Cluster Formation Triggered by Filament Collisions in Serpens South
  The Serpens South infrared dark cloud consists of several filamentary ridges,
some of which fragment into dense clumps. On the basis of CCS ($J_N=4_3-3_2$),
HC$_3$N ($J=5-4$), N$_2$H$^+$ ($J=1-0$), and SiO ($J=2-1, v=0$) observations,
we investigated the kinematics and chemical evolution of these filamentary
ridges. We find that CCS is extremely abundant along the main filament in the
protocluster clump. We emphasize that Serpens South is the first
cluster-forming region where extremely-strong CCS emission is detected. The
CCS-to-N$_2$H$^+$ abundance ratio is estimated to be about 0.5 toward the
protocluster clump, whereas it is about 3 in the other parts of the main
filament. We identify six dense ridges with different $V_{\rm LSR}$. These
ridges appear to converge toward the protocluster clump, suggesting that the
collisions of these ridges may have triggered cluster formation. The collisions
presumably happened within a few $\times \ 10^5$ yr because CCS is abundant
only in such a short time. The short lifetime agrees with the fact that the
number fraction of Class I objects, whose typical lifetime is $0.4 \times \
10^5$ yr, is extremely high as about 70 percent in the protocluster clump. In
the northern part, two ridges appear to have partially collided, forming a
V-shape clump. In addition, we detected strong bipolar SiO emission that is due
to the molecular outflow blowing out of the protostellar clump, as well as
extended weak SiO emission that may originate from the filament collisions.
