Modeling a High‐Mass Turn‐Down in the Stellar Initial Mass Function

Statistical sampling from the stellar initial mass function (IMF) for allstar-forming regions in the Galaxy would lead to the prediction of ~1000 Msunstars unless there is a rapid turn-down in the IMF beyond several hundred solarmasses. Such a turndown is not necessary for dense clusters because the numberof stars sampled is always too small. Here we explore several mechanisms for anupper mass cutoff, including an exponential decline of the star formationprobability after a turbulent crossing time. The results are in good agreementwith the observed IMF over the entire stellar mass range, and they give agradual turn down compared to the Salpeter function above ~100 Msun for normalthermal Jeans mass, M_J. The upper mass turn down should scale with M_J indifferent environments. A problem with the models is that they cannot give boththe observed power-law IMF out to the high-mass sampling limit in denseclusters, as well as the observed lack of supermassive stars in whole galaxydisks. Either there is a sharper upper-mass cutoff in the IMF, perhaps fromself-limitation, or the IMF is different for dense clusters than for themajority of star formation that occurs at lower density. Dense clusters seem tohave an overabundance of massive stars relative to the average IMF in a galaxy.Comment: 19 pages, 2 figures, Astrophysical Journal, Vol 539, August 10, 200