The Massive Star Forming Region Cygnus OB2. II. Integrated Stellar
  Properties and the Star Formation History
  Cygnus OB2 is the nearest example of a massive star forming region,
containing over 50 O-type stars and hundreds of B-type stars. We have analysed
the properties of young stars in two fields in Cyg OB2 using the recently
published deep catalogue of Chandra X-ray point sources with complementary
optical and near-IR photometry. Our sample is complete to 1 Msun (excluding A
and B-type stars that do not emit X-rays), making this the deepest study of the
stellar properties and star formation history in Cyg OB2 to date. From Siess et
al. (2000) isochrone fits to the near-IR color-magnitude diagram, we derive
ages of 3.5 (+0.75/-1.0) and 5.25 (+1.5/-1.0) Myrs for sources in the two
fields, both with considerable spreads around the pre-MS isochrones. The
presence of a stellar population somewhat older than the present-day O-type
stars, also fits in with the low fraction of sources with inner circumstellar
disks (as traced by the K-band excess) that we find to be very low, but
appropriate for a population of age ~5 Myrs. We also find that the region lacks
a population of highly embedded sources that is often observed in young star
forming regions, suggesting star formation in the vicinity has declined. We
measure the stellar mass functions in this limit and find a power-law slope of
Gamma = -1.09 +/- 0.13, in good agreement with the global mean value estimated
by Kroupa (2002). A steepening of the slope at higher masses is observed and
suggested as due to the presence of the previous generation of stars that have
lost their most massive members. Finally, combining our mass function and an
estimate of the radial density profile of the association suggests a total mass
of Cyg OB2 of ~3 x 10^4 Msun, similar to that of many of our Galaxy's most
massive star forming regions.
