We want to determine the lithium abundances of solar twin stars as a function of stellar age to provide constraints for stellar evolutions models and to investigate whether there is a connection between low Li abundance and the occurrence of planets.

For a sample of 21 solar twins observed with the HARPS spectrograph at high spectral resolution (R ≃ 115.000) and very high signal-to-noise ratio (600 ≤ S/N ≤ 2400), precise lithium abundances were obtained by spectral synthesis of the Li i 6707.8 Å line and compared to stellar ages, masses, and metallicities determined from a spectroscopic analysis of the same set of HARPS spectra.

We show that for the large majority of the solar twins there is a strong correlation between lithium abundance and stellar age. As the age increases from 1 to 9 Gyr, the Li abundance decreases by a factor of ~50. The relation agrees fairly well with predictions from non-standard stellar evolution models of Li destruction at the bottom of the upper convection zone. Two stars deviate from the relation by having Li abundances enhanced by a factor of ~10, which may be due to planet engulfment. On the other hand, we find no indication of a link between planet hosting stars and enhanced lithium depletion.