Resumen: 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.
Resumen: We present the results of an optical polarization survey toward the galactic anticenter, in the area 5(h) greater than or equal to alpha greater than or equal to 2(h) and 6 degrees less than or equal to delta less than or equal to 12 degrees. This region is characterized by the presence of a stream of high-velocity H I as well as high galactic latitude molecular clouds. We
used our polarization data together with 100 mu m IRAS maps of the region to study the relation between the dust distribution and the geometry of the magnetic field. We find that there is a correlation between the percent polarization and the 100 mu m flux such that P(%) less than or equal to (0.16 +/- 0.05)F-100. When the IRAS flux is converted into H I column densities this becomes P(%) less than or equal to (0.13 +/- 0.03)N-20 - 0.22, which is consistent with previous interstellar medium studies on the relation between reddening and polarization. This correlation indicates that our survey is as deep as IRAS and that the magnetic field geometry does not change strongly with the optical depth in the lines of sight that we have studied. The implied lower limit to the distance of our survey is 500-700 pc at galactic latitudes b = -20 degrees and 100 pc at b = -50 degrees. Our main finding is that the magnetic field is perpendicular to the H I high-velocity stream as well as the molecular cloud MBM 16 in the high-latitude region. The field is also perpendicular to the velocity gradient in the stream. Closer to the plane, the magnetic field is parallel to the dust filaments that extend like a plume toward the halo. Our observations indicate that the galactic magnetic field toward the high-latitude clouds is toroidal. We propose a model in which flux tubes that rise out of the galactic plane become force-free and predominantly toroidal at high latitudes. The high-latitude clouds may be gas streams that are falling back toward the galactic plane within such buoyant braided ropes of magnetic flux.
