H2D+ in the high mass star-forming Region Cygnus-X
  H2D+ is a primary ion which dominates the gas-phase chemistry of cold dense
gas. Therefore it is hailed as a unique tool in probing the earliest,
prestellar phase of star formation. Observationally, its abundance and
distribution is however just beginning to be understood in low-mass prestellar
and cluster-forming cores. In high mass star forming regions, H2D+ has been
detected only in two cores, and its spatial distribution remains unknown. Here
we present the first map of the 372 GHz ortho-H2D+ and N2H+ 4-3 transition in
the DR21 filament of Cygnus-X with the JCMT, and N2D+ 3--2 and dust continuum
with the SMA. We have discovered five very extended (<= 34000 AU diameter) weak
structures in H2D+ in the vicinity of, but distinctly offset from embedded
protostars. More surprisingly, the H2D+ peak is not associated with either a
dust continuum or N2D+ peak. We have therefore uncovered extended massive cold
dense gas that was undetected with previous molecular line and dust continuum
surveys of the region. This work also shows that our picture of the structure
of cores is too simplistic for cluster forming cores and needs to be refined:
neither dust continuum with existing capabilities, nor emission in tracers like
N2D+ can provide a complete census of the total prestellar gas in such regions.
Sensitive H2D+ mapping of the entire DR21 filament is likely to discover more
of such cold quiescent gas reservoirs in an otherwise active high mass
star-forming region.
