The study of young stellar populations has revealed that most stars are in binary or higher order multiple systems .
In this study the influence on the stellar initial mass function ( IMF ) of large quantities of unresolved multiple massive stars is investigated by taking into account stellar evolution and photometrically determined system masses .
The models where initial masses are derived from the luminosity and colour of unresolved multiple systems show that even under extreme circumstances ( 100 % binaries or higher order multiples ) the difference between the power-law index of the mass function of all stars and the observed mass function is small ( { { { { \mathrel { \mathchoice { { \mbox { \lower 2.15 pt \vbox { \halign { \cr } $ \displaystyle% \hfil < $ \cr$ \displaystyle \hfil \sim$ } } } } } { { \mbox { \lower 2.15 pt \vbox { \halign { \cr } % $ \textstyle \hfil < $ \cr$ \textstyle \hfil \sim$ } } } } } { { \mbox { \lower 2.15 pt \vbox { % \halign { \cr } $ \scriptstyle \hfil < $ \cr$ \scriptstyle \hfil \sim$ } } } } } { { \mbox { \lower 2 % .15 pt \vbox { \halign { \cr } $ \scriptscriptstyle \hfil < $ \cr$ \scriptscriptstyle \hfil% \sim$ } } } } } } 0.1 ) .
Thus , if the observed IMF has the Salpeter index \alpha = 2.35 then the true stellar IMF has an index not flatter than \alpha = 2.25 .
Additionally , unresolved multiple systems may hide between 15 and 60 % of the underlying true mass of a star cluster .
While already a known result , it is important to point out that the presence of a large number of unresolved binaries amongst pre-main-sequence ( PMS ) stars induces a significant spread 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 the massive stars by about 0.6 Myr .