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Stellar birth is defined as when nuclear fission begins in a
star’s core. Salpeter in 1955 introduced the concept of initial
mass function (IMF), which represents the number of stars with a
given mass M at stellar birth per unit volume of space (Salpeter,
1955; Moore, 2002). The IMF is assumed to be a power law in the
form:
IMF (M) = cM-(1+x) , where x is the parameter of the power law.
Salpeter set x at 1.35.
In the Solar neighborhood the IMF = M-2.35 (Moore, 2002). This
means that low mass stars are much more likely to form than high
mass stars.
The exact nature of the Salpeter constant [x] remains a subject
of much investigation, especially for massive stars (Kroupa,
2004). A massive star often has more than one companion which
constrains their formation, and a significant fraction of all
massive OB stars are found far from their probable birth site,
presumably because they were ejected from cores of binary-rich
star systems (Kroupa, 2004).
There are theoretical considerations and some observational
evidence to suggest star formation in higher metallicity
environments appears to produce more low mass stars (Kropa,
2001). Thus, we might postulate that earlier generations of
massive stars, such as the purported very early Population III
stars, were truly massive compared to the largest Supergiant
stars we see today, because, in general, star formation today
takes place in the presence of a higher metal content than in
earlier epochs of star formation (Mackey, 2003). Unfortunately,
there is no strong data thus far to support this conclusion. It
appears that the IMF is very uniform over a wide variety of
conditions from the formation of low mass brown dwarf stars to
very massive stars. According to Kroupa (2004): “This general
insight appears to hold for populations including present-day
star formation in small molecular clouds, rich and dense massive
star-clusters forming in giant clouds, through to ancient and
metal-poor exotic stellar populations that may be dominated by
dark matter. This apparent universality of the IMF is a
challenge for star formation theory, because elementary
considerations suggest that the IMF ought to systematically vary
with star-forming regions.” Recent work has also shown no
deficiency of massive stars at high metallicity (Schaerer,
2003). |
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