next previous
Up: Multicolour deep CCD

Possible pre-MS cluster members

To see whether the present observations have reached to the faintness level of PMS stars or not, the tex2html_wrap_inline3536, tex2html_wrap_inline3538 diagrams for the stars observed by us (filled circles), as well as for the members (generally proper motion) observed photoelectrically by others (crosses) but not present in our sample have been plotted in Fig. 18 (click here) for all the star clusters under study. In plotting this figure, we used uniform interstellar extinction with values of E(B-V) = 0.03, 0.31, 0.34 and 0.22 for NGC 3228, NGC 4103, NGC 5662 and NGC 6087 respectively. The corresponding values used for their true distance moduli are 8.5, 11.5, 9.5 and 9.9. In this diagram, we have also plotted Iben's (1965) isochrones for contracting stars with ages 10 and 30 Myr and the theoretical stellar birthline given by Palla & Stahler (1992), which marks the luminosity and temperature at which contracting stars are expected first to become visible at optical wavelengths, together with the ZAMS. The following inferences can be drawn from this figure:

(i)
 A few stars in NGC 4103 and NGC 6087 are located well above the theoretical stellar birthline. They are probably field stars.
(ii)
 No PMS is clearly visible in the clusters under study. The low-mass stars which formed at the same time as the more massive stars are expected to join the ZAMS at tex2html_wrap_inline3542, +4.1, +6.0 and +5.8 for the clusters NGC 3228, NGC 4103, NGC 5662 and NGC 6087 respectively. So the present observations have barely reached to the faintness level for detecting the PMS stars. Also to make sure, one should look for signatures of PMS stars like tex2html_wrap_inline3550 emission, irregular variability or ultraviolet excess in them, as Adams et al.  (1983) did in NGC 2264. On the other hand, our observations in NGC 4755 have been able to detect PMS stars (see Sagar & Cannon 1995). NGC 4755 is the youngest (tex2html_wrap_inline3552 Myr) open star cluster observed under this programme. The clearly visible kink away from the ZAMS around tex2html_wrap_inline3554 in tex2html_wrap_inline3556, tex2html_wrap_inline3558 diagram of NGC 4755 (see Fig. 7 (click here) in Sagar & Cannon 1995) is interpreted as the MS turn-on for PMS stars. Most of the PMS stars have an age around 6 Myr, indicating that massive stars in NGC 4755 formed about 4 Myr before the bulk of the low-mass stars.


next previous
Up: Multicolour deep CCD

Copyright by the European Southern Observatory (ESO)
web@ed-phys.fr