For a little more depth on the subject, from the Astrophysical Journal;
(if you're short for time, speed read the last 4 paragraphs)
The Dynamical State of Barnard 68: A Thermally Supported, Pulsating Dark Cloud
Charles J. Lada and Edwin A. Bergin , João F. Alves , Tracy L. Huard
The Astrophysical Journal, 586:286-295, 2003 March 20
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 (10), C32S (21), C34S (21), and N2H+ (10) 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, V = 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 45 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 12 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.
1. INTRODUCTION
Barnard 68 (B68) is a fine example of a small, round, and optically opaque dark molecular cloud known as a Bok globule. Recently Alves, Lada, & Lada (2001) used deep near-infrared extinction measurements to map the structure of this globule in a level of detail previously unsurpassed for any molecular cloud. In particular, they were able to construct a highly resolved radial density profile of the cloud, spanning its entire 100 (104 AU) extent with 5 (500 AU) angular resolution. They found the measured radial density distribution of the globule to be in impressive agreement with the theoretical predictions for a Bonnor-Ebert sphere, that is, a truncated isothermal sphere bounded by a fixed external pressure.
The extremely close agreement between observation and theory for this cloud implies a number of interesting observable consequences (or predictions) for its physical state. In particular, thermal pressure is likely to be an important, if not dominant, source of support for the cloud against gravitational collapse. In this sense, the physical state of B68 would not be typical, since turbulence is the dominant source of internal bulk motions and support in most molecular clouds. Further evidence in support of the idea that thermal, not turbulent, motions dominate the internal pressure of the cloud followed from a more detailed analysis of the extinction data. J. Alves et al. (2003, in preparation) found the surface-density structure of the cloud to be extremely smooth, placing a limit on the magnitude of random fluctuations in the surface density of 5% over angular scales ranging from 3 to 200 (300--20,000 AU). Such smooth structure seems incompatible with expectations for turbulent cloud models (Padoan, Jones, & Nordlund 1997; Juvela 1998). An important observational consequence for a thermally dominated cloud would be the presence of very narrow molecular line widths. One might also expect that random variations in molecular line-center velocities (of optically thin species) across the cloud would be small compared to the sound speed in the cloud. Both of these aspects of a cloud's velocity field can be directly investigated with sensitive, high frequency resolution molecular line observations.
To further examine and better constrain the physical nature of this unusual cloud, we have obtained extensive, high angular resolution and high signal-to-noise ratio observations of B68 in a number of interesting molecular species, including C18O, C32S, C34S, and N2H+. These observations were designed, in part, to assess the relative roles of turbulence and thermal pressures for the support of this cloud against gravity. Because B68 was found to be formally unstable and near the critical condition or pivotal point for gravitational collapse (Alves et al. 2001), we also designed our observational program to measure the cloud's dynamical state (i.e., is B68 rotating, expanding, collapsing, or static?).
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The most significant feature of the map is the striking spatial segregation of the blue- and redshifted asymmetry. The redshifted inflow asymmetry dominates the central regions of the cloud map but is almost entirely surrounded by a contiguous region dominated by the blueshifted, outflow asymmetry. This cloud is apparently experiencing simultaneous but spatially coordinated infall and outflow motions of its outer layers. Such an alternating spatial pattern may be suggestive of global, nonradially symmetric surface oscillations or pulsations of the cloud about an equilibrium configuration as discussed later in this paper.
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4.1. A Thermally Supported Cloud
The striking agreement between the observed density distribution in the B68 cloud and that predicted for a Bonnor-Ebert sphere near critical stability, suggests that thermal pressure is a primary source of support for this cloud against collapse.
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Nontheless, the radial profile of angular velocity for this cloud differs from the inward spin-up one would expect for the more Keplerian-like motion of an isothermal sphere in near hydrostatic equilibrium (Kiguchi et al. 1987; Tassoul 1978) and the angular velocity profile predicted for an evolving magnetized protostellar core (Basu & Mouschovias 1995a, 1995b). However, in the absence of significant ambipolar diffusion in the cloud core, one might expect magnetic breaking to force corotation between the inner and outer parts of the cloud. This would require the existence of a magnetic field that permeates the cloud and is coupled to the gas. The lack of a significant nonthermal component to the observed line widths in the central regions would seem to preclude a significant turbulent magnetic field. Although a rigid, static field could be present, it might be relatively weak, given the close balance between gravitation and thermal pressure required by the agreement of the cloud's density profile and the predictions of a near critical thermally supported Bonnor-Ebert sphere (Alves et al. 2001). Thus, it is presently unclear whether or not magnetic breaking or some other cause is responsible for the lack of increased rotation in the central regions of the cloud.
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The B68 cloud appears to be embedded in the Loop I supernova superbubble (Alves et al. 2001; J. Alves et al. 2003, in preparation). Its derived surface pressure is an order of magnitude greater than that of the general interstellar medium but comparable to that derived for the Loop I superbubble from X-ray observations (Breitschwerdt, Freyberg, & Egger, 2000). Therefore, the cloud likely interacted with the shock of a supernova remnant sometime during the not too distant past. We speculate that this external interaction with the supernova provided the peturbation that set at least the outer layers of the cloud into oscillation. Although the interaction did not destablize the cloud, it may have excited other, higher mode oscillations that have since damped out. One possibility is that B68 was a dense core region of a much more massive cloud complex. In this case it would have been surrounded by a lower density, cold molecular envelope, the weight of which provided a confining external pressure similar to that characterizing the cloud now. This envelope would have been stripped with the passage of the supernova shock, leaving behind the denser core region, which then came into a new pressure equilibrium with the hot gas in the supernova shell. The oscillations we now observe might then be the remaining signature of that physical interaction.