Experimental MgI oscillator strengths and radiative lifetimes for astrophysical applications on metal-poor stars - New data for the MgI b triplet

Context. The stellar abundance ratio of Mg/Fe is an important tool in diagnostics of galaxy evolution. In order to make reliable measurements of the Mg abundance of stars, it is necessary to have accurate values for the oscillator strength (f- value) of each of the observable transitions. In metal-poor stars the MgI 3p-4s triplet around 5175 angstrom (Fraunhofer's so-called b lines) are the most prominent magnesium lines. The lines also appear as strong features in the solar spectrum. Aims. We present new and improved experimental oscillator strengths for the optical MgI 3p-4s triplet, along with experimental radiative lifetimes for six terms in Mg I. With these data we discuss the implications on previous and future abundance analyses of metal-poor stars. Methods. The oscillator strengths have been determined by combining radiative lifetimes with branching fractions, where the radiative lifetimes are measured using the laser induced fluorescence technique and the branching fractions are determined using intensity calibrated Fourier Transform (FT) spectra. The FT spectra are also used for determining new accurate laboratory wavelengths for the 3p-4s transitions. Results. The f-values of the MgI 3p-4s lines have been determined with an absolute uncertainty of 9%, giving an uncertainty of +/- 0.04 dex in the log g f values. Compared to values previously used in abundance analyses of metal-poor stars, rescaling to the new values implies an increase of typically 0.04 dex in the magnesium abundance.