Chemical Signature Indicating A Lack of Massive Stars in Dwarf Galaxies
  Growing evidence supports an unusual elemental feature appearing in nearby
dwarf galaxies, especially dwarf spheroidals (dSphs), indicating a key process
of galaxy evolution that is different from that of the Galaxy. In addition to
the well-known deficiency of alpha-elements in dSphs, recent observations have
clearly shown that s-process elements (Ba) are significantly enhanced relative
to Fe, alpha-, and r-process elements. This enhancement occurs in some dSphs as
well as in the Large Magellanic Cloud, but is unseen in the Galaxy. Here we
report that this feature is evidence of the lack of very massive stars (> 25
solar mass) as predicted in the low star formation rate environment, and we
conclude that the unique elemental feature of dwarf galaxies including a
low-alpha/Fe ratio in some low-metallicity stars is, at least in some part,
characterized by a different form of the initial mass function. We present a
detailed model for the Fornax dSph galaxy and discuss its complex chemical
enrichment history together with the nucleosynthesis site of the light
s-process element Y.
