nearly all of the kinetic energy of the falling envelope is transformed into inter- 

 nal energy of the gas and radiated away. For a central object of approximately 

 one solar mass, the shock velocity on its surface is about 300 km/sec, while the 

 shock luminosity is about 40 L^. There will also be an accretion shock in the 

 disk, with a range of velocities depending on the position within the disk. How- 

 ever, the main heating of the disk is thought to be viscous heating driven by tur- 

 bulence. Close to the central object the dust initially present in the collapsing 

 envelope will have been vaporized by the strong radiation field generated by the 

 shock, creating a dust-free zone. The different zones in a collapsing, rotating 

 interstellar cloud are illustrated in figure 3-4, which shows the typical sizes for 

 these zones. 



This global picture of star formation has recently been modified with the 

 discovery of strong stellar winds from protostars. The effect of the protostellar 

 wind on the molecular cloud is thought to cause two shocks. First, the wind will 

 shock against the shell of swept-up material (i.e., the wind shock, <400 km/sec). 

 Second, this shell of swept-up material will be driven supersonically into the 

 surrounding molecular cloud material (v ~ 15 km/sec). The size of the wind- 

 acceleration zone is about protostellar size or perhaps the size of the circum- 

 stellar disk. The size of the wind shock and the molecular cloud shock is much 

 larger, possibly even larger than the size of the collapsing envelope. This is shown 

 in figure 3-5. 



Carbon monoxide observations in the vicinity of pre-main-sequence stars 

 reveal blue-shifted and red-shifted emission in two opposing lobes. The observed 

 outflow velocities around T Tauri stars (pre-main-sequence stars with M ~ 1Mg) 

 are about 15 km/sec or less. Emission from shocked, vibrational^ excited H 2 

 has also been detected in these regions. The most spectacular sign of outflow 

 from protostars is, however, the phenomenon of Herbig-Haro (HH) objects. 

 These nebulae show a strong emission-line spectrum due to cooling behind 

 the shocks. Proper motion studies and optical line studies reveal space velocities 

 of up to 400 km/sec. The proper motion vectors of the HH objects in a family 

 point back toward the exciting star. These shocked clumps of gas are either 

 ejected directly from the circumstellar disk of the protostars, or they may 

 represent the interaction between the directed flows from these objects and their 

 molecular cloud environment. It is believed that the responsible stars are the 

 precursors of even the T Tauri stars, which themselves are pre-main-sequence 

 stars. The evolution of the protosolar nebula is thus envisaged to incorporate 

 phases of both infall and outflow, possibly simultaneously. The importance of 

 the HH-exciting stars is twofold: first, they indicate hitherto unsuspected violent 

 activity around the protostar, and second, their study reveals clues to the physi- 

 cal and chemical nature of protostellar disks and circumstellar nebulae. 



Both the nebular morphology of the HH objects themselves and their proper 

 motions, where these can be determined, indicate that the outflows from their 

 exciting stars are highly anisotropic. The flows are either unipolar or bipolar 



47 



