3. Determination of the age
We now proceed and will fit isochrones by eye to the colour-magnitude diagram of each field separately, with and as fit parameters. We used the isochrones of the Geneva group (Schaerer et al. 1993) for LMC metallicity ( or [Fe/H] dex). These isochrones include the effect of convective core overshooting and are based on the new opacities. The use of the isochrones of the Padova group (Alongi et al. 1993) would lead to slightly younger ages (about 0.05 in logarithmic age); but we stuck to the Geneva data to stay consistent with the analyses of other regions in and around LMC 4 by Olsen et al. (1997), Petr (1994) and Wilcots et al. (1996), as discussed in Sect. 5(see Table 6). In all, we estimate that the accuracy of the determined ages is in logarithmic units.
According to Ratnatunga & Bahcall (1985) the number of foreground stars is negligible in respect of the fit. Thus one can expect about 3 stars of our Galaxy towards the LMC in the colour range of mag and in the apparent visual magnitude range of mag mag (corresponding to the absolute visual magnitude range for LMC objects of approximately mag mag).
As examples of this fit to the 1st dataset we show four CMDs in Fig. 2: the CCD field 0 with the largest age of the analysed area (a), field 6 with one of the smallest ages (b), and field 22 to demonstrate the impossibility of fitting an isochrone to the last five fields (i.e. fields 20-24 corresponding to region e, see Sect. 4) because of a poorly populated upper main sequence (c). We also show the CMD for the whole 'J'-shaped region (i.e. all CCD fields, 0-24, without overlap) with the appropriate isochrones and the main sequence cut used to set up the mass function (see Braun 1996 for all CMDs). We find that all our fields indicate an age between 9 and 16 Myr. The results of all 25 fields are listed in Table 4.
Table 4. Age (t), reddening () and number of stars () of all 25 CCD fields of the 1st dataset containing stars with B, V magnitudes (inclusive multiple counts for stars in the overlapping regions)
The top of the main sequence region may contain data points for evolved stars. Given the age derived we expect their number to be small so that they cannot effect the age determination in a significant way.
The values for the reddening are in accordance with the foreground reddening of mag (Oestreicher et al. 1995). In performing the calibration of the fields in sequence from 3 to 11 we found small systematic inconsistencies. Since field 3 and 11 could be calibrated in an absolute manner, we have interpolated the calibration for the fields in between, leading to less certain values of the stellar colour and thus of , as given in Table 4. We judge the effect of this uncertainty on the ages as negligible.
For all our fields one has to realize that we implicitly assume that all stars seen in a field belong to the same population. However, projection effects can mask age gradients, because in fitting isochrones one is only sensitive to the youngest star population. Considering that star formation has been going on in all regions of the LMC some influence has to be expected. To handle this problem we investigate the luminosity and mass function for our region in the next section.
© European Southern Observatory (ESO) 1997
Online publication: March 24, 1998