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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
M0.”
Is there an upper limit for formation of massive stars? Not
enough is known about massive stars either theoretically or
observationally to draw a firm conclusion. There are no indirect
or direct observations of stars with masses greater than ~150
Solar masses. Does this mean the upper limit has been reached or
does this merely reflect an observational bias, because massive
stars are very rare and have very short lives? If the present
generally accepted IMF for most of the observed Milky Way and
Magellanic Clouds is true for most star formation, then Weidner
and Kroupa (2004) are “…led to conclude that a fundamental
maximum stellar mass near 150 M0 exits…” This is supported by
recent work that the supernova progenitors responsible for
extremely metal-poor (EMP) stars most likely had masses of
20-130 Solar, but not more than 130 Solar masses (Nomoto, 2004).
On the other hand, Massey and Hunter (1997) in studying R136
feel “…we have yet to encounter any physical limit to how
massive a star may form in nature, that the only limit we see is
a statistical one, depending upon the richness (and age) of the
cluster.” More observations and theoretical work needs to be
done before there is a definitive answer on how heavy a star can
get. Such work may include infrared and visual observations from
the James Webb telescope when it becomes operational in several
years, and the Spitzer Space Telescope is beginning to provide
valuable observations of star forming regions (Spitzer, 2005).
Infrared and radio observations of young star clusters and star
forming regions is very important and will add new information
about massive star formation. Observations of known massive
stars, such as Eta Carina, P Cygni, and Rho Cassiopeiae provide
us a daily view of the life and behavior of mature, living,
massive stars. Continuing supernovae observations will provide
information about their progenitor stars and the fate of massive
stars in general.
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Essay posted January 22, 2005.
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