a memory of the gas phase and grain surface chemistry that had been slowly 

 accumulating within the parent cloud. Whether complex organic chemistry origi- 

 nated early in the history of the solar system or arrived from elsewhere will 

 profoundly influence the likelihood of life arising elsewhere. Analogs of the 

 protosolar nebula are critical to our understanding. 



Stars form through the gravitational collapse of interstellar molecular clouds. 

 This process starts with the fragmentation of the parent cloud into a number of 

 clumps, many of which may collapse to form stars. According to present models, 

 the initial collapse phase is isothermal, but when the cloud becomes opaque to 

 visible radiation it heats up. Since the initial cloud is rotating, the cloudlet will 

 flatten during the collapse. The core collapses faster than the outside, resulting 

 in a core-envelope structure, with the envelope falling freely onto the core. The 

 core itself consists of a central object (the protostar) in hydrodynamical equilib- 

 rium surrounded by an accretion disk (the protoplanetary disk). The core is 

 separated from the envelope by an accretion shock. In this accretion shock, 



Dusty 

 Envelope 



Cloud 

 Boundary 



Dust Destruction 

 Region 



Dust Free 

 Envelope 



Accretion 

 Shock 



Dust Free 

 Disk 



Hydrostatic 

 Core 



3x10" cm 



Dusty 

 Disk 



3x10 10 cm 2x10 14 cm 2x10 17 cm 



Size scale for 1M@ protostar 



Figure 3-4. The different regions in a collapsing, rotating interstellar cloud. 



46 



