  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 LiI 6707.8 A 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.
