Molecular cloud fragment in Carina
33
For this class continue to read chapters 1,2 and 3 in "Galaxies in the Universe"
We cover star formation, the IMF, binary formation and the secondary mass function rather quickly.
Star formation is covered in depth in Astro 420
Cold gas, becomes Jeans unstable, collapses, fragments, reaches T, rho high enough for fusion.
Molecular cloud fragment in Carina
Bate's star formation simulation
How do you get different mass stars?
There are three primary mechanisms suggested for generating such a mass function,
all essentially presume that there is local star formation within a larger mass of gas.
Memo to self: insert figures later.
Last updated 08/11
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The Kolmogorov index of the spectrum of fluctuations in turbulent fluids is 2.33,
interestingly close to the Salpeter index.
An obvious implication is that star forming gas is turbulent (generally true) and
that the IMF follows from the spectrum of fluctuations in the turbulent flow of the gas.
The microphysics of how this would happen are not known, though there has been
some interesting theoretical work on the issue.
There are claims that the structure of the ISM is best represented by
a fractal structure, in which case the IMF is recovered from the mass spectrum
of cool gas on small scales, given an interpenetrating fractal hot ISM.
Again the microphysics are not well understood, though there is some
theoretical work on the issue.
A major open issue is whether the ISM is actually anything like fractal
on the relevant scales or if it is actually smooth.
A contrasting view, represents the mass spectrum of stars essentially being due to
the spectrum of masses expected from a bottom-up formation scenario, where there is
some universal minimum mass scale for protostars, and the more massive stars form from
mergers of small lumps.
The main problem with this view is that the time scale to build up the more
massive stars is short, requiring a very high space density of protostars.
On the other hand, massive stars are preferentially observed in high density regions,
on the third hand, high density regions are where most of the mass is, and that is
where one would expect to observe high mass stars in any case. On the fourth hand,
dynamical relaxation drives high mass stars towards high density regions.
One interesting possibility is that mergers are relevant for stars with
masses above 10 solar masses or so, which have difficulty forming from direct
gas accretion due to high photon pressure at early times. In which case
mergers are mostly relevant for the highest mass stars only (possibly accounting
for the alleged break in the high mass end of the IMF?).
