2014ApJ...790..129C

We present single-dish observations of the L1689-SMM16 core in the Ophiuchus molecular cloud in NH₃(1, 1) and (2, 2) emission using the Green Bank Telescope, in N₂H⁺ (1-0) emission using the Nobeyama Radio Observatory, and in NH₂ D (1_1,1^a - 1_0,1^s), HCN (1-0), HNC (1-0), H¹³CO⁺ (1-0), and HCO⁺ (1-0) emission using the Mopra telescope. The morphologies of the integrated NH₃(1, 1) and N₂H⁺ (1-0) emission well match that of 250 µm continuum emission. Line widths of NH₃(1, 1) and N₂H⁺ (1-0) show the presence of transonic turbulence across the core. Jeans and virial analyses made using updated measurements of core mass and size confirm that L1689-SMM16 is prestellar, i.e., gravitationally bound. It also has accumulated more mass compared to its corresponding Jeans mass in the absence of magnetic fields and therefore is a "super-Jeans" core. The high levels of X(NH₃)/X(N₂H⁺) and deuterium fractionation reinforce the idea that the core has not yet formed a protostar. Comparing the physical parameters of the core with those of a Bonnor-Ebert sphere reveals the advanced evolutionary stage of L1689-SMM16 and shows that it might be unstable to collapse. We do not detect any evidence of infall motions toward the core. Instead, red asymmetry in the line profiles of HCN (1-0) and HNC (1-0) indicates the expansion of the outer layers of the core at a speed of ∼0.2 km/s to 0.3 km/s. For a gravitationally bound core, expansion in the outer layers might indicate that the core is experiencing oscillations.