2010ApJ...708.1002F

We present a Nobeyama 45 m Radio Telescope map and Australia Telescope Compact Array pointed observations of N₂H⁺ 1-0 emission toward the clustered, low-mass star-forming Oph B Core within the Ophiuchus molecular cloud. We compare these data with previously published results of high-resolution NH₃ (1,1) and (2,2) observations in Oph B. We use 3D CLUMPFIND to identify emission features in the single-dish N₂H⁺ map, and find that the N₂H⁺ "clumps" match well similar features previously identified in NH₃(1,1) emission, but are frequently offset to clumps identified at similar resolution in 850 µm continuum emission. Wide line widths in the Oph B2 sub-Core indicate that non-thermal motions dominate the Core kinematics, and remain transonic at densities n ∼ 3x10⁵/cm3 with large scatter and no trend with N(H₂). In contrast, non-thermal motions in Oph B1 and B3 are subsonic with little variation, but also show no trend with H₂ column density. Over all of Oph B, non-thermal N₂H⁺ line widths are substantially narrower than those traced by NH₃, making it unlikely NH₃ and N₂H⁺ trace the same material, but the v_LSR of both species agree well. We find evidence for accretion in Oph B1 from the surrounding ambient gas. The NH₃/N₂H⁺ abundance ratio is larger toward starless Oph B1 than toward protostellar Oph B2, similar to recent observational results in other star-forming regions. The interferometer observations reveal small-scale structure in N₂H⁺ 1-0 emission, which are again offset from continuum emission. No interferometric N₂H⁺ emission peaks were found to be coincident with continuum clumps. In particular, the ∼1 M_☉B2-MM8 clump is associated with an N₂H⁺ emission minimum and surrounded by a broken ring-like N₂H⁺ emission structure, suggestive of N₂H⁺ depletion. We find a strong general trend of decreasing N₂H⁺ abundance with increasing N(H₂) in Oph B which matches that found for NH₃.