1 and $v\,\sin{i}$ values between 60 and 150 km s$^{-1}$. Using a least squares deconvolution technique we extracted high $S/N$ broadening profiles and determined the loci of the Fourier transform zeros $q_{1}$ and $q_{2}$ where the $S/N$-ratio was high enough. For 78 stars $q_{2}$ could be determined and the ratio $q_{2}/q_{1}$ was used as a shape parameter sensitive to solar-like differential rotation (the equatorial velocity is faster than the near polar velocities). Seventy-four of the 78 stars have values of $q_{2}/q_{1}$ consistent with solid body rotation; in four of the 78 cases, values of $q_{2}/q_{1}$ are not consistent with rigid rotation. Although these stars may be binaries, none of their profiles shows any signatures of a companion. The Fourier transforms do not indicate any distortions and the broadening profiles can be considered due to single objects. One of these candidates may be an extremely rapid rotator seen pole-on, but for the other three stars of spectral types as early as A6, differential rotation seems to be the most plausible explanation for the peculiar profiles.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The rotational evolution of the radiative zone of magnetic Ap stars is investigated with numerical simulations. An angular-velocity profile decreasing with axis distance in combination with a magnetic field leads to a magnetorotational instability. The resulting flows efficiently transport angular momentum outwards. The corresponding decay of angular-velocity gradients in the radiative zone is estimated to take about 10–100 million years.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Using an homogeneous sample of $v\sin i$ values for A-type main sequence stars (Royer et al. 2002), the equatorial velocity ($v$) distributions are determined as function of spectral class, from B9 to F2. The chemically peculiar and binary stars are discarded. These distributions of “normal” stars are discussed in terms of stellar formation and evolution, in particular the remaining bimodality observed for the earliest spectral types of the sample. We show that late B and early A-type main-sequence stars have genuine bimodal distributions of true equatorial rotational velocities probably due to the phenomena of angular momentum loss and of redistribution the star underwent before reaching the main sequence. A striking lack of slow rotators is noticed among intermediate and late A-type stars. The bimodal-like shape of their true equatorial rotational velocity distributions could be due to evolutionary effects.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

PISKUNOV: I would like to start this discussion with evolutionary aspects. There are a few recent developments both observational and theoretical. So, some might disagree and others might agree that there are many things happening concerning the pre-Main sequence evolution of early A type and Ap stars and that is what makes them different. Would anybody like to comment?To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Convection is one of the most intricate processes studied in stellar astrophysics and has challenged both theorists and observers since the beginnings of astrophysics. But during the last two decades observational data of unprecedented resolution and accuracy have been collected in solar and stellar research which permit a new look at the field. An enormous increase of computer speed now permits solving more complete model equations with more accurate numerical approximations. Modelling and theoretical understanding of convection, however, are lagging behind observational progress and are still wanting.

As a background to the contributions to this session on convection, I first provide an overview on its basic physics and its observational evidence. I point out why astrophysicists have a general interest in improvements of our understanding of stellar convection and then focus on convection in A-stars with their unique combination of convection zones. I summarise how this richness of different manifestations can arise in A-stars, such as convection zones near the surface and in the core, several on top of each other, or some of them depleted by diffusion processes, suppressed by or even creating magnetic fields, suspected to create a chromosphere in some of them, or influenced by binaries, to name just a few. In the last part I will present a few recent results on modelling of convection in A-stars.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Convection and turbulence in stellar atmospheres have a significant effect on the emergent flux from A-type stars. The recent theoretical advancements in convection modelling have proven to be a challenge to the observers to obtain measurements with sufficient precision and accuracy to permit discrimination between the various predictions.

A discussion of the current observational techniques used to evaluate the various convection theories is presented. These include filter photometry, spectrophotometry, hydrogen lines, and metal lines. The results from these techniques are given, along with the successes and limitations.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Radiation hydrodynamics simulations have been used to produce numerical models of the convective surface layers of a number of stars, including the Sun and other stars on or above the main-sequence, white dwarfs of type DA, and red supergiants.

While granulation of main-sequence solar-type stars resembles that of the Sun, the convective velocity fields of F-type stars are much more violent and accompanied by strong pulsations. The properties of the thin convection zone(s) of A-type stars differ again (see Fig. 1). In this contribution, the pattern and dynamics of their surface granulation, the photospheric velocity fields and their effect on line profiles are investigated, based on new 3-D models of surface convection in main-sequence A-type stars with $T_{\rm eff}$=8500 K and 8000 K. Furthermore, we will look below the surface to study overshoot and the interaction of the surface convection zone and the deeper helium II convection zone.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We present the results of 3–D nonlinear simulations of magnetic dynamo action by core convection within A-type stars of 2 $M_{\odot}$ with a range of rotation rates. We consider the inner 30% by radius of such stars, with the spherical domain thereby encompassing the convective core and a portion of the surrounding radiative envelope. The compressible Navier-Stokes equations, subject to the anelastic approximation, are solved to examine highly nonlinear flows that span multiple scale heights, exhibit intricate time dependence, and admit magnetic dynamo action. Small initial seed magnetic fields are found to be amplified greatly by the convective and zonal flows. The central columns of strikingly slow rotation found in some of our progenitor hydrodynamic simulations continue to be realized in some simulations to a lesser degree, with such differential rotation arising from the redistribution of angular momentum by the nonlinear convection and magnetic fields. We assess the properties of the magnetic fields thus generated, the extent of the convective penetration, the magnitude of the differential rotation, and the excitation of gravity waves within the radiative envelope.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The convection code of Nordlund & Stein has been used to evaluate the 3D, radiation-coupled convection in a stellar atmosphere with $T_{\rm eff} = 7300$K, log $g = 4.3$ and $[{\rm Fe}/{\rm H}]= 0.0$, corresponding to a main-sequence A9 star. I present preliminary comparisons between the 3D-simulation and a conventional 1D stellar structure calculation, and elaborate on the consequences of the differences.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html