Colliding Filaments and a Massive Dense Core in the Cygnus OB 7
  Molecular Cloud
  We report results of molecular line observations carried out toward a massive
dense core in the Cyg OB 7 molecular cloud. The core has an extraordinarily
large mass ($\sim1.1 \times 10^4$ $M_\odot$) and size ($\sim2 \times 5$
pc$^2$), but there is no massive young star forming therein. We observed this
core in various molecular lines such as C$^{18}$O($J=1-0$) using the 45m
telescope at Nobeyama Radio Observatory. We find that the core has an elongated
morphology consisting of several filaments and core-like structures. The
filaments are massive ($10^2-10^3$ $M_\odot$), and they are apparently
colliding against each other. Some candidates of YSOs are distributed around
their intersection, suggesting that the collisions of the filaments may have
influenced on their formation. To understand the formation and evolution of
such colliding filaments, we performed numerical simulations using the adaptive
mesh refinement (AMR) technique adopting the observed core parameters (e.g.,
the mass and size) as the initial conditions. Results indicate that the
filaments are formed as seen in other earlier simulations for small cores in
literature, but we could not reproduce the collisions of the filaments simply
by assuming the large initial mass and size. We find that the collisions of the
filaments occur only when there is a large velocity gradient in the initial
core in a sense to compress it. We suggest that the observed core was actually
compressed by an external effect, e.g., shocks of nearby supernova remnants
including HB21 which has been suggested to be interacting with the Cyg OB 7
molecular cloud.
