Dr. Paul Goldsmith will use this award to pursue a novel probe of molecular cloud structure for the study of HI narrow line self absorption, or HINSA. While there have been many advances in our understanding of the dense, cold regions where new stars form, there remain many important questions. These include (1) the structure of molecular clouds including heating and cooling processes which determine their temperature and ionization state; (2) the lifetime of molecular clouds, in particular related to the time scale for star formation; and (3) the role of the magnetic field. The observations of the 21cm