Initial conditions for star formation in clusters: physical and
  kinematical structure of the starless core Oph A-N6
  We present high spatial (<300 AU) and spectral (0.07 km/s) resolution
Submillimeter Array observations of the dense starless cluster core Oph A-N6,
in the 1 mm dust continuum and the 3-2 line of N2H+ and N2D+. The dust
continuum observations reveal a compact source not seen in single-dish
observations, of size ~1000 AU and mass 0.005-0.01 M\odot. The combined line
and single-dish observations reveal a core of size 3000 \times 1400 AU
elongated in a NW-SE direction, with almost no variation in either line width
or line center velocity across the map, and very small non-thermal motions. The
deuterium fraction has a peak value of ~0.15 and is >0.05 over much of the
core. The N2H+ column density profile across the major axis of Oph A-N6 is well
represented by an isothermal cylinder, with temperature 20 K, peak density 7.1
\times 10^6 cm^{-3}, and N2H+ abundance 2.7 \times 10^{-10}. The mass of Oph
A-N6 is estimated to be 0.29 M\odot, compared to a value of 0.18 M\odot from
the isothermal cylinder analysis, and 0.63 M\odot for the critical mass for
fragmentation of an isothermal cylinder. Compared to isolated low-mass cores,
Oph A-N6 shows similar narrow line widths and small velocity variation, with a
deuterium fraction similar to "evolved" dense cores. It is significantly
smaller than isolated cores, with larger peak column and volume density. The
available evidence suggests Oph A-N6 has formed through the fragmentation of
the Oph A filament and is the precursor to a low-mass star. The dust continuum
emission suggests it may already have begun to form a star.
