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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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