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 \sim 1 M _ { \odot } ( 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 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 \sim 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 \pm 0.13 , in good agreement with the global mean value estimated by .
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 \sim 3 \times 10 ^ { 4 } M _ { \odot } , similar to that of many of our Galaxy ’ s most massive star forming regions .