Molecular Cloud Cores with High Deuterium Fraction: Nobeyama
  Single-Pointing Survey
  We present the results of a single-pointing survey of 207 dense cores
embedded in Planck Galactic Cold Clumps distributed in five different
environments ($\lambda$ Orionis, Orion A, B, Galactic plane, and high
latitudes) to identify dense cores on the verge of star formation for the study
of the initial conditions of star formation. We observed these cores in eight
molecular lines at 76-94 GHz using the Nobeyama 45-m telescope. We find that
early-type molecules (e.g., CCS) have low detection rates and that late-type
molecules (e.g., N$_2$H$^+$, c-C$_3$H$_2$) and deuterated molecules (e.g.,
N$_2$D$^+$, DNC) have high detection rates, suggesting that most of the cores
are chemically evolved. The deuterium fraction (D/H) is found to decrease with
increasing distance, indicating that it suffers from differential beam dilution
between the D/H pair of lines for distant cores ($>$1 kpc). For $\lambda$
Orionis, Orion A, and B located at similar distances, D/H is not significantly
different, suggesting that there is no systematic difference in the observed
chemical properties among these three regions. We identify at least eight high
D/H cores in the Orion region and two at high latitudes, which are most likely
to be close to the onset of star formation. There is no clear evidence of the
evolutionary change in turbulence during the starless phase, suggesting that
the dissipation of turbulence is not a major mechanism for the beginning of
star formation as judged from observations with a beam size of 0.04 pc.
