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

Charles J. Lada, Edwin A. Bergin, João F. Alves, Tracy L. Huard

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.

External organisation(s)
Harvard-Smithsonian Center for Astrophysics, European Southern Observatory (Germany)
The Astrophysical Journal
Publication date
Peer reviewed
Austrian Fields of Science 2012
103004 Astrophysics
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