2012ApJ...745..117B

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_☉. The combined line and single-dish observations reveal a core of size 3000x1400 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 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 7.1x10⁶/cm3, and N₂H⁺ abundance 2.7x10^–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.