Fifty Years of Imf Variation: the Intermediate-mass Stars

I track the history of star count estimates of the Milky Way field star and open cluster IMFs, concentrating on the neglected mass range from 1 to 15 M⊙. The prevalent belief that the IMF is universal appears to be without basis for this mass range. Two recent estimates of the field star IMF using different methods and samples give values of the average logarithmic slope Γ between -1.7 and -2.1 in the mass range 1.1 to 4 M⊙. Two older estimates between 2 and 15 M⊙, using much smaller samples, disagree severely; the field IMF in this range is essentially unknown from star counts. Variations in Γ among open cluster IMFs in this same mass range have not decreased despite numerous detailed studies, with a rather uniformly distributed spread of about 1.0 to 1.4, even for studies using homogeneous data and reduction procedures and including only clusters with a significant mass range. These cluster variations might be accommodated by the combined effects of sampling, systematic errors, stellar evolution uncertainties, dynamical evolution, and unresolved binaries. If so, then the cluster data are consistent with a universal IMF, but are also consistent with sizeable variations. The data do not allow an estimate of an average IMF or Γ because the average depends on the choice of weighting procedure and other effects. There is little basis for claims that the field star IMF is steeper or flatter than the cluster IMF in this mass range. If the spread in cluster IMFs is in excess of the effects listed above, real IMF variations must occur that do not depend much on environmental physical conditions explored so far. The complexity of the star formation process seen both in observations and in simulations of turbulent cluster formation suggests that large realization-to-realization differences might be expected. In this case an individual cluster IMF would be in part the product of evolutionary contingency in star formation, and the significant function of interest is the probability distribution of IMF parameters. In either case an average IMF can probably be better determined from the field star IMF using data from future space astrometric missions, or from combinations of indirect constraints using integrated light or chemical evolution applied to whole galaxies.