Experimental Ca I oscillator strengths for the 4p–5s triplet

¹ Atomic Astrophysics, Lund Observatory, Lund University, Box 43, 221 00 Lund, Sweden e-mail: r.blackwell@astro.lu.se
² Atomic Physics, Department of Physics, Lund Institute of Technology, Box 118, 221 00 Lund, Sweden

Received: 13 February 2009
Accepted: 7 May 2009

Abstract

Context. Transition lines of neutral calcium are observed in the spectra of stellar and substellar objects. In particular, the abundance of α-elements in metal-poor stars can place important constraints on the galactic chemical evolution. Such stellar abundance analyses rely heavily on accurate values for the oscillator strength of the observable transitions. Theoretical oscillator strengths are available for most neutral calcium lines visible in stellar spectra, but there are a limited number of experimental values in the literature.

Aims. We present new and improved experimental oscillator strengths for the optical Ca i 4p–5s triplet (6102.7, 6122.2, 6162.2 Å). In addition, we present experimental radiative lifetimes for seven energy levels in the triplet system of Ca i.

Methods. The oscillator strengths were determined by combining radiative lifetimes with branching fractions. The radiative lifetimes were measured using laser-induced fluorescence, and the branching fractions were determined using intensity calibrated spectra measured with Fourier transform spectrometry. In addition, the spectra were used to determine accurate (dλ = 0.001 Å, dσ = 0.002 cm^-1) laboratory wavelengths for the 4p–5s transitions.

Results. Oscillator strengths for the Ca i 4p–5s lines were determined with an absolute uncertainty of 9%, an uncertainty of ± 0.04 dex in the log(gf) values. The branching fractions were determined with a higher accuracy, resulting in relative uncertainties of 2–3%.