Initial conditions for star formation in clusters: Physical and kinematical structure of the starless core Oph A-N6

Abstract

We present high spatial (<300AU) and spectral (0.07kms-1) resolution Submillimeter Array observations of the dense starless cluster core Oph A-N6 in the 1mm 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 1000AU and mass 0.005-0.01 M. The combined line and single-dish observations reveal a core of size 3000 × 1400AU 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 20K, peak density 7.1 × 106cm -3, and N2H+ 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. © 2012. The American Astronomical Society. All rights reserved.