We report sensitive , high resolution molecular-line observations of the dark cloud Barnard 68 obtained with the IRAM 30-m telescope .
We analyze spectral-line observations of C ^ { 18 } O ( 1–0 ) , C ^ { 32 } S ( 2–1 ) , C ^ { 34 } S ( 2–1 ) , and N _ { 2 } H ^ { + } ( 1–0 ) in order to investigate the kinematics and dynamical state of the cloud .
We find extremely narrow linewidths in the central regions of the cloud , \Delta V = 0.18 \pm 0.01 km s ^ { -1 } and 0.15 \pm 0.01 km s ^ { -1 } for C ^ { 18 } O and C ^ { 34 } S , respectively .
These narrow lines are consistent with thermally broadened profiles for the measured gas temperature of 10.5 K. We determine the thermal pressure to be a factor 4 – 5 times greater than the non-thermal ( turbulent ) pressure in the central regions of the cloud , indicating that thermal pressure is the primary source of support against gravity in this cloud .
This confirms the inference of a thermally supported cloud drawn previously from deep infrared extinction measurements ( 1 ) .
We also find the molecular linewidths to systematically increase in the outer regions of the cloud , where we calculate the thermal pressure to be between 1 – 2 times greater than the turbulent pressure .
We find the distribution of line-center radial velocities for both C ^ { 18 } O and N _ { 2 } H ^ { + } to be characterized by systematic and well-defined linear gradients across the face of the cloud .
The rotational kinetic energy is found to be only a few percent of the gravitational potential energy , indicating that the contribution of rotation to the overall stability of the cloud is insignificant .
However , the C ^ { 18 } O and N _ { 2 } H ^ { + } velocity gradients differ from each other in both magnitude and direction , suggesting that the cloud is differentially rotating , with the inner regions rotating slightly more slowly than the outer regions .
Finally , our observations show that C ^ { 32 } S line is optically thick and self-reversed across nearly the entire projected surface of the cloud .
The shapes of the self-reversed profiles are asymmetric and are found to vary across the cloud in such a manner that the presence of both inward and outward motions are observed within the cloud .
Moreover , these motions appear to be globally organized in a clear and systematic alternating spatial pattern which is suggestive of a small amplitude , non-radial oscillation or pulsation of the outer layers of the cloud about an equilibrium configuration .