Quiescent Dense Gas in Protostellar Clusters: The Ophiuchus A Core

We present combined BIMA interferometer and IRAM 30 m telescope data of N2H+ 1-0 line emission across the nearby dense star-forming core Ophiuchus A at high linear resolution (e.g., ~1000 AU). Six maxima of integrated line intensity are detected, which we designate Oph A-N1 through N6. The N4 and N5 maxima coincide with the starless continuum objects SM1 and SM2, respectively, but the other maxima do not coincide with previously identified objects. In contrast, relatively little N2H+ 1-0 emission coincides with the starless object SM2 and the Class 0 protostar VLA 1623. The FWHM of the N2H+ 1-0 line, ΔV, varies by a factor of ~5 across Oph A. Values of ΔV < 0.3 km s-1 are found in 14 locations in Oph A, but only that associated with N6 is both well defined spatially and larger than the beam size. Centroid velocities of the line, V LSR, vary relatively little, having an rms of only ~0.17 km s-1. Small-scale V LSR gradients of less than 0.5 km s-1 over ~0.01 pc are found near SM1, SM1N, and SM2 but not N6. The low N2H+ abundances of SM2 or VLA 1623 relative to SM1, SM1N, or N6 may reflect relatively greater amounts of N2 adsorption onto dust grains in their colder and probably denser interiors. The low ΔV of N6, i.e., 0.193 km s-1 FWHM, is only marginally larger than the FWHM expected from thermal motions alone, suggesting that turbulent motions in the Oph A core have been reduced dramatically at this location. The nondetection of N6 in previous thermal continuum maps suggests that interesting sites possibly related to star formation may be overlooked in such data.