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 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 ∼ 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 vLSR 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 Msun 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.