The literature concerning spectroscopic studies, abundances, photometry and systematics of Hg-Mn stars is reviewed since the last international meeting on chemically peculiar stars in the Crimea in 1985, and is intended to be complete to the end of 1991.

The spatial inhomogeneities, Zeeman broadening, and extreme abundance anomalies have thus far prevented definitive analyses of magnetic CP stars. Nevertheless, the abundance anomalies are so large that many of them have been known for decades. Abundance excesses of iron-peak elements of factors of 10 to 100 are common. Relative abundances on the iron peak are not constant. The lines of vanadium and nickel are often weak, and these elements may even be deficient in some stars. In spitè of the large variations, the odd-even effect persists; there is only minor evidence that chemical separation has perturbed the nuclear pattern. The lanthanide rare earths can have excesses of 100 to 1000 or even more in extreme cases. For these elements there is some evidence of fractionation. The actinide rare earth elements uranium and thorium are weakly (but surely!) present in a few of the magnetic CP stars: the best case is HR 465.

I review abundance determinations of normal B5-F4 and peculiar stars published since 1984. Several analyses performed with photographic spectrograms indicate interesting stars which should be analyzed with high signal-to-noise data. Studies of stars of known ages which belong to clusters, associations, and moving groups should led to the most direct confrontations with theory. The increase in signal-to-noise ratio provided by electronic detectors with respect to photographic plates should allow accurate analyses of moderating rotating stars. High resolution, high signal-to-noise ratio studies have revealed crucial information about the line profiles of Sirius, Vega, and other A stars. It would aid comparison of analyses if we could agree on a standard set of gf-values and line damping constants. A computer bulletin board would be a useful means to provide and maintain such data as well as model atmosphere codes.

We are using the HST/GHRS in a long-term program to obtain UV spectra of unprecedented resolution and precision for bright, ultra-sharp-lined Bp (HgMn) stars and comparable normal stars. To date we have doubled the number of heavy elements for which abundances may be estimated in χ Lupi, and have obtained the first observations of Hg III lines with which to test diffusion scenarios for its extreme Hg isotope anomaly.

This brief review discusses the similarities between CP stars, focusing on magnetic fields and chemical abundances. New evidence for the presence of a magnetic field with a complex structure in some CP1 stars is presented. Small-scale structures on the surface of these stars may be connected with such a field, and the resulting effects on the curve-of-growth have been investigated. Finally, the changes during evolutionary time-scales on surface abundances are advocated as one possible reason of the large observed scatter in the chemical anomalies inside given groups of CP Stars.

Recent determinations of abundances of heavy elements in CP stars are examined. An emphasis is placed on those elements which are found in a group of CP Stars but not in others. Fragmentary data are available for most of heavy elements. Systematic surveys (especially in the UV region) using a large sample of stars are needed before we understand the physics of the apparently erratic behavior of heavy elements in CP stars.

I have developed a new version of my model atmosphere program called ATLAS12. It recognizes more than 1000 species, each in up to 10 isotopic forms, including all ions of the elements up through Zn and the first 5 ions of heavier elements up through Es. The elemental abundances are treated as variable with depth. ATLAS12 has 6 input files of line data containing 58,000,000 atomic and molecular lines. For each line the wavelength, identification, lower energy level, gf, radiative, Stark, and van der Waals damping constants are packed into 16 bytes. At each wavelength point in a frequency integration the profiles of all the significant nearby lines are computed and summed. The program and line files will be distributed in the fall of 1992.

There are no significant differences at A0 between an opacity-sampled model computed with ATLAS12 and opacity-distribution-function model computed with ATLAS9. ATLAS12 allows arbitrary abundances but is slower. The new program can be used to produce improved models for Am and Ap stars that include the effects of millions of lines.

A brief review on NLTE models of normal and chemically peculiar stars of spectral types A and late B is presented. We will concentrate on three basic question about NLTE modeling, namely i) Why (calculate NLTE models)? ii) How (to calculate NLTE models efficiently)? and iii) What (we learn from NLTE models already calculated)?

We summarize the newly available results which improve both the accuracy and the completeness of atomic data to be used for analysis and modelization of A stars. A major progress is obtained from the large amount of theoretical work that has been performed through the recent revision of stellar opacities. Extensive lists of accurate line data are available and we present their general characteristics. Detailed photoionization cross-sections are now available for many initial states of different atomic ions. They currently show deep resonance structures and strongly depart from the hydrogenic shapes which were often assumed. Their importance is illustrated by the example of the Si II ultra-violet features. Further implications about computation of radiative forces are pointed out.

Echelle spectra obtained with the HST/GHRS are used to understand the blending in lower resolution IUE sprectra in the region of the raie ultime line of Au II. Laboratory analysis of the Au II transition has provided an accurate wavelength and transition probability from which the first Au abundances are determined for stars other than the Sun.

A comparison of various physical parameters of the normal A stars with those obtained by using the peculiarity-free zones of Ap stars indicate that the peculiar A stars may be treated as normal stars as far as their effective temperatures, radii, bolometric corrections and the mass range are concerned.

Elemental abundances from recent spectrum-synthesis analyses of the ultraviolet spectra of normal and HgMn-type late-B stars are examined for inter-correlations and for correlations with stellar parameters by use of standard statistical techniques. A few marginally significant correlations are apparent in the normal-star sample. In the HgMn sample, significant correlations with Teff are identified for Mn, Cu, and Ga. Strong inter-correlations are also identified in this sample for the abundances of two triads of elements (Mg-Al-Cr and Mn-Cu-Ga). Trivariate correlation analysis is used to demonstrate that these element-element inter-correlations are statistically independent of both Teff and log g. The correlated behaviour of these characteristically anomalous elements casts new light on the apparent heterogeneity of the HgMn class of peculiar stars, and may have important implications for the diffusion model which has been advocated as an explanation of the HgMn phenomenon.

This paper presents a new statistical investigation of peculiar A-type stars (Am, Ap, Hg-Mn) among spectroscopic binary (SB) stars. The relative frequency of Am (CP 1) stars is 55% in the spectral range A1 to A6 of main-sequence stars. The Ap (CP 2) stars amount to 15% in the range B9 to A2. The Hg-Mn stars are concentrated to the spectral types B8 to AO and reach a relative frequency of 23%. The Am SB stars have the shortest orbital periods and the smallest eccentricities (30% circular) whereas the Ap SB stars show a strong tendency to long periods and highly eccentric orbits (only 10% circular). The masses of the Am stars agree with the masses of non-peculiar SB stars of corresponding spectral type.