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Title: The dense ring in the Coalsack: the merging of two subsonic flows
Authors: J. M. Rathborne, C. J. Lada, W. Walsh, M. Saul, H. M. Butner

A recent high angular resolution extinction map toward the most opaque molecular globule, Globule 2, in the Coalsack Nebula revealed that it contains a strong central ring of dust column density. This ring represents a region of high density and pressure that is likely a transient and possibly turbulent structure. Dynamical models suggest that the ring has formed as a result of a sudden increase in external pressure which is driving a compression wave into the Globule. Here we combine the extinction measurements with a detailed study of the C18O (1-0) molecular line profiles toward Globule 2 in order to investigate the overall kinematics and, in doing so, test this dynamical model. We find that the ring corresponds to an enhancement in the C18O non-thermal velocity dispersion and non-thermal pressure. We observe a velocity gradient across the Globule that appears to trace two distinct systematic subsonic velocity flows that happen to converge within the ring. We suggest, therefore, that the ring has formed as two subsonic flows of turbulent gas merge within the Globule. The fact that the outer layers of the Globule appear stable against collapse yet there is no centrally condensed core, suggests that the Globule may be evolving from the outside in and has yet to stabilise, confirming its youth.

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Title: Near-Infrared Extinction in The Coalsack Globule 2
Authors: Takahiro Naoi, Motohide Tamura, Tetsuya Nagata, Yasushi Nakajima, Hiroshi Suto, Koji Murakawa, Ryo Kandori, Sho Sasaki, Shogo Nishiyama, Yumiko Oasa, Koji Sugitani

We have conducted J, H, and Ks imaging observations for the Coalsack Globule 2 with the SIRIUS infrared camera on the IRSF 1.4 m telescope at SAAO, and determined the colour excess ratio, E(J-H)/E(H-Ks). The ratio is determined in the same photometric system as our previous study for the rho Oph and Cha clouds without any colour transformation; this enables us to directly compare the near-infrared extinction laws among these regions. The current ratio E(J-H)/E(H-Ks) = 1.91 0.01 for the extinction range 0.5 < E(J-H) <1.8 is significantly larger than the ratios for the rho Oph and Cha clouds (E(J-H)/E(H-Ks) = 1.60-1.69). This ratio corresponds to a large negative index alpha = 2.34 0.01 when the wavelength dependence of extinction is approximated by a power law which might indicate little growth of dust grains, or larger abundance of dielectric non-absorbing components such as silicates, or both in this cloud. We also confirm that the colour excess ratio for the Coalsack Globule 2 has a trend of increasing with decreasing optical depth, which is the same trend as the rho Oph and Cha clouds have.

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