Emile Biémont

A new database of astrophysical interest

The aim of the new database D.R.E.A.M. is to supply the astrophysicists with accurate atomic data (wavelengths, energy levels, oscillator strengths, radiative lifetimes) of neutral, singly or multiply ionized lanthanides. Up to now, it contains about 50000 lines but it will be updated and extended in the near future.

Lifetime calculations in Yb II

Oscillator strengths and lifetime values for Yb II levels have been calculated within the framework of a pseudo-relativistic Hartree-Fock (HFR) approximation combined with a least-squares fitting of the calculated eigenvalues to the observed energy levels. A considerable amount of configuration interaction together with core-polarization effects have been included in the calculations. The quality of the HFR results has been assessed through a comparison with data obtained using the completely independent relativistic quantum defect orbital (RQDO) method. The theoretical lifetimes reported in the present paper are now in good agreement with most of the accurate experimental values available. The origin of the persisting discrepancies is suggested.

Experimental and theoretical radiative lifetimes, branching fractions, and oscillator strengths for Lu I and experimental lifetimes for Lu II and Lu III

Radiative lifetimes, accurate in most cases to ±5%, from time-resolved, laser-induced fluorescence measurements on a slow beam of lutetium atoms and ions are reported for 22 odd-parity levels and 4 even-parity levels of Lu I and 14 odd-parity levels of Lu II. In addition, we report the radiative lifetime of one odd-parity level and an upper bound on the radiative lifetime of a second odd-parity level of Lu III. Experimental branching fractions for Lu I from emission spectra covering the near ultraviolet to the near infrared and recorded using the US National Solar Observatory 1.0 m Fourier transform spectrometer are reported. The branching fractions are combined with the radiative lifetimes to produce 44 experimentally determined transition probabilities or oscillator strengths, accurate generally to ±10%, for Lu I. New theoretical values for Lu I radiative lifetimes and branching fractions from a relativistic Hartree-Fock calculation that includes core polarization effects are also reported. These experimental and theoretical results, as well as older published results, are compared.

Radiative lifetime measurements and transition probability calculations in lanthanide ions

We have undertaken a systematic investigation of spectroscopic properties of lanthanide ions (Z=57-71). Using time-resolved laser-induced fluorescence following one- or two-photon excitations, a large number of radiative lifetimes have been measured at the Lund Laser Centre for singly, doubly and trebly ionized atoms. These new measurements have been used for testing theoretical calculations performed within the framework of a relativistic Hartree-Fock approach taking core-polarization effects into account. Using the experimental lifetimes (when available) and the theoretical branching fractions, a large number of transition probabilities, most of them of astrophysical interest, have been deduced and are stored in a new database (DREAM). Up to now, the results obtained concern the following ions: La2+, Ce2+, Ce2+, Pr2+, Ho2+, Er2+, Tm2+, Tm2+, Yb2+, Yb2+, Yb2+, Lu2+ and Lu2+

Cerium: The lithium substitute in post-AGB stars ?

In this letter we present an alternative identification for the line detected in the spectra of s-process enriched low-mass post-AGB stars around 6708 A and which was interpreted in the literature as due to Li. Newly released line lists of lanthanide species reveal, however, the likely identification of the line to be due to a Ceii transition. We argue that this identification is consistent with the Ce abundance of all the objects discussed in the literature and conclude that in none of the low-mass s-process enriched post-AGB stars there is indication for Li-production.

Time-resolved laser-induced fluorescence lifetime measurements and relativistic Hartree–Fock calculations of transition probabilities in Sm II

Radiative lifetime measurements were performed with time-resolved laserinduced fluorescence techniques for 47 levels of the astrophysically important ion Sm 1+ over the energy range 21 000–36 000 cm −1 .T henew results have been compared with previous measurements but also with theoretical calculations taking configuration interactions and core-polarization effects into account, and as atisfying agreement has been found for many levels of this complex ion. New calculated transition probabilities are deduced from the experimental lifetimes and from the theoretical branching fractions for 162 transitions of astrophysical interest. These results will help astrophysicists in the quantitative investigation of the chemical composition of CP stars.

Radiative lifetimes in La III and oscillator strengths in La III and Lu III

New radiative lifetime measurements performed with time-resolved laser spectroscopy for the 6p1/2 and 6p3/2 levels of La III provide a unique opportunity for testing the adequacy of the relativistic Hartree-Fock approach for atomic structure calculations in this ion. As a consequence of the comparison, an extensive set of accurate f-values is provided for many La III and Lu III UV and visible transitions of astrophysical interest.

Theoretical Oscillator Strengths in Pr III and Application to Some CP Stars

A set of transition probabilities is obtained for the first time for many transitions of Pr III of astrophysical interest. A relativistic Hartree-Fock method, combined with a least-squares fitting of the available experimental levels, including a large number of newly determined values, has been used for the calculations in which configuration interaction and core-polarization effects have been introduced. The interest of the new data set is illustrated by considering the chemical composition of the chemically peculiar (CP) stars HD 101065 (Przybylskis star) and HD 141556 (χ Lupi).

Experimental and theoretical energy levels, transition probabilities and radiative lifetimes in Yb III

The analysis of the spectrum of Yb III has been extended allowing us to establish 11 new energy level values. In addition, radiative lifetimes of two excited states of Yb III have been measured for the first time using time-resolved laser-induced fluorescence following two-photon excitation. The good agreement between experimental results and semi-empirical calculations performed with the relativistic Hartree-Fock method including core-polarization effects allows the determination of transition probabilities for 15 lines.

Doubly Ionized Thorium: Laser Lifetime Measurements and Transition Probability Determination of Interest in Cosmochronology

The first lifetime measurements have been performed for six levels of doubly ionized thorium, an important cosmochronometer in astrophysics for estimating the age of the Galaxy. The levels, belonging to the 5f2, 5f7p, 7s7p, and 6d7p configurations of Th III, have been measured by the time-resolved laser-induced fluorescence method and compared with relativistic Hartree-Fock calculations including configuration interaction and core-polarization effects. Taking advantage of the excellent agreement between theory and experiment, a first set of transition probabilities of astrophysical interest has been deduced for this ion from a combination of the experimental lifetimes and of the theoretical branching fractions.