The Initial Conditions of Clustered Star Formation. II. N2H+ Observations of the Ophiuchus B Core

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 Delta *mm 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 ~ 3 X 105 cm-3 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.