The influence of multiple stars on the high-mass stellar initial mass function and age-dating of young massive star clusters

摘要

The study of young stellar populations has revealed that most stars are inbinary or higher order multiple systems. In this study the influence on thestellar initial mass function (IMF) of large quantities of unresolved multiplemassive stars is investigated by taking into account stellar evolution andphotometrically determined system masses. The models where initial masses arederived from the luminosity and colour of unresolved multiple systems show thateven under extreme circumstances (100% binaries or higher order multiples) thedifference between the power-law index of the mass function of all stars andthe observed mass function is small (~0.1). Thus, if the observed IMF has theSalpeter index alpha = 2.35 then the true stellar IMF has an index not flatterthan alpha = 2.25. Additionally, unresolved multiple systems may hide between15 and 60% of the underlying true mass of a star cluster. While already a knownresult, it is important to point out that the presence of a large number ofunresolved binaries amongst pre-main-sequence (PMS) stars induces a significantspread in the measured ages of these stars even if there is none. Also,lower-mass stars in a single-age binary-rich cluster appear older than themassive stars by about 0.6 Myr.