New constraints on the chemical evolution of the dwarf spheroidal galaxy
  LeoI from VLT spectroscopy
  (Abridged) We present the spectroscopy of red giant stars in the dwarf
spheroidal galaxy LeoI, aimed at further constraining its chemical enrichment
history. Intermediate-resolution spectroscopy in the CaII triplet spectral
region was obtained for 54 stars in LeoI using FORS2 at the ESO Very Large
Telescope. The equivalent widths of CaII triplet lines were used to derive the
metallicities of the target stars on the [Fe/H] scale of Carretta & Gratton, as
well as on a scale tied to the global metal abundance, [M/H].
  The metallicity distribution function for LeoI stars is confirmed to be very
narrow, with mean value [M/H]~-1.2 and intrinsic dispersion, sigma_[M/H]=0.08.
We find a few metal-poor stars (whose metallicity values depend on the adopted
extrapolation of the existing calibrations), but in no case are stars more
metal-poor than [Fe/H]=-2.6.
  Our measurements provide a hint of a shallow metallicity gradient of -0.27
dex/Kpc among LeoI red giants.
  By combining the metallicities of the target stars with their photometric
data, we provide age estimates and an age-metallicity relation for a subset of
red giant stars in LeoI. Our age estimates indicate a rapid initial enrichment,
a slowly rising metal abundance and an increase of ~0.2 dex in the last few
Gyr.
