SIMBAD references

Lack of reliable estimates of distances to most of the local dark clouds has, so far, prevented a quantitative study of their kinematics. Using a statistical approach, we have been able to extract the average spatial distribution as well as the kinematical behaviour of the local dark clouds from their measured radial velocities. Since radial velocities were available only for the northern dark clouds, we have obtained them for 115 southern ones to make a more complete kinematical study of the local clouds. The thesis mainly deals with the presentation of this new data, analysis of the new as well as the existing data and the comparison of the results with those arrived at by earlier studies.

The local clouds are found to be expanding at a speed of ∼4 km s^-1 which is in general agreement with the estimates from optical and HI studies. However, it is found that the kinematics of the local clouds is not described by the model proposed for the local HI gas where a ring of gas expanding from a point gets only sheared by the galactic rotation. Rather, the observed distribution of their radial velocities is best understood in terms of a model in which the local clouds are participating in circular rotation appropriate to their present positions with a small expansion also super-imposed. This possibly implies that cloud-cloud collisions are important. The spatial distribution of clouds derived using such a model is in good greement with the local dust distribution obtained from measurements of reddening and extinction towards nearby stars. In particular, a region f size ∼350pc in diameter enclosing the Sun is found to be devoid of clouds. Intriguingly, most clouds in the longitude range 100 degrees to 145 degrees appear to have negative radial velocities implying that they are approaching us.

Two other related research efforts are also reported in the thesis: (i) Since the clouds observed are distributed over the entire longitude range and wide latitude range, the resultant database of the spectral line parameters is well-suited for studying the average physical properties of the local clouds. Our investigation in this regard shows that, with respect to latitude, the medians of both the brightness temperatures of the clouds and their ^13CO column densities increase marginally while the median of the 12CO linewidths shows clear decrease. Some explanations for these trends are given. (ii) Maps of one of the Orion system of cometary clouds, L1616, in J=1 -> 0 transitions of ^12CO and ^13CO are also presented. The distribution of the "wing" emission shows evidence for mass motions. Also, the "virial" mass of the cloud is found to be five times the actual cloud mass determined from the ^13CO column density map. It is argued that this cloud has abnormal star formation efficiency and is possibly disintegrating. The morphology and the location of the cloud indicates that it is being affected by the star epsilon Orionis which is also possibly responsible for the cloud's unusual star formation efficiency.