Magnetic intensification of the Li I lambda 6708 line and the abundance and age spread in young cool stars

The lithium abundance is considered the best age indicator of young cool stars and its spread is fundamental for the understanding of star formation mechanisms, in particular, of possible multiple formation events within a single region. The observed spread in the equivalent widths of lithium lines cannot be understood on the basis of present stellar evolutionary models, and proposed explanations invoke stellar activity. Despite the fact that magnetic fields of several tenths of a tesla are expected theoretically and are observed in these stars, their effect on the formation of spectral lines has always been neglected. In this Letter, the possibility that the magnetic intensification of spectral lines results in an overestimation of lithium abundances is quantitatively explored through the computation of Stokes I profiles of the Li i λ6708 doublet, using the polarized line synthesis code COSSAM. Spectral synthesis for fields up to 1 T reveals the doublet to be sensitive to magnetic fields, with equivalent widths up to 60% larger than the zero field value and a possible overestimation of lithium abundances by 1 dex. Consequently, the contribution of magnetic fields to the lithium spread appears to be significant. Neglecting the magnetic intensification of the Li i λ6708 doublet could make stars appear millions of years younger and make stellar formation mechanisms appear to last much longer than usually supposed. To conclude, for lithium abundance determinations and estimates of lithium based stellar ages, one has to obtain magnetic field measurements and to include them in the spectrum analysis.