The Dynamical State of Barnard 68: A Thermally Supported, Pulsating Dark Cloud

Abstract

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 C18O (1-0), C32S (2-1), C34S (2-1), and N2H+ (1-0) in order to investigate the kinematics and dynamical state of the cloud. We find extremely narrow line widths in the central regions of the cloud, ΔV = 0.18 ± 0.01 km s-1 and 0.15 ± 0.01 km s-1 for C18O and C34S, 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 nonthermal (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 (Alves, Lada, & Lada). We also find the molecular line widths 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 C18O and N2H+ 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 C18O and N2H+ 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 C32S line is optically thick and self-reversed across nearly the entire projected surface of the cloud. The shapes of the 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 is observed within the cloud. Moreover, these motions appear to be globally organized in a clear and systematic alternating spatial pattern that is suggestive of a small-amplitude, nonradial oscillation or pulsation of the outer layers of the cloud about an equilibrium configuration.