THE REGULATORY AND DIAGNOSTIC ROLES OF CHEMISTRY IN LOW-MASS STAR-FORMATION

We describe the different types of interstellar clouds and those sub-u nits in them that develop in the process of low-mass star formation. A n introduction to the ion-molecule chemistry that plays a role in the production of the simple molecules in those objects is given. Within t he large clouds containing star forming regions, molecular clumps with visual extinctions of about one magnitude are observed to be numerous . We consider these clumps to be the progenitors of star forming regio ns. We argue that they are longer-lived than more opaque starless clum ps because the ionization level, controlled by the S+ abundance, falls with increasing visual extinction, leading to a larger decay rate of turbulence and a decreasing contribution of turbulent pressure to clou d support. Once the collapse of a molecular clump with most gas phase sulphur in the form of S+ is initiated, the collapse probably occurs o n a free-fall timescale and results in the formation of a cluster cons isting of dense cores and an intercore medium. We present theoretical results for the chemical evolution during the collapse that leads to t he appearance of such a cluster. Low-mass stars are born in dense core s, and the winds of the stars react on the core cluster, so that ablat ion of cores occurs. Molecular diagnostics of the wind-core interface structure, relevant to the study of the structures and evolution of a large variety of dynamic diffuse astronomical sources consisting of co ndensations embedded in more tenuous media, have been identified in th eoretical investigations. For instance, one may determine whether mixi ng of gas occurs at a magnetized interface, or if mixing is inhibited and only momentum and energy transfer between wind and core gas takes place. The ablated core gas and stellar wind may settle in the interco re medium, and a subsequent generation of cores partially composed of the ablated gas and wind may form. We consider the chemical signatures that if observed would establish whether later generation cores actua lly form. The detailed mapping of emissions from a number of molecular species, and the interpretation of those data, would permit the infer ence of the time required for intercore gas to collapse to create core s and the survival time of the cores. Infall within a core to form a s tar has not been detected though young stars exist in many cores. Broa d components should arise in infalling gas, and searches for broad win gs associated with NH3 emission features have been unsuccessful. The a bsence of broad wings is possibly due to the freeze-out of NH3 onto du st grain surfaces. The abundances of some other species should actuall y rise as moderate freeze-out onto dust of heavy elements from the gas phase occurs. We identify species that should have emission features with broad components in collapsing cores, and discuss the insight tha t such detections will bring to our understanding of star formation.