5 IMF

We used the term initial mass function (IMF) for an observed mass function that is already evolved and we assume that the stars are coeval (Parker & Garmany 1993) although if we refer to our previous and present results (Testor & Schild 1993) this is not completely valid. To construct the IMF we counted between mass tracks shown in the H-R diagram the number of stars. Using Scalo's (1986) notation, the slope of the initial mass function is

$$\rm\Gamma = d \log \xi(\log {\it m})/d log {\it m}$$

where m is the stellar mass in units of the solar mass and the mass function $\xi = \xi[(\log m)^{-1}$ kpc^-2] is the number of stars born per square kiloparsec and per unit logarithmic mass interval.

We combine stars with known spectral types and stars of which $M_{\rm bol}$ and $T_{\rm eff}$ were derived by photometry only and consider that the incompleteness becomes significant below $10\,M_{\odot}$.We find IMF slopes of $\Gamma = -1.29 \pm 0.20$ and $\Gamma = -1.05$$\pm$ 0.12 for LH 101 and LH 104, respectively. The IMF slope for LH101 is close to that of Salpeter (1955). As its stars constitute two age groups the IMF should represent rather the PDMF (Massey 1997), i.e. the mass function of the massive stars observed at the present day. The derived mass function $\xi$ is shown in Figs. 7a and b. The flatter slope of the IMF in LH 104 is probably a consequence of our approximate method in deriving the parameters of the O and WR components in the massive binary systems. Some of these systems could contain more than two stars.

The IMF slopes found are comparable with those of numerous LMC OB associations in the LMC studied by different authors and listed by Massey (1997).

  

Figure 7: a-b) The initial mass function for LH 104 and LH 101. The slope determined from a fit to the mass bins with 10 $M_{\odot}$ $\leq$ M $\leq$ 60 $M_{\odot}$ is represented by a dashed line and with 10 $M_{\odot}$ $\leq$ M $\leq$ 85 $M_{\odot}$ by a solid curve