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-mainsequence (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 ≃0.3 M _⊙ 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.