A study is undertaken to determine abundances in rapidly rotating main sequence A stars (v sin i = 100 − 180 km/s) in order to investigate if non-solar abundance patterns found in slow rotators (Lemke 1989, 1990) persist toward higher rotational velocities. This will provide some insight into how the Am phenomenon and other chemical peculiarities are connected with slow rotation and diffusion processes.

Most stars were found to have a rather solar composition. At least one star, HR5023, is enriched in Ba by approximately one order of magnitude relative to the sun at a v sin i of 110km/s. Both C and Ba will be discussed.

The Teff value, according to the definition of this parameter, is computed with the procedure described; the observations are then compared with the flux distribution from Kurucz’ LTE blanketed models for various values of the metallicity parameter [Z/H].

Then we derive the bolometric luminosity L/L⊙ by combining the stellar bolometric flux with the values of the trigonometric parallaxes and of the colour excess found in the literature. These luminosities are then plotted on evolutionary sequences; from the position of some λ Boo stars (ρ Vir and π1 Ori), we argue that their evolutionary stage, if interpreted as that of stars having left the main sequence, is too late to keep the λ Boo abundance anomalies.

These same λ Boo stars, plotted on pre-main sequence evolutionary tracks, would indicate increasing λ Boo character when the stars are approaching the main sequence showing the strongest character in λ Boo and 29 Cyg, the two stars nearest the ZAMS.

A search for λ Bootis stars in two OB associations (Orion and Lacerta OB1) has been carried out. One clear example of a ZAMS λ Bootis star has been discovered in Orion OB1. Both Orion OB1 and Lacerta OB1 contain, as well, a number of mildly metal-weak A-stars which may be λ Bootis stars in-the-making.

The observed hydrogen-line profiles of four λ Boo type stars— HD 111786, HD 125162, HD 179791 and HD 192640 are presented. Existence of two separate groups based on the shape of these lines is confirmed. Radial velocity (Vr = −5 ± 3 kms−1) and projected rotational velocity (vsini = 130 ± 10 kms−1) for HD 111786 are measured.

It was shown by Maitzen & Pavlovski (1989) that a majority of λ Bootis stars stand out by negative Δα indices. Using this as detection tool, three candidate stars with λ Bootis characteristics are picked up in open cluster Praesepe.

Among 67 early A-type stars at high galactic latitudes, selected to have both small m1 and large c1 Strömgren photometric indices, we find about one third to have quite normal dwarf or subgiant spectra at MK classification dispersion. Those stars with peculiarities included 10 new λ Bootis stars, though none of these appeared significantly evolved. We wonder whether some of the metal-weak dwarf stars in our set may be field horizontal-branch equivalents of the blue, downward-curving, low-luminosity end of the blue horizontal-branch stars.

Dwarf Cepheids (large-amplitude δ Sct variables and SXPhe variables in the GCVS) are late A-type stars near light maximum and early-F stars near light minimum. In the lower part of the instability strip where these stars are found the pulsation constant Q varies only slightly. As pointed out by Andreasen, Hejlesen, and Petersen (1983) this makes it easy to transform a theoretical HR or alternatively a (logTe, logg) diagram to a theoretical logTe, logP0) diagram by the use of the relation. This follows from the equations:

where P0 is the fundamental period of pulsation. The periods, of course, can be determined with very high accuracy.

Since the last IAU Colloquium devoted to Ap stars, die continuous development of high signal to noise observations and of computerized analysis tools have greatly increased the effectiveness of abundance anomalies as a probe of hydrodynamical processes in stars. Until the 1980s, they seemed limited to ApBp and AmFm stars where the effects are largest. More recently the abundances of Li and He have been suggested to play important diagnostic roles for hydrodynamics in both the Sun (Proffitt and Michaud 1991b, Pinsonneault et al. 1989) and Halo stars (Proffitt and Michaud 1991a, Pinsonneault et al. 1992, Vauclair 1988, Charbonnel et al. 1992, Deliyannis and Demarque 1991). The observation of a Li abundance gap in F stars of clusters has raised the question of the link between these objects and the AmFm stars (Michaud 1986, Charbonneau and Michaud 1988, 1991) or alternately with solar type stars. The observed Li abundance on die subgiant branch sheds some light on this problem (Charbonneau et al. 1989; Charbonnel and Vauclair 1992). In solar type stars, the observed Li abundance constrains the superficial turbulence that may inhibit surface He gravitational settling (Proffitt and Michaud 1991b) whose effects can be detected using solar seismology (Guzik and Cox 1992, Dziembowski et al. 1992).

We present the main aspects of diffusion calculations in the simplest realistic diffusion model for main-sequence CP stars. The results obtained with this kind of computations are generally used in modelling AmFm and HgMn stars. Important improvements in this field, essentially due to the new atomic data banks, may be expected in the near future.

Progress made in spectroscopy and in the diffusion theory permits now to make severe comparisons, based on line profiles, between theory and observation.

We first review transport processes which are present in the atmospheric layers of Ap stars and discuss their relative importance. We then show that mass loss could play a key role for the creation of abundance maps. A mass loss model is proposed for 53 Cam and is compared, by spectrum synthesis, with visible and IUE high resolutions observations. The model accounts well for the line profiles of several elements with an exception for Ti. Furthermore, the abundance stratification predicted by this model gives close agreement with the large variation of the abundances of Cr and Fe found between the visible and UV domains. The diffusion-mass loss model finally permits to give a simple interpretation of the peculiar Ca II K lines observed in many Ap SrCrEu stars and in particular in 53 Cam, β CrB and HD 191742.

The role played by rotationally-induced mixing in the diffusion-based models for non-magnetic chemically peculiar stars is investigated. This paper focuses on one specific rotationally controlled mixing mechanism, namely thermally-driven meridional circulation. Its effects on the time evolution of chemical abundances are illustrated by means of three specific examples. The first two concern the diffusion model for FmAm stars, where it is shown that while circulation has a determining influence on the settling of Helium, it has no significant effect on the diffusion of heavier metals once the He superficial convection zone has disappeared. The third example is concerned with the diffusion/mass loss model for λBootis stars. It is shown that the inclusion of circulation prevents the appearance of generalized underabundances at any epoch of the evolution, indicating that the diffusion/mass loss model for these objects must be abandoned.

Non–LTE calculations for neutral C, N, and O are performed to determine the accurate abundances of these elements in the atmosphere of Vega by taking the non–LTE effects realistically into consideration. It is shown that C and O are mildly deficient by ∼0.2–0.3dex while N shows an appreciably large underabundance by ∼0.8dex.

By using the ATLAS9 code which requires the new opacity data from Kurucz (1992), we derived a new atmospheric model for Vega with parameters Teff=9500 K, log g=3.95, [M/H]=−0.5, ξ=2 kms s−1. The model was fixed by comparing observed and computed energy distributions and Balmer profiles.

The stationary state of the element stratification under appropriate turbulence is investigated. Equation of the stationary state is derived and solved under several simplifications. Cases of Ga and Al are studied. Al is predicted to be underabundant, but the abundance is rising with decreasing effective temperatures of the stars. Different results obtained using two methods of finding the stationary Ga stratification are indicated.

Conventional wisdom holds that early-type and late-type stars have very different outer atmospheres, because the early-type stars lack deep convective zones. I argue that the magnetic chemically peculiar (CP) stars hotter than about spectral type A2 display many of the activity phenomena seen in the most active late-type stars. In particular, many CP stars are luminous nonthermal radio and coronal x-ray sources like the RS CVn systems. A wind-fed magnetosphere model has been proposed to explain both the nonthermal radio and the x-ray emission. In this model the stellar wind plays the role of a mechanical energy source analogous to the role played by convection in the active late-type stars.

So far, neither chromospheres nor stellar winds have been directly detected in main-sequence A stars. While radiative diffusion requires extremely weak stellar winds to reproduce chemical anomalies (10−15 to 10−12M⊙yr−1), two independent direct searches for mass loss set up upper limits to 10−10 M⊙yr−1, which is still several orders of magnitude higher. We discuss some new recent possibilities to detect chromospheres which arise thanks to new NLTE model atmospheres. In the near future, some progress is also expected from new observations of Lyman α with HST and from the increased sensitivity of ROSAT in the X-ray domain.

A brief review is presented of the evidence for magnetospheres surrounding the upper main sequence helium peculiar stars. Some of the underlying physical processes are discussed. The helium strong stars all appear to possess comparatively dense magnetospheres. The phenomenon is also present among some helium weak stars, but is quite rare.