Is the Stellar Initial Mass Function Universal?

Stars are believed to form out of gas and dust clouds by gravitational instability. The probability distribution function of the initial mass of the stars is known in astrophysics as the initial mass function, or IMF. In the decades since 1955, when Edwin Salpeter first determined the IMF in the neighborhood of the Sun, only relatively small variations of the IMF have been found within our own Milky Way, despite enormous variations in the physical conditions within star-forming regions - and so the IMF has been assumed to be universal. However, in the past two years a number of independent extragalactic studies have found significant deviations from the IMF as measured in the Milky Way. The observations are based on a variety of independent techniques, ranging from gravitational lensing, to stellar kinematics of elliptical galaxies, to gas kinematics of spiral galaxies, and to spectral diagnostics of stellar populations. Based on these observations, the IMF may in fact depend on the mass of the galaxy and hence on the cosmological time at which the stars formed, possibly reflecting the evolving physical conditions in the expanding universe. The non-universal IMF is a remarkable discovery that, if confirmed, would have profound implications for many areas of astrophysics. In addition to its relevance for star formation theory, a systematic variation of the IMF with galaxy stellar mass, age, or metallicity would have profound implications for understanding the structure and evolution of galaxies and for the reconstruction of the cosmic star formation history.

This workshop will bring together a focused group of key people including star formation theorists, galaxy evolution theorists, Galactic star formation observers, stellar spectroscopists, and some of the extragalactic observers who have published results on deviations from a universal IMF. The aims of the workshop are to

- critically analyze both galactic and extragalactic observations of the IMF, with a particular focus on assumptions and possible systematic errors;

- compare current theories of star formation with these observations and understand the physical origin of any predicted IMF non-universality;

- investigate the connection between nuclear activity and the IMF;

- explore the implications of a non-universal IMF for galactic structure and evolution, and for the current theory of star formation. Are changes needed to accommodate the new observations? Are there other observables that correlate with the IMF and can therefore be used to constrain the unknown IMF in extragalactic systems?; and

- create cross-subdisciplinary links, forge new collaborations, and begin new studies on the IMF.