Publication: Global aspects of dynamics and star formation in Taurus
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1992-08
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IOP Publishing
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Abstract
We employ a number of data sets that include the proper motions of T-Tauri stars, molecular and H I velocities and densities, optical polarimetry, and H-alpha-luminosities of young stars in order to derive a global view of gas dynamics and star formation in the Taurus molecular cloud. We find strong correlations among cloud properties. TTS stars move in a direction that is roughly parallel to the gaseous filaments in the cloud. Assuming that stars and gas are well coupled, we infer that the cloud moves through the ISM at approximately 17 km s-1. The Taurus cloud is a member of Gould's Belt, and it is moving in the same direction as the other members. We propose that the cloud's motion arises as a consequence of its formation through Parker instability in the ISM. The bulk of gas displaced by this instability moves laterally at several times the Alfven speed of the undisturbed gas as it falls back toward the galaxy. It is this motion that is consistent with that of Taurus. An odd (antisymmetric with respect to reflection through the Galactic plane) mode Parker instability produces a warped gas layer which we interpret to be the origin of Gould's Belt, of which Taurus is a small constituent. The infalling gas will shock with material in the plane of the Belt. MHD waves are generated in this process, and we suggest that this is the origin of large-scale Alfven waves in Taurus. Such waves do not damp for the life of the cloud and keep it supported against global gravitational collapse. The gravitational energy release during the Parker instability is sufficient to account for the kinematics of the cloud as well as its internal stirring. The motion of the Taurus cloud is mildly super-Alfvenic with respect to the surrounding undisturbed WNM, and this must be associated with a shock. There are indications of the presence of a shock of speed V(sh) congruent-to 6.7 km s-1 in Taurus. This shock is suggested to be responsible of the relative velocity drift of 3.8 km s-1 between the weaker H I self-absorption component and the rest of the cloud (CO, H I emission, PMS stars). Observational support for the existence of a large-scale Alfven wave in Taurus comes from both the wavelike distortion of the cloud magnetic field (wavelength of 17 pc) and the wavelike disturbance in the velocity field of this H I component which matches up with the magnetic data. Finally, the influence of the shock in the star formation process is analyzed. It is shown that there are no significant differences in the evolutionary state of groups of PMS stars located at very different positions in the cloud. It appears as if many features of the dynamics and physics of the cloud are determined by global issues such as the process of cloud formation and its interaction with the larger scale ISM.