Dense Cores in Dark Clouds. XIV. N₂H⁺ (1-0) Maps of Dense Cloud Cores

We present results of an extensive mapping survey of N₂H⁺ (1-0) in about 60 low-mass cloud cores already mapped in the NH₃ (1, 1) inversion transition line. The survey has been carried out at the FCRAO antenna with an angular resolution of 54'', about 1.5 times finer than the previous ammonia observations made at the Haystack telescope. The comparison between N₂H⁺ and NH₃ maps shows strong similarities in the size and morphology of the two molecular species, indicating that they are tracing the same material, especially in starless cores. Cores with stars typically have map sizes about a factor of 2 smaller for N₂H⁺ than for NH₃, indicating the presence of denser and more centrally concentrated gas compared to starless cores. The mean aspect ratio is ~2. Significant correlations are found between NH₃ and N₂H⁺ column densities and excitation temperatures in starless cores, but not in cores with stars, suggesting a different chemical evolution of the two species. Starless cores are less massive ( ≃ 3 M_☉) than cores with stars ( ≃ 9 M_☉). Velocity gradients range between 0.5 and 6 km s^-1 pc^-1, similar to what has been found with NH₃ data, and the ratio β of rotational kinetic energy to gravitational energy has magnitudes between ~10^-4 and 0.07, indicating that rotation is not energetically dominant in the support of the cores. "Local" velocity gradients show significant variation in both magnitude and direction, suggesting the presence of complex motions not interpretable as simple solid-body rotation. Integrated intensity profiles of starless cores present a "central flattening" and are consistent with a spherically symmetric density law n ∝ r^-α, where α = 1.2 for r < r_break and α = 2 for r > r_break, with r_break ~ 0.03 pc. Cores with stars are better modeled with single density power laws with α ≥ 2, in agreement with observations of submillimeter continuum emission. Line widths change across the core, but we did not find a general trend: there are cores with significant positive as well as negative linear correlations between Δv and the impact parameter b. The deviation in line width correlates with the mean line width, suggesting that the line of sight contains ~10 coherence lengths. The corresponding value of the coherence length, ~0.01 pc, is similar to the expected cutoff wavelength for MHD waves. This similarity may account for the increased "coherence" of line widths on small scales. Despite finer angular resolution, the majority of N₂H⁺ and NH₃ maps show a similar "simple" structure, with single peaks and no elongation.