An analysis of the deuterium fractionation of star-forming cores in the
  Perseus molecular cloud
  We have performed a pointed survey of N2D+ 2-1 and N2D+ 3-2 emission toward
64 N2H+-bright starless and protostellar cores in the Perseus molecular cloud
using the Arizona Radio Observatory Submillimeter Telescope and Kitt Peak 12 m
telescope. We find a mean deuterium fractionation in N2H+, R_D =
N(N2D+)/N(N2H+), of 0.08, with a maximum R_D = 0.2. In detected sources, we
find no significant difference in the deuterium fractionation between starless
and protostellar cores, nor between cores in clustered or isolated
environments. We compare the deuterium fraction in N2H+ with parameters linked
to advanced core evolution. We only find significant correlations between the
deuterium fraction and increased H_2 column density, as well as with increased
central core density, for all cores. Towards protostellar sources, we
additionally find a significant anti-correlation between R_D and bolometric
temperature. We show that the Perseus cores are characterized by low CO
depletion values relative to previous studies of star forming cores, similar to
recent results in the Ophiuchus molecular cloud. We suggest that the low
average CO depletion is the dominant mechanism that constrains the average
deuterium fractionation in the Perseus cores to small values. While current
equilibrium and dynamic chemical models are able to reproduce the range of
deuterium fractionation values we find in Perseus, reproducing the scatter
across the cores requires variation in parameters such as the ionization
fraction or the ortho- to para-H_2 ratio across the cloud, or a range in core
evolution timescales.
