A comprehensive study of the rich open star cluster
NGC 2099 based on deep 
CCD observations*
1
State Observatory, Manora Peak, Nainital 263129, India
2
Indian Institute of Astrophysics, Bangalore 560034, India
Corresponding author: R. Sagar, sagar@upso.ernet.in
Received:
12
July
2001
Accepted:
15
October
2001
The CCD observations of the rich open star cluster NGC 2099 and its
surrounding field region have been carried out up to a limiting magnitude of
mag in B, V and I passbands for the first time. A total of
~12 000 stars have been observed in the area of about 
in the cluster region, as well as ~2180 stars in the ~
area of the field region located ~
away from the cluster center. The cluster parameters determined by fitting the
convective core overshoot isochrones in the 
and 
diagrams
are 
mag, distance = 
pc, age = 400 Myr and
metallicity 
. A well-defined cluster main sequence spread over about
8 mag in range is observed for the first time. Its intrinsic spread amounting to
~0.06 mag in colour is almost the same over the entire brightness and can
be understood in terms of the presence of physical/optical binaries. The core
and cluster radii determined from the radial stellar density profiles are
185´´ and 1000´´ respectively. Only about 22% of cluster members
are present in the core region. The effects of mass segregation, most probably
due to dynamical evolution, have been observed in the cluster. The mass function
slope of the entire cluster is ~
. It becomes closer to the
Salpeter value of -1.35, if flattening in the cluster mass function due
to presence of both binaries and a much more extended corona is considered.
Key words: open clusters and associations: individual: NGC 2099 / stars: HR diagram / evolution / formation
© ESO, 2002