Improved oscillator strengths and wavelengths for Os I and Ir I, and new results on early r-process nucleosynthesis ^*

¹ Atomic Astrophysics, Lund Observatory, Lund University, PO Box 43, 22100 Lund, Sweden
² Astronomical Observatory, Niels Bohr Institute for Astronomy, Physics & Geophysics, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
³ Nordic Optical Telescope Scientific Association, La Palma, Canary Islands, Spain
⁴ Atomic Physics, Department of Physics, Lund Institute of Technology, PO Box 118, 22100 Lund, Sweden
⁵ GEPI, Observatoire de Paris-Meudon (UMR 8111), DASGAL, 2 pl. Jules Janssen, 92195 Meudon Cedex, France
⁶ Laboratoire Aimé Cotton, Centre National de la Recherche Scientifique, Orsay, France 


Corresponding author: S. Ivarsson, stefan.ivarsson@astro.lu.se

Received: 28 May 2003
Accepted: 25 July 2003

Abstract

The radioactive decay of ²³⁸U and ²³²Th has recently been used to determine ages for some of the oldest stars in the Universe. This has highlighted the need for accurate observational constraints on production models for the heaviest r-process elements which might serve as stable references, notably osmium and iridium. In order to provide a firmer basis for the observed abundances, we have performed laser-induced fluorescence measurements and Fourier Transform Spectroscopy to determine new radiative lifetimes and branching fractions for selected levels in Os I and Ir I. From these data, we determine new absolute oscillator strengths and improved wavelengths for 18 Os I and 4 Ir I lines. A reanalysis of VLT spectra of CS 31082-001 and new results for other stars with Os and Ir detections show that (i): the lines in the UV and λ 4260 Å yield reliable Os abundances, while those at 4135, 4420 Å are heavily affected by blending; (ii): the Os and Ir abundances are identical in all the stars; (iii): the heavy-element abundances in very metal-poor stars conform closely to the scaled solar r-process pattern throughout the range 56 ≤ Z ≤ 77; and (iv): neither Os or Ir nor any lighter species are suitable as reference elements for the radioactive decay of Th and U.