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The time formation for massive stars is so short they reach the
zero-age Main Sequence (ZAMS) while still embedded in their
birth clouds (Feldt, 2003). This makes it quite difficult to
follow the birth and development of massive stars. Moreover,
massive star formation takes place in distant complex molecular
clouds. It is thought massive stars form almost exclusively in a
clustered mode (Beuther, 2004). Observational and theoretical
work has shown that massive stars can be formed via disk
accretion (McKee, 2003; Yorke, 2002). It is also possible
intermediate-mass protostars may merge to form more massive
stars through collisions and interactions in the molecular
clouds. Recent, high-spatial-resolution interferometric dust
continuum observations have enabled the derivation of a
protocluster mass distribution function for the massive star
forming region IRAS 19410+2336 (Beuther, 2004). The initial
results show a mass distribution consistent with the commonly
accepted stellar initial mass function. Fragmentation of the
initial mass cores in stellar forming regions probably
determines the masses of the final stars. According to Beuther
(2004): “This implies that stars of all masses can form via
accretion processes, and coalescence of intermediate-mass
protostars appears not to be necessary.”
Massive stars generally end their lives as type II supernovae.
The yields of Type II supernovae alpha elements, such as, O, Mg,
and Ne, are a function of the progenitor’s mass, whereas the
yield of a supernova’s explosive reaction elements, such as Fe,
Si, and Ca is not as closely related to the star’s original mass
prior to its explosion (Gibson, 1998). Looking at the yields of
these two groups of elements for various Type II supernovae and
tying these yields to element abundances in metal poor stars in
the Galactic halo gives an “indirect probe for the upper mass
limit to the IMF” (Gibson, 1998). Unfortunately, there are
substantial uncertainties for these yields. Looking at this
question in detail, Gibson (1998) states: “…we simply cannot
constrain the upper limit to mU to anything better than ~ 60-200
Mo.” |
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