The Slope of the Upper End of the IMF and the Upper Mass Limit: An Observer's Perspective

TL;DR

This paper reviews methods for measuring the upper end slope of the IMF and the upper mass limit, concluding current data show consistent IMF slopes and that the supposed 150 solar mass upper limit is not supported by recent evidence.

Contribution

It critically examines observational methods for IMF slope and upper mass limit, highlighting uncertainties and invalidating previous claims of a strict upper mass limit.

Findings

IMF slopes are consistent within errors

The 150 solar mass upper limit is unsupported by recent data

Measurement uncertainties significantly affect IMF and mass limit estimates

Abstract

There are various ways of measuring the slope of the upper end of the IMF. Arguably the most direct of these is to place stars on the H-R diagram and compare their positions with stellar evolutionary models. Even so, the masses one infers from this depend upon the exact methodology used. I briefly discusssome of the caveats and go through a brief error analysis. I conclude that the current data suggest that the IMF slopes are the same to within the errors. Similarly the determination of the upper mass "limit" is dependent upon how well one can determine the masses of the most massive stars within a cluster. The recent finding by Crowther et al (2010) invalidates the claim that there is a 150Mo upper limit to the IMF, but this is really not surprising given the weakness of the previous evidence.