  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, has
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 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 different Initial Mass
Functions of two regions (excess of ~0.3 solar mass 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. The X-ray source #294 joins a
number of similar superflare PMS stars where long magnetic structures may
connect the protoplanetary disk to the stellar surface.

Astrophysics