The nature of the dense core population in the Pipe nebula:: Core and cloud kinematics from C18O observations

We present molecular line observations of 94 dark cloud cores identified in the Pipe nebula through near-IR extinction mapping. Using the Arizona Radio Observatory 12m telescope, we obtained spectra of these cores in the J = 1-0 transition of (CO)-O-18. We used the measured core parameters, T-R*, Delta(v), v(lsr,) radius, and mass, to explore the internal kinematics of the cores, as well as their radial motions through the larger molecular cloud. We find that the vast majority of the dark extinction cores are true cloud cores, rather than the superposition of unrelated filaments. While we identify no significant correlations between the cores' internal gas motions and their other physical parameters, we identify spatially correlated radial velocity variations that outline two main kinematic components of the cloud. The largest is a 15 pc long filament that is surprisingly narrow both in spatial dimensions and in radial velocity. Beginning in the Stem'' of the Pipe, this filament displays uniformly small (CO)-O-18 line widths (Delta v similar to 0.4 km s(-1)), as well as core-to-core motions only slightly in excess of the gas sound speed. The second component outlines what appears to be part of a large (2 pc; 10(3)M(circle dot)) ringlike structure. Cores associated with this component display both larger line widths and core-to-core motions than cores in the main cloud. The Pipe molecular ring may represent a primordial structure related to the formation of this cloud.