From Clump to Protostar

HH-30 (see text next to figure 1.2 for explanation)  

It can be shown that as a clump collapses under its own gravity half of the gravitational energy released will go into heating the collapsing cloud (this result, known as the Virial Theorem, is probably the most repeated result in star formation), the other half is radiated away. The direct result of this is that as a clump collapses it heats up. This effect will be most extreme in the centre of the cloud, where the highest temperatures exist. In order for the protostar to become a star it has to get these temperatures high enough for nuclear fusion to start. However, there are several processes that work to prevent this. Probably the most important of these is due to the initial gas cloud being in orbit around the galactic centre which means that it had some initial angular momentum. As it collapses this angular momentum is conserved. This causes the rotation speed of the protostar to increase. If no other effects were present this rotation speed would eventually reach the point where it would halt the collapse of the protostar. In order to continue its collapse the protostar must lose most of its initial angular momentum. It is this requirement that leads to many of the interesting effects that we observe. As an initially spherical but rotating clump collapses, it will tend to become flattened since the material falling in from the poles is not held back by the rotation. This eventually causes a rotating disk to be formed which will gradually accrete onto the central protostar. The key to removing the excess angular momentum is that protostars which have evidence for accretion disks also have very powerful bi-polar outflows. These outflows, which may contain up to 10% of the mass of the accreting material (Hartman, p13 [12]), carry away the angular momentum allowing the rest of the material to fall onto the protostar. The mechanism by which this occurs is not yet fully understood but most theories invoke magnetic fields in the disk to power the outflows. These outflows can be seen in several different guises: it is clear that at least some of them precess over time, some of them are very highly collimated, others can have quite wide opening angles. In some cases the jets can be seen to be interacting with the surrounding envelope material causing heated shock fronts. The extra heat generated allows chemical reactions to take place that would not otherwise be seen enabling unusual molecules to be observed (eg. SiO).

HH30   Figure 1.1^1.2 is a picture of the Herbig-Haro object HH30 which is almost a textbook example of the processes described above. The picture on the left of the figure shows two images taken by the Hubble Space Telescope just under 1 year apart. The diagram on the right shows the main components visible in the pictures. The protostar at the centre is surrounded by a large flared accretion disk. The bipolar outflow is not only clearly visible but also has distinguishable clumps in it which can be tracked over time to give a direct measurement of the speed of the outflow (which is around 200 km $s^{-1}$). In recent years numerous such images have been taken by the HST showing a wide variety of examples of different stages of star formation. Figure 1.2 shows two examples of edge-on protoplanetary disks in the Orion molecular cloud. These are stars in the final stages of star formation. In the right image the bright areas are light from the central star reflecting off the dust above the disk. The disk itself is still opaque thus preventing a direct view of the newly formed star.