  There is mounting evidence that the stellar initial mass function (IMF) could
extend much beyond the canonical Mi ~100, Msun limit, but the impact of such
hypothesis on the chemical enrichment of galaxies still remains to be
clarified. We aim to address this question by analysing the observed abundances
of thin- and thick-disc stars in the Milky Way with chemical evolution models
that account for the contribution of very massive stars dying as
pair-instability supernovae. We built new sets of chemical yields from massive
and very massive stars up to Mi ~ 350 Msun, by combining the wind ejecta
extracted from our hydrostatic stellar evolution models with explosion ejecta
from the literature. Using a simple chemical evolution code we analyse the
effects of adopting different yield tables by comparing predictions against
observations of stars in the solar vicinity. After several tests, we focus on
the [O/Fe] ratio which best separates the chemical patterns of the two Milky
Way components. We find that with a standard IMF, truncated at Mi ~ 100 Msun,
we can reproduce various observational constraints for thin-disc stars, but the
same IMF fails to account for the [O/Fe] ratios of thick-disc stars. The best
results are obtained by extending the IMF up to Mi = 350 Msun and including the
chemical ejecta of very massive stars, in the form of winds and
pair-instability supernova explosions.Our study indicates that PISN played a
significant role in shaping the chemical evolution of the Milky Way thick disc.
By including their chemical yields it is easier to reproduce not only the level
of the alpha-enhancement but also the observed slope of thick-disc stars in the
[O/Fe] vs [Fe/H] diagram. The bottom line is that the contribution of very
massive stars to the chemical enrichment of galaxies is potentially quite
important and should not be neglected in chemical evolution models.
