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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.
Stars form with a range of masses. Jeans mass collapse leads to higher density,
fragmentation and sub-collapse.
Well approximated as a power law - Salpeter index.
Mechanism determining the distribution of masses poorly understood, several
semi-heuristic theories, may be related to turbulence or competitive accretion.
Major open question is how many stars form in associations and clusters.
How do binaries and multiples form?
Summary of Orion Nebula including mass function (revised)
We really need a good figure explaining Present Day Mass Functions both in isolation and with continuous star formation.
Bother.
Theoretical Luminosity Function - from Schombert
Real Luminosity Functions - from Reyel - for illustration
We want to know what the Initial Mass Function is, the number of stars per unit mass formed initially before any stellar evolution has taken place.
What we observe is a present day luminosity function.
We are confounded by severe observational biases, and the need to
map an observed luminosity function of an evolved multi-age population (in one or more wavebands) into
an intrinsic mass function.
Then we have to deconvolve the present day mass function, which is composed
of multiple generations of partially evolved stars, of differing metallicity,
and deduce the IMF.
Underlying all this is the implicit premise that there exists ``the IMF'', ie we are assuming that in different circumstances and at different times the distribution of the numbers of stars of different mass has a robust functional form.
Whether the IMF is universal, or whether there can be radically different local IMFs,
with the global IMF formed from the summed of different IMFs is a major open question.
Particular open issues is whether populations of very different metallicities
have significantly different IMFs, and whether there exist more than one ``mode'' of
star formation (eg in dense cool regions vs dense hot regions vs low density cool regions)
and whether the IMF might be very different in different current star forming situations.
Having said that, the observational evidence, as originally noted by Salpeter, is that the inferred IMF is well fit by a power law over a large mass range.
dN/dm = C m-x
Where ``x'' is usually referred to as the Salpeter index.
Note that x=2 is a ``special value'' for x.
Observationally, x=2.35 for intermediate mass stars.
There are suggestions of breaks at both ends of the mass range,
with the slope being flatter at low masses and steeper at high masses.
There are also claims that the IMF is flatter (more high mass stars)
at the high mass end in starbursts and rich clusters, although this
claim is vulnerable to severe selection effects.
Bastian et al ARAA 2010 - conclusion(?)
Hillenbrand - IMF slope in details
We will want to add more figures here.
Last updated 08/11
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