Observations of variable broadband linear polarization in magnetic Ap stars (due to the transverse Zeeman effect), when combined with measurements of the mean longitudinal field Bɩ can in some cases allow one to determine the angles i and β (which describe the inclination of the stellar axis of rotation and the obliquity of the magnetic axis to the rotation axis) much more accurately than these angles can be determined from observations of Bɩ alone. Such variable intrinsic linear polarization has been observed for a number of stars; the effect is generally detectable only in cool Ap stars of unusually large field strength. We discuss the data and simple modelling for the stars HD 24712 = HR 1217, HD 137909 = β CrB, and HD 62140 = 49 Cam.

Diamagnetic effects as heating and cooling mechanisms at stellar surfaces have been ignored in the past. This is due to the misconseption that the effect, when averaged over a velocity distribution, vanishes. We explore here the relevance of the effect to the observed luminosity and color modulations observed in CP stars.

Different drift velocities of electrons and ions in response to viscous forces exerted by neutral atoms generate current density j and magnetic field B, where B is proportional to vorticity ω of the fluid. Magnetic vortex tubes (MVTs) form arrays on a hierarchy of scales. MVTs are basic to the magnetism of all astrophysical systems, conferring a structure of aligned filaments. In the Sun a torsional oscillation generates oscillatory vorticity, and hence an oscillatory magnetic field. The same mechanism is proposed for the Ap stars, but with “pole-on” viewing. Resonance-line radiation pressure segregates elements within MVTs of Ap stars, where the anomalous concentrations are preserved. However, variation of the 30 MG magnetic fields of AM Her white dwarfs may be due to precession of an MVT. There is reason to attribute common magnetic flux to the Sun, Ap stars, white dwarfs and neutron stars

A package of computer-programs is presented, which is suited for the evaluation of stellar spectra in regard of line positions in order to determine the magnetic field strength and the radial velocity. The programs have been developed for the calculation of the measuring data obtained from photographic spectrograms but could be generalized for the online observation in a spectrograph at the telescope using an electronic detector.

The network of programs may be amplified deliberately as required for any special application. For the exchange of data among cooperating institutes we propose the standardization of programs and data-files as outlined below.

A method for determining the strength of a stellar magnetic field is presented, which is based on the requirement that the scatter of abundances from various lines should be minimized on the best choice of the field strength and the microturbulence. This procedure is applied to o Peg and cool Ap stars, showing that the resulting solutions are consistent with those derived from other methods.

Ap stars whose spectral lines are resolved in several magnetically split components are of particular interest, because their magnetic field can be diagnosed with unrivalled precision, in an essentially approximation-free manner. We are pursuing a systematic programme of search and study of such stars. At present 25 of them are known, of which 13 have been discovered in the course of the present project (Mathys 1990, hereafter Paper I; Mathys & Lanz 1992, hereafter Paper II; this paper).

Up to now, we have obtained observations of 22 of these Ap stars with resolved magnetically split lines. We have recorded high-resolution spectra of a region containing, among others, the three lines Cr ɪɪ λ6147.154, Fe ɪ λ6147.741, and Fe II λ 6149.258. The Zeeman patterns of these three lines are, respectively, close to a triplet, close to a quadruplet, and a doublet. The doublet Fe ɪɪ λ 6149.258 is particularly interesting, because of its large Landé factor (2.7): it can be resolved even in stars with fairly modest fields.

Broadband linear polarization in the spectra of Ap stars is believed to be due to differential saturation between σ and π Zeeman components in spectral lines. This mechanism has been known for a long time to be the main agent of a similar phenomenon observed in sunspots. Since this phenomenon has been carefully calibrated in the solar case, it can be confidently used to deduce the magnetic field of Ap stars.

Given the magnetic configuration of a rotating star, it is possible to deduce the broadband polarization at any phase. Calculations performed for the oblique dipole model show that the resulting polarization diagrams are very sensitive to the values of i (the angle between the rotation axis and the line of sight) and β (the angle between the rotation and magnetic axes). The dependence on i and β is such that the four-fold ambiguity typical of the circular polarization observations ((i,β), (β,i), (π-i,π-β), (π-β,π-i)) can be removed.

Magnetic fields in Vega, ϒ Geminorum, and Sirius are searched by using empirical and theoretical relations of magnetic-field strength to the relative differences between the equivalent widths of each pair of the two Fe II lines near 6150 and 4400 Å. We estimated the possible strengths of their magnetic fields as follows: 0.0 kG or 2.5 – 3.5 kG for Vega, 0 – 2 kG or 2.8 – 3.0 kG for ϒ Gem, and 0.0 kG or ∼ 2(?) – 2.5 kG for Sirius. The empirical relation yields the former value in each star, which is consistent with those determined through polarization measurements, while the theoretical relation the latter with systematically large values.

The existence of non-magnetic CP stars among the ones in the CP2 and CP4 groups is discussed. Assuming to be non-magnetic a star in which the magnetic field has been measured but no value in excess of the 3σ level has been detected, the implications of the spectrum and/or light variability observed in some such stars are discussed. Since the overall properties of non-magnetic stars do not differ significantly from those of the magnetic ones and a similar variability phenomenology has been observed in several such stars, the probable presence of a weak large scale organized magnetic field is argued.

