The IMF Revisited: A Case for Variations

TL;DR

This paper reviews the current state of research on the initial mass function (IMF) across various stellar environments, highlighting uncertainties and potential variations, and discusses implications for theoretical models.

Contribution

It provides a comprehensive survey of IMF results, emphasizing uncertainties and presenting evidence for possible IMF variations across different clusters and galaxies.

Findings

Uncertainties like binaries and mass segregation can steepen the true IMF.

About 60 clusters show a spread in IMF index of at least one.

Evidence suggests either large uncertainties or real IMF variations not linked to environment.

Abstract

A survey of results concerning the IMF derived from star counts is presented, including work up to, but not including, that presented in these proceedings. The situation regarding low-mass stars in the field and in clusters, high-mass stars and intermediate-mass stars in clusters and associations of the Milky Way and LMC, pre-main sequence objects in visible and embedded clusters, and the IMF in galaxies more distant than the Magellanic Clouds is discussed, with an emphasis on the sources of uncertainty. Most of these uncertainties, especially radial mass segregation and unresolved binaries, would steepen the true IMF relative to the apparent IMF. Several cases of apparently large variations in cluster IMFs are pointed out, and a graphical comparison of results for about 60 clusters shows a spread of at least unity in the logarithmic IMF index for all mass ranges above about 1 $M_{\sun}$. I conclude that either: 1. The uncertainties are so large that very little can be said about an average IMF or IMF variations; or 2. If the observations are taken at face value, there are strong indications of IMF variations, which do not seem to correlate with obvious environmental conditions like metallicity or stellar density. If there is an average IMF, I suggest that it is steepest at intermediate masses. If the variations are real, they offer a useful test of theoretical models.