Chemical evolution of the HC3N and N2H+ molecules in dense cores of the
  Vela C giant molecular cloud complex
  We have observed the HC3N (J=10-9) and N2H+ (J=1-0) lines toward the Vela C
molecular clouds with the Mopra 22 m telescope to study chemical
characteristics of dense cores. The intensity distributions of these molecules
are similar to each other at an angular resolution of 53", corresponding to
0.19 pc suggesting that these molecules trace the same dense cores. We
identified 25 local peaks in the velocity-integrated intensity maps of the HC3N
and/or N2H+ emission. Assuming LTE conditions, we calculated the column
densities of these molecules and found a tendency that N2H+/HC3N abundance
ratio seems to be low in starless regions while it seems to be high in
star-forming regions, similar to the tendencies in the NH3/CCS, NH3/HC3N, and
N2H+/CCS abundance ratios found in previous studies of dark clouds and the
Orion A GMC. We suggest that carbon chain molecules, including HC3N, may trace
chemically young molecular gas and N-bearing molecules, such as N2H+, may trace
later stages of chemical evolution in the Vela C molecular clouds. It may be
possible that the N2H+/HC3N abundance ratio of ~ 1.4 divides the star-forming
and starless peaks in the Vela C, although it is not as clear as those in
NH3/CCS, NH3/HC3N, and N2H+/CCS for the Orion A GMC. This less clear separation
may be caused by our lower spatial resolution or the misclassification of
star-forming and starless peaks due to the larger distance of the Vela C. It
might be also possible that the HC3N (J=10-9) transition is not a good chemical
evolution tracer compared with CCS (J=4-3 and 7-6) transitions.
