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 60low 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 resolutionabout 1.5 times finer than the previous ammonia observations. Cores with starstypically have map sizes about a factor of two smaller for N2H+ than for NH3,indicating the presence of denser and more centrally concentrated gas comparedto starless cores. Significant correlations are found between NH3 and N2H+column densities and excitation temperatures in starless cores, but not incores 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 beenfound with NH3 data. ``Local'' velocity gradients show significant variation inboth magnitude and direction, suggesting the presence of complexmotions notinterpretable as simple solid body rotation. Integrated intensity profiles ofstarless cores present a ``central flattening'' and are consistent with aspherically 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 powerlaws with n ~ r^{-2}. Line widths change across the core but we did not find ageneral trend. The deviation in line width correlates with the mean line width,suggesting that the line of sight contains ~ 10 coherence lengths. Thecorresponding value of the coherence length, ~ 0.01 pc, is similar to theexpected cutoff wavelength for MHD waves. This similarity may account for theincreased ``coherence'' of line widths on small scales. Despite of the finerangular resolution, the majority of N2H+ and NH3 maps show a similar ``simple''structure, with single peaks and no elongation.Comment: 62 pages, 11 figures, ApJ, in pres