Probing the initial conditions of high-mass star formation. II.
  Fragmentation, stability, and chemistry
  We present a new high-resolution study of pre-protocluster regions in tracers
exclusively probing the coldest and dense gas (NH_2D). The data are used to
constrain the chemical, thermal, kinematic, and physical conditions (i.e.,
densities) in G29.96e and G35.20w. NH_3, NH_2D, and continuum emission were
mapped using the VLA, and PdBI. In particular, NH_2D is a unique tracer of
cold, precluster gas at high densities, while NH_3 traces both the cold and
warm gas of modest-to-high densities. In G29.96e, Spitzer images reveal two
massive filaments, one of them in extinction (infrared dark cloud). We observe
very low line widths in NH_3 (FWHM <1km/s). These multi-wavelength,
high-resolution observations of high-mass pre-protocluster regions show that
the target regions are characterized by (i) turbulent Jeans fragmentation of
massive clumps into cores (from a Jeans analysis); (ii) cores and clumps that
are "over-bound/subvirial", i.e. turbulence is too weak to support them against
collapse, meaning that (iii) some models of monolithic cloud collapse are
quantitatively inconsistent with data; (iv) accretion from the core onto a
massive star, which can (for observed core sizes and velocities) be sustained
by accretion of envelope material onto the core, suggesting that (similar to
competitive accretion scenarios) the mass reservoir for star formation is not
necessarily limited to the natal core; (v) high deuteration ratios
([NH_2D/NH_3]>6%), which make the above discoveries possible; (vi) and the
destruction of NH_2D toward embedded stars. [abridged]
