We report on the chemical abundances derived from high-dispersion spectra of 14 red giant stars in the Sagittarius dwarf spheroidal ( Sgr dSph ) galaxy .
The stars span a wide range of metallicities , -1.6 \leq { [ Fe / H ] } \leq - 0.1 dex , and exhibit very unusual abundance variations .
For metal-poor stars with { [ Fe / H ] } < -1 , { [ \alpha / Fe ] } \approx + 0.3 similar to Galactic halo stars , but for more metal-rich stars the relationship of [ \alpha /Fe ] as a function of [ Fe/H ] is lower than that of the Galactic disk by 0.1 dex .
The light elements [ Al/Fe ] and [ Na/Fe ] are sub-solar by an even larger amount , approximately 0.4 dex .
The pattern of neutron-capture heavy elements , as indicated by [ La/Fe ] and [ La/Eu ] , shows an increasing s -process component with increasing [ Fe/H ] , up to [ La/Fe ] \sim + 0.7 dex for the most metal-rich Sgr dSph stars .
The large [ La/Y ] ratios show that the s -process enrichments came from the metal-poor population .
We can best understand the observed abundances with a model in which the Sgr dSph formed stars over a many Gyr and lost a significant fraction of its gas during its evolution .
Low-mass , metal-poor , AGB stars polluted the more metal-rich stars with s -process elements , and type Ia SN from low-mass progenitors enriched the ISM with iron-peak metals .
The type II/type Ia SN ratio was smaller than in the Galactic disk , presumably due to a slower star formation rate ; this resulted in the observed low [ \alpha /Fe ] , [ Al/Fe ] and [ Na/Fe ] ratios .
The fact that Sgr stars span such a wide range in metallicity leads us to conclude that their age spread is even larger than previously inferred .
We derive ages for these red giants using the Padova models ( Girardi et al .
2000 ) .
The ages span \sim 0.5 to 13 Gyr , which implies a very long duration of star formation in the central regions of the Sgr dSph .