Abstract. Optical region spectra of 31 HgMn stars have been studied for the abundance and isotope mixture of mercury. In the course of the investigation the lines Hg  λ4358 and Hg  λλ3984,6149 have been studied, with abundances established for all three lines in several HgMn stars. The mercury isotope mixture has been determined from high resolution spectra of the λ3984 line. Possible signs of an ionization anomaly have been detected by the comparison of the abundance derived from the Hg  line and the Hg  lines in seven of the observed HgMn stars. A possible correlation of the mercury abundance with T eff has been detected. Possible signs of a weak anticorrelation of the manganese and mercury abundance in HgMn stars have been found, which could be interpreted as a sign of inhomogeneous surface distribution of these elements. For a number of the HgMn stars in this study the mercury abundance and isotope mixture are reported for the first time.

Abstract.Optical region spectra for a number of upper main sequence chemically peculiar (CP) stars have been observed to study singly and doubly ionized praseodymium and neodymium lines. In order to improve existing atomic data of these elements, laboratory measurements have been carried out with the Lund VUV Fourier Transform Spectrometer (FTS). From these measurements wavelengths and hyperfine structure (hfs) have been studied for selected Pr ii, Pr iii and Nd iii lines of astrophysical interest. Radiative lifetimes for some excited states of Pr ii have been determined with the aid of laser spectroscopy at the Lund Laser Center (LLC) and have been combined with branching fractions measured in the laboratory to calculate gf values for some of the stronger optical lines of Pr ii. With the aid of the derived gf values and laboratory measurements of the hfs, a praseodymium abundance was derived from selected Pr ii lines in the spectrum of the Am star 32 Aqr. This abundance was used to derive astrophysical gf values for selected Pr iii lines in 32 Aqr, and these gf values were used to get a praseodymium abundance for the HgMn star HR 7775. The praseodymium abundance in HR 7775 was then utilized to derive astrophysical gf values for all observable Pr iii lines in this star. The neodymium abundance, derived from unblended lines of Nd ii in HR 7775, has been utilized to establish astrophysical gf values for observed Nd iii lines in the optical region of this star. Selected Pr iii and Nd iii lines have been identified and studied in a number of HgMn stars and three hot Am stars. The praseodymium and neodymium abundance change rapidly from an approximate 1-1.2 dex enhancement for the hot Am stars to 1.5-3 dex enhancement for the cool HgMn stars, indicating a well-defined boundary between the hot Am and HgMn stars in the vicinity of 10 500 K. The enhancement of praseodymium and neodymium in Am and HgMn stars may be explained by diffusive processes active in the stellar atmosphere, while the observed discontinuity might be explained by a thin hydrogen convection zone thought to be present for the Am stars, but absent in the HgMn stars. The absence of a convection zone would cause the diffused elements to gather higher in the atmosphere of HgMn stars compared to Am stars, and explain the observed increase in abundance.

Very heavy elements ( Pt, Au, Hg, Tl, and Bi) are found to be enhanced in the atmospheres of the chemically peculiar stars of the upper main sequence by up to a million times the solar system levels. Such enhancements are believed to result from atmospheric dynamics (i.e., diffusion) rather than scenarios that dredge up nuclearprocessed material to the surface or transfer processed material between binary companions. However, the theoretical framework needs to be further constrained by observations beyond the realm of the spectral types for which such abundance enhancements are observed at optical wavelengths.The International Ultraviolet Explorer (IUE ) satellite collected spectra of bright stars for which chemical peculiarities have been derived from ground-based data. For several elements the abundance enhancements have only been recently measured using Hubble Space Telescope data and have therefore not yet been exploited in the IUE data. We have initiated a program to analyze IUE high-dispersion spectra to more fully characterize the pattern of very heavy element enhancement for many mercury-manganese ( HgMn) stars and to potentially extend the spectral class (effective temperature) boundaries over which these abundance anomalies are known to exist. The abundances of very heavy elements in chemically normal B and A-type stars provide a base level that may be compared with the solar system abundances. These early spectral type stars may therefore reveal clues for galactic chemical evolution studies since they were formed at a later epoch than the Sun in the history of the Galaxy. This first paper presents the motivation for the analyses to follow, outlines our spectral co-addition technique for IUE spectra, and discusses the choice of model atmospheres and the synthetic spectrum procedures, while initiating the study by highlighting the abundance of gold in several HgMn stars.

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Abstract. The bismuth spectrum emitted from a hollow cathode discharge has been recorded with a Fourier Transform Spectrometer (FTS). Accurate wavelengths have been determined for 104 Bi ii lines, and several new energy levels have been found, while the accuracy of previously known Bi ii level energies have been improved. The hyperfine structure of all observed Bi ii lines has been analyzed, yielding hyperfine constants A and B for 56 Bi ii levels. With the aid of the laboratory measurements the optical region spectrum of the HgMn star HR 7775 has been studied for all observable Bi ii lines. The wavelengths and hfs constants established from the laboratory work have been combined with theoretical gf values to identify spectral lines and make an abundance estimation of bismuth. It has been established that bismuth is present in HR 7775 at an enhancement level of approximately 5 orders of magnitude relative to the meteoritic abundance, consistent with previous observations in the ultraviolet region of this star. Astrophysical gf values are presented for a number of Bi ii lines.