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 about 1.5 times finer than the previous ammonia observations. Cores with stars typically have map sizes about a factor of two smaller for N2H+ than for NH3, indicating the presence of denser and more centrally concentrated gas compared to starless cores. 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. Velocity gradients range between 0.5 and 6 km/s/pc, similar to what has been found with NH3 data. ``Local'' velocity gradients show significant variation in both magnitude and direction, suggesting the presence of complexmotions 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^{-1.2} for r < ~0.03 pc and n ~ r^{-2} at larger r. Cores with stars are better modelled with single density power laws with n ~ r^{-2}. Line widths change across the core but we did not find a general trend. 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 of the finer angular resolution, the majority of N2H+ and NH3 maps show a similar ``simple'' structure, with single peaks and no elongation