
Up: Multicolour deep CCD
New UBVRI CCD photometry down to
is presented for
about 2000 stars altogether near the young open clusters NGC 3228, NGC
4103, NGC 5662 and NGC 6087. The present work in combination with
earlier proper motion, radial velocity, photoelectric and
spectroscopic observations leads to the following conclusions:
- (i)
- Visual fitting of the ZAMS to the bluest envelope of the
cluster sequence in the
;
;
and
diagrams over a broad range of V (
) gives
distances of
to NGC 4103,
to NGC 5662 and
to NGC 6087.
- (ii)
- Reddening across the observed regions in NGC 4103, NGC
5662 and NGC 6087 seems to be uniform with mean values of E(B-V) =
0.31, 0.34 and 0.22 respectively. The law of interstellar extinction
in the direction of these cluster regions is normal.
- (iii)
- The present work supports earlier finding that the Cepheid
variables V Cen and S Norm are probably members of NGC 5662 and NGC 6087
respectively.
- (iv)
- Bertelli et al. (1994) isochrones fitted to the bright
cluster members in the
;
;
and
diagrams indicate that the oldest cluster in the sample is NGC
3228 with an age of
and the youngest is NGC 4103 with
an age of
30 Myr. Both NGC 5662 and NGC 6087 are of nearly the
same age, having values
and 65 Myr respectively.
- (v)
- It is difficult to separate cluster members from field
stars on the basis of present observations alone; kinematic data are
also required. Deep higher precision photometric and multi-object
spectroscpic observations in the optical as well as in the near infrared
are required to identify the PMS stars in the clusters under discussion.
We thank the Mount Stromlo and Siding Spring Observatories of the
Australian National University for the allotment of observing time for
this project and the referee Dr. J.J. Clariá for constructive comments.
Financial support for RS provided by the then Department
of Industry, Technology and Commerce of Australia, by the IAU and by
the AAO are gratefully acknowledged.
This research has made use of the Open Cluster Data Base compiled by
J.-C. Mermilliod, Institut d'Astronomie de l'Université de
Lausanne, Switzerland.

Up: Multicolour deep CCD
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