We compare observed ultraviolet and visible flux distributions, for a sample of A-type stars, with computed fluxes obtained with version 9 of the ATLAS code (Kurucz, 1970) in order to asses the impact of reddening. We also make a comparison between determinations of reddening from UV colour indices and visible wide band (Johnson) and intermediate band (Strömgren) colour indices. We discuss the implications on the study of slightly reddened high galactic latitude stars.

A fine analysis performed using four 4.3 Å/mm IIaO Mt. Wilson Observatory spectrograms shows that HD 43819 has abundance anomalies similar to those of other Silicon stars, namely it is helium, carbon, magnesium, and sulfur poor while being silicon, titanium, chromium, manganese, iron, strontium, zirconium, and rare earth rich.

Leckrone and Adelman have established an IUE observing strategy that has yielded co-added spectra with enhanced S/N ratios for several A and B stars. New observations by Roby and Adelman using the same technique have added two new Hg-Mn stars into this sample. We have begun a long-term study of elemental abundances in this uniform, high-quality set of IUE spectra for 13 stars. We report on the first stages of this project: abundances for N, Cr, Mn, Fe, Co, and Ni. The study of the Fe- peak elements show that our data set can provide accurate abundances and that abundances obtained from UV and optical spectra often are in good agreement. This study provides the groundwork for self-consistent abundance analyses of more exotic elements in our long term project.

The CP-star 53 Aur (HD 47152, HR 2425) has been assigned in the literature different types of peculiarity. Bertaud (1959) classified it as A0p-EuCr, Cowley et al. (1969) as B9p-Eu(Cr?), Osawa (1965) as Hg and Zverko (1974) as Mn -type. MacAlister (1978) discovered its binary nature and Tokovinin (1986) first determined its orbit with a period of 13.7 y. Subsequent interferometric observations by Baize (1989) yielded new orbital elements with a period of 25.816 y, total mass of 5.8 M⊙ and magnitude difference of two B9 components 0.3 mag. Adelman (1982) performed spectrophotometry and fitted the Balmer jump with Teff=10 500 K, while the Paschen continuum with Teff=9 500 K, log g=4 in both cases. Palmer (1965) estimated the projected rotational velocity to 325 km s−1 from the width of the Ca II-K line, while Wolff and Preston (1975) obtained 33 km s−1 from the Mg II 448.1 nm line. In this work we analyze high-dispersion spectra obtained in two distant orbital phases, namely in 1975 and in 1992 to distinguish the components of the system.

We report here results of Bi II spectral lines Stark broadening research within the modified semiempirical approach. The strong absorption Bi II lines observed in Hg – Mn star atmospheres have been investigated as well as the influence of the departure from LS – coupling and the correct knowledge of ionization potential. The obtained results have been compared with other estimations and with experimental results.

Spectroscopic analyses of the β CrB spectrum have been performed in the past without taking into consideration the double nature of this well-known cool Ap star (e.g. Hiltner, 1945; Hack, 1958; Adelman, 1973). Recently, Kamper et al. (1990) have re-examined the astrometric (interferometric and visual) and spectroscopic (radial velocity) data and have given an improved orbit for the β CrB system (P=10.55 yr) and the passage at periastron at 1991.02.

I present the Hβ profiles of 25 CP2 and CP4 stars, together with their equivalent widths. Surface gravities are estimated for stars with known effective temperature from the equivalent width of Hβ.

The line spectrum of BD +24°3675 is investigated from 4730 to 4990 Å. I discuss the presence of individual elements, including Rare Earths. The equivalent widths of all strong lines are given. Estimates of Teff and logg indicate that the star might be evolved with a low surface gravity. Indications for spectral line variability have been found.

The atmospheric parameters of metallic-lined (Am) stars have been the subject of much debate. Using spectrophotometric flux fitting, Lane & Lester (1984) obtained values of Teff and log g for several classical Am stars that were systematically lower than those obtained by Dworetsky & Moon (1986), who used empirically calibrated uvbyß photometry. We have used the standard procedures of photometry, spectrophotometry and hydrogen-line profile fitting to determine the atmospheric parameters of many A and F stars, including several Am stars. Modifications to the standard techniques have been used to allow for the effects of cooler companion stars in the determination of such parameters. It is found that spectrophotometric methods give results for Teff and log g that are highly sensitive to the adopted metal abundance. This explains the anomalous results of Lane & Lester.

A and Am stars were observed in open clusters for their Li content. The Li abundance is therefore studied on the hot side of the Li gap along isochrones. It is discussed in relation to evolutionary model atmospheres including particle transport processes.

Duplicity is a very widespread phenomenon among Am-stars. For instance, Abt (1961) investigating 25 such stars found out that 22 of them are spectroscopic binaries. However this important phenomenon is ignored usually in chemical composition investigations of Am-stars. Consequently some “mean” element abundances are determined, which can noticeably differ from real abundances in atmospheres of components. Moreover false chemical anomalies can appear, as shown by the theoretical modelling of spectra of binary stars (Lyubimkov, 1989, 1992). Meanwhile accurate data on chemical composition of Am-stars must be considered as observational test for any hypothesis suggested for explanation of these objects.

