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 −1 ) 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 N 2 H + and N 2 D + . 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 ☉ . The combined line and single-dish observations reveal a core of size 3000 × 1400 AU elongated in a NW–SE direction, with almost no variation in either line width nor 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 N 2 H + 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 × 10 6 cm −3 , and N 2 H + abundance 2.7 × 10 −10 . The mass of Oph A-N6 is estimated to be 0.29 M ☉ , compared to a value of 0.18 M ☉ from the isothermal cylinder analysis, and 0.63 M ☉ 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 that 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 that it may already have begun to form a star.