Isotope structure of Sm II as an indicator of r- vs. s-process nucleosynthesis

¹ Atomic Astrophysics, Lund Observatory, Lund University, Box 43, 22100 Lund, Sweden e-mail: martin.lundqvist@astro.lu.se
² GEPI, Observatoire de Paris Meudon, 92125 Meudon Cedex, France

Received: 11 August 2006
Accepted: 14 November 2006

Abstract

Aims.The isotope composition of samarium provides a previously unexploited means of determining the r- versus s-process content of e.g. metal-poor stars. The seven stable isotopes of samarium can be divided into two groups, where in one group the r-process is the dominant production mechanism, while for the second group it is the s-process that is dominant. The center-of-gravity for spectral lines produced by these two groups are slightly shifted with respect to each other, where the heavier and most abundant isotopes are mainly produced by the r-process. This relative shift makes the line position dependent upon the content of r- and s-processed samarium. The isotope composition also affects the width and the shape of the line, making it narrower and slightly more asymmetric when the composition deviates from the solar system composition.

Methods.The behaviour of lines in stellar spectra is studied using synthetic spectrum techniques with line data accounting for both isotope and hyperfine structure. Isotope shifts in 11 lines of have been measured from a laboratory spectrum obtained with a Fourier transform spectrometer. Hyperfine structure constants have been taken from the literature.

Results.The new atomic data are applied to the spectrum of the metal-poor halo star CS 31082-001 and suggests that this star contains predominantly r-processed material. We also discuss the uncertainties involved in this type of high precision measurement.