The MK morphology of the spectra in the region λλ 3850-4400 Å of 28 Am stars has brought to light the tendency of these stars to form into groups in contrast to earlier findings by various authors (S. Sreedhar Rao & K. D. Abhyankar, 1991). The potential of such a study is seen in the revelation of the inherent flexibility of the stars in this sample, apparently leading to a selective binning of the species with distinctly similar signatures, viz., Ca ll K and Sr ll 4077 features, the blue and violet regions and the Am-shell features. These are found to be the essential discriminants on the basis of which a multi-dimensional classification of Am spectra in the domain of the MK process is attempted.

The energy distribution of the Am stars are affected by the line blanketing, which appears as a deviation in the behaviour of color indices with reference to the normal stars. In this paper we study the various color indices along with the data from IRAS and try to identify a realistic representative of the effective temperature. An excess in the IR also becomes apparent in some cases.

The values of the effective temperature derived for Am stars by the infra-red flux method and from the Balmer line profiles are in very good agreement. They are hotter by more than 300K than the Teff given by Lane and Lester (1984) and cooler by about 200K than those obtained by Dworetsky et al (1986).

We are performing elemental abundances of sharp-lined metallic-lined stars and normal late A through middle F stars using 2.4 Å mm−1 photographic region spectrograms taken with the coudé spectrograph of the 1.4-m telescope of the Dominion Astrophysical Observatory. Our initial studies of the Am stars 15 Vul and 32 Aqr and the normal stars θ Cyg (F4 V) and ɩ Psc (F7 V) showed that the elemental abundances of the normal stars were close to solar while 15 Vul shows smaller abundance anomalies than does 32 Aqr. These analyses are being done consistently with SJA’s studies of the HgMn and hotter normal stars.

To understand the complex physical processes revealed by the wide variety of surface chemical abundances and the inhomogeneous distributions of various elements over the surfaces of magnetic Ap and Bp stars, it is important to be able to describe the surface magnetic field structure. For most magnetic stars, the available observations are far from sufficient to define the surface magnetic structure even with low resolution, but more extensive and constraining observations are gradually becoming available for a small set of stars. This paper discusses the extent to which the surface field structure can be defined for stars for which only the usual measurements (essentially the variation of the mean longitudinal field strength Bɩ with rotational phase) are available, and how further information (variation of mean field modulus Bs with phase, line profile observations, and circular and linear spectropolarimetry of absorption lines or broad-band radiation) can be used to obtain a much more detailed picture of the magnetic field structure.

An original approach, the moment technique, is applied to analyze the shapes of spectral lines of Ap stars recorded in both circular polarizations. The longitudinal magnetic field, the asymmetry of the longitudinal magnetic field, and the quadratic field of the studied stars are derived. From the consideration of these quantities and of their variations through the stellar rotation cycle, constraints on the spatially unresolved structure of the magnetic fields are obtained.

The mapping of stellar surfaces using the observed profiles of spectral lines is described. The problem is expressed in terms of an integral equation to be solved, using Tikhonov’s method. The local line profiles may be given either by an analytic approximation or by the solution of the equation of transfer with a model atmosphere. The model atmospheres commonly used may not correctly represent the stellar atmosphere which may lead to errors in the derived surface maps. Tests of mapping programs and applications to real stars show the capabilities and limitations of surface mapping. Mapping stellar magnetic fields places more severe demands on the data and computational programs than mapping the abundance distributions.

A technique for deriving the distribution of elements on the surface of Ap stars using maximum entropy reconstruction principles is described. The technique is applied to deriving the silicon distribution on 56 Ari, CU Vir, 11 Ori and the chromium distribution on ϒ2 Ari. Silicon on these stars is depleted at the magnetic poles and is enhanced in regions between the magnetic equator and poles. The chromium distribution on ϒ2 Ari is markedly different than the chromium distribution seen on other Ap stars. It shows depletions at one of the magnetic poles (as do other Ap stars) but it does not show the depleted band at the equator as has been seen on θ Aur, 45 Her, and ω Her. The silicon distribution on 11 Ori also differs from that found on other stars in that it shows evidence for a depleted band, similar to what has been seen in the chromium distribution is some stars. Characteristic features in the abundance maps such as spots or bands appear to mark the location of the magnetic poles or equator so that these maps can be used to infer the magnetic field geometries on these stars. Dipole decentering parameters derived from the abundance maps yield decentering parameters of about 0.2 stellar radii. The amount of decentering seems to be correlated with rotation period (longer period Ap stars have less decentering). Horizontal diffusion can complicate the use of abundance maps to determine the field geometry. The effects of horizontal diffusion can only be understood by a proper theoretical study of its effects or by mapping the elemental distribution on Ap stars of known age.