2009ApJ...698.1914P

We have observed the Oph A-N6 prestellar core in the following transitions: N₂D⁺ J= 3 -> 2, DCO⁺ J= 3 -> 2 and J= 5 -> 4, HCO⁺ J= 3 -> 2, CS J= 5 -> 4 and J= 7 -> 6, and H¹³CO⁺ J= 3 -> 2 and J= 4 -> 3, using the James Clerk Maxwell Telescope. We also observed the NH₃(1, 1) and (2, 2) inversion transitions toward the Oph A-N6 peak with the Green Bank Telescope. We have found that the N6 core is composed of shells of different chemical composition due to the freezing out of chemical species at different densities. The undepleted species N₂D⁺ appears to trace the high-density interior of the core, DCO⁺ and H¹³CO⁺ trace an intermediate region, and CS traces the outermost edges of the core. A distinct blue-red spectral asymmetry, indicative of infall motion, is clearly detected in the HCO⁺ spectra, suggesting that N6 is undergoing gravitational collapse. This collapse was possibly initiated by a decrease in turbulent support suggested by the fact that the nonthermal contribution to the line widths is smaller for the molecular species closer to the center of the core. We also present a temperature profile for the core. These observations support the claim that the Oph A-N6 core is an extremely young prestellar core, which may have been recently cutoff from magnetohydrodynamic support and begun to collapse.