The present paper concentrates on several seemingly nonstandard cases of variability related to CP stars. Variability of magnetic CP stars with time scales different from rotational period, variability among the HgMn stars, variability of normal stars with variations similar to CP stars, and a relation between Be and CP stars are shortly discussed.

UBV(RI)c photoelectric observations of four variable magnetic Bp stars (HD 21699, HD 177003, HD 184927 and HD 217833) were carried out between 1989 and 1992. Random fluctuation of 0m.1 range was detected in the U colour band in the case of HD 177003 and HD 184927. The U light curve of HD 21699 shows 0m.02 peaks superposed on the periodic light variation which definitely exceeds the typical error of the observations. It is not likely that atmospheric extinction or observational error would cause the phenomena described. On the other hand there are no physical processes known which lead to such changes of the U band continuum radiation in Bp stars.

From the rotational variation of UV resonance lines of carbon, nitrogen, and oxygen, we study the irregular distribution of these elements upon the surface of two magnetic Ap stars: HD 18296 (21 Per) and HD 25823 (41 Tau).

The spectrum of 33 Gem exhibits a great similarity with that of a CP3 star, but in contrast to other members of this group, light variations and variability in helium lines were reported. Even an effective magnetic field strength up to 1.4 kG was measured in the lines of elements He I, Si n, P H, Mg II, Ti H, Mn II, Fe H (Glagolevskij et al., 1985). For these far-reaching differences high resolution (R≃ 40000), high signal-to-noise (> 100) CCD spectra of 33 Gem were observed with the Coudé spectrograph of 2.2 m telescope at Calar Alto. As well for the profiles of the He I lines as for those of the metallic lines no changes could be found. Our rough inspection of Zeeman spectograms taken with the 6 m telescope at Zelenschuk in which Glagolevskij et al. noticed a magnetic field revealed unexpected splitting of several lines into two components probably caused by a companion. The line components are separated approximately by 0.1 Å. For this splitting a confirmation of the presence of a magnetic field was difficult.

The anticorrelation of C and Si previously inferred from high-resolution spectroscopy of sharp-lined A stars classified as normal or mild Am is also found among the metal-deficient, more rapidly rotating λ Boo stars. The entire sample of program stars covers the range [C/Si] = − 1.14 (Sirius, mild Am star) to + 1.35 (HR 7400, λ Boo-type). Iron varies parallel with silicon. With respect to surface chemistry the λ Boo stars do not form a distinct group but rather represent the lower end in a continuous sequence of ‘metal’ abundance extending over more than 1.5 dex.

We argue that differential accretion of gas and dust in late stages of star formation or during the main-sequence phase is responsible for most of the abundance variations of the lighter elements up to the iron group. Enhanced or impeded accretion of dust leads to a metal-rich or metal-poor surface composition, respectively. The heavy trace elements Sr and Ba show additional anomalies indicating that other processes like gravitational and radiative separation are also involved.

The main properties of this class of rare objects are described, and the criteria selected for discovering new members are indicated. The abundance anomalies, resulting from the scanty analyses made up to now, are discussed together with the theoretical interpretation proposed in the framework of the modified diffusion theory and of the recent selective accretion proposal.

Finally, we discuss the evolutionary stage of the λ Boo stars which, in the HR diagram, lie near, but not on the main sequence.

In 1968 van den Heuvel showed that the rotational velocities at many A types have peaks below V sin i = 45 km s−1 and above 90 km s−1. We explored this bimodal behavior by obtaining new rotational velocities of 1761 A-type stars with coudé CCD spectra and, to date, new MK classifications for 956 of those. We find that the slow rotators consist of Ap, Am, and short-period binaries while the rapid rotators are normal and λ Bootis stars. The overlap is only 8–10% and can probably be attributed to undetected abnormal stars and binaries.

The λ Bootis stars discovered purely by their having weak λ4481 Mg II constitute about 18% of the early A-type stars but 4% at the late ones. Statistically all such λ Bootis stars have V > 100 km s−1 but they constitute only a small fraction of those stars.

More than half of the stars classified as A2 IV (and some at Al IV and A3 IV) seem to be a hitherto unrecognized class of slowly-rotating abnormal-abundance stars of luminosities like those of class V stars.

The He-rich variable HD 37479 has 2 axes of symmetry, one characterized by the depletion of metals, and one by the enrichment of He. The former is oriented along the axis of the magnetic dipole, while the latter is off set by some 45°. The 2 axes represent 2 different modes of diffusion, the first one being controlled solely by the magnetic field, irrespective of wind, the second one being due to stellar wind that is controlled by the magnetic field and intertial forces. It has been attempted to formulate simple physical conditions that allow to determine the diffusion regions on the surface. It is shown that the resulting surface map can well reproduce the observed phase variations of the equivalent widths of HeI 4471, of UV resonance lines of C IV and Si IV, and the effective magnetic field.