Chandra study of the cepheus B star-forming region:: Stellar populations and the initial mass function

The Cepheus B ( Cep B) molecular cloud and a portion of the nearby Cep OB3b OB association, one of the most active regions of star formation within 1 kpc, have been observed with the ACIS detector on board the Chandra X-Ray Observatory. We detect 431 X-ray sources, of which 89% are confidently identified as clustered pre-main-sequence ( PMS) stars. Two main results are obtained. First, we provide the best census to date for the stellar population of the region. We identify many members of two rich stellar clusters: the lightly obscured Cep OB3b association and the deeply embedded cluster in Cep B, whose existence was previously traced only by a handful of radio sources and T Tauri stars. Second, we find a discrepancy between the X-ray luminosity functions of the Cep OB3b and the Orion Nebula cluster. This may be due to the different initial mass functions of the two regions ( an excess of similar or equal to 0.3M(circle dot) stars) or different age distributions. Several other results are obtained. A diffuse X-ray component seen in the field is attributed to the integrated emission of unresolved low-mass PMS stars. The X-ray emission from HD 217086 ( O7n), the principle ionizing source of the region, follows the standard model, involving many small shocks in an unmagnetized radiatively accelerated wind. X-ray source 294 joins a number of similar superflare PMS stars for which long magnetic structures may connect the protoplanetary disk to the stellar surface.