A Universal Stellar Initial Mass Function: A Critical Look at Variations

Few topics in astronomy initiate such vigorous discussion as whether
or not the initial mass function (IMF) of stars is universal, or
instead sensitive to the initial conditions of star formation.  The
distinction is of critical importance: variations in the IMF could
provide deep insights into the process by which stars form, and nearly
all of the observable properties of a stellar population or galaxy
depend to some extent on the underlying IMF.  I will describe results
that appear in a recent review (Bastian, Covey, & Meyer, ARAA 2010 in
press) that take a critical look at the case for IMF variations, with
a view towards whether other explanations are sufficient given the
evidence.  I focus on new results from associations and young
clusters, the companion mass ratio distribution of multiple star
systems, and very young super-star clusters in starburst galaxies.  We
conclude that there is currently no strong evidence that the IMF
varies strongly and systematically as a function of initial conditions
of star formation.