We present the first high spatial resolution X-ray study of NGC 2244 , the 2 Myr old stellar cluster immersed in the Rosette Nebula , using the Chandra X-ray Observatory .
Over 900 X-ray sources are detected ; 77 % have optical or FLAMINGOS near-infrared ( NIR ) stellar counterparts and are mostly previously uncatalogued young stellar cluster members .
All known OB stars with spectral type earlier than B1 are detected and the X-ray selected stellar population is estimated to be nearly complete between 0.5 and 3 M _ { \odot } .
The X-ray luminosity function ( XLF ) ranges from 29.4 < \log L _ { x } < 32.0 ergs s ^ { -1 } in the hard ( 2 - 8 keV ) band .
By comparing the NGC 2244 and Orion Nebula Cluster XLFs , we estimate a total population of \sim 2000 stars in NGC 2244 .
A number of further results emerge from our analysis : 1 ) The XLF and the associated K -band luminosity function indicate a normal Salpeter initial mass function ( IMF ) for NGC 2244 .
This is inconsistent with the top-heavy IMF reported from earlier optical studies that lacked a good census of < 4 M _ { \odot } stars .
2 ) The spatial distribution of X-ray stars is strongly concentrated around the central O5 star , HD 46150 .
The other early O star , HD 46223 , has few companions .
The cluster ’ s stellar radial density profile shows two distinctive structures : a power-law cusp around HD 46150 that extends to \sim 0.7 pc , surrounded by an isothermal sphere extending out to 4 pc with core radius 1.2 pc .
This double structure , combined with the absence of mass segregation , indicates that this 2 Myr old cluster is not in dynamical equilibrium .
Our results will strongly constrain models of the cluster formation process .
The spatial distribution of X-ray selected K -excess disk stars and embedded stars is asymmetric with an apparent deficit towards the north .
3 ) The fraction of X-ray-selected cluster members with K -band excesses caused by inner protoplanetary disks is 6 % , slightly lower than the 10 % disk fraction estimated from the FLAMINGOS study based on the NIR-selected sample .
This is due to the high efficiency of X-ray surveys in locating disk-free weak-lined T Tauri stars .
4 ) X-ray luminosities for 24 stars earlier than B4 confirm the long-standing \log ( L _ { x } / L _ { bol } ) \sim - 7 relation .
The Rosette OB X-ray spectra are soft and consistent with the standard model of small-scale shocks in the inner wind of a single massive star .
5 ) About 50 intermediate-mass ( 2 < M < 8 M _ { \odot } ) cluster members are identified ; they exhibit a wide range of X-ray luminosities consistent with previously studied samples of Herbig Ae/Be stars .
Their K -excess disk fraction is \sim 10 % , indicating that the Herbig Ae/Be phenomenon is rare or short-lived .