The relation between the most-massive star and its parental star cluster mass
Abstract
We present a thorough literature study of the most-massive star, mmax, in several young star clusters in order to assess whether or not star clusters are populated from the stellar initial mass function (IMF) by random sampling over the mass range 0.01 < m < 150 Msol without being constrained by the cluster mass, Mecl. The data reveal a partition of the sample into lowest mass objects (Mecl < 102 Msol), moderate mass clusters (102 Msol < Mecl < 103 Msol) and rich clusters above 103 Msol. Additionally, there is a plateau of a constant maximal star mass (mmax ~ 25 Msol) for clusters with masses between 103 Msol and 4 103 Msol. Statistical tests of this data set reveal that the hypothesis of random sampling from the IMF between 0.01 and 150 Msol is highly unlikely for star clusters more massive than 102 Msol with a probability of p ~ 2 10-7 for the objects with Mecl between 102 Msol and 103 Msol and p ~ 3 10-9 for the more massive star clusters. Also, the spread of mmax values at a given Mecl is smaller than expected from random sampling. We suggest that the basic physical process able to explain this dependence of stellar inventory of a star cluster on its mass may be the interplay between stellar feedback and the binding energy of the cluster-forming molecular cloud core. Given these results, it would follow that an integrated galactic initial mass function (IGIMF) sampled from such clusters would automatically be steeper in comparison to the IMF within individual star clusters.
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