4. Luminosity function and mass function
To get a first hint about the possibility of a superposition of stellar populations with different ages - the isochrone fitting would only be sensitive for the youngest population if the turn-off points (TOPs) of the main sequence (MS) are not too different - we combine 5 CCD fields to one region (with an area of on the sky, see Fig. 1). For each region we construct the luminosity function (with slope ) and the mass function (with slope ), as well as for the whole analysed area. This was made for equidistant intervals I with the usual approach (see Scalo 1986):
As an example we present in Fig. 4 the luminosity and the mass function of region b. The translation of brightness to mass for stars on the MS was done with the best fitting Geneva isochrone of 10 Myr, which can be described with the numerical relation:
with a rms error of 0.031, beeing valid up to the TOP at mag (corresponding to ).
The resulting slopes are, as shown in Table 5, very close to the standard values of and (Will 1996). A higher negative slope would indicate the superposition of different star populations (note that incompleteness in the faint bins causes a flatter distribution than it really would be).
Table 5. Per region the number of stars in the photometry is given. For the determination of the luminosity function with slope only the indicated number of stars on the main sequence is used. The same is given for the mass function (with the derived slope ). No correction for incompleteness at the faint end is made.
Since the slopes we find are the same as those found in general, we conclude that there is no significant contribution of older age groups in these fields up to 300 Myr (equivalent to stars of 3 still being on the main sequence). Even older star formation events may be present but they would contribute at fainter levels than the useable limit of our photometry.
© European Southern Observatory (ESO) 1997
Online publication: March 24, 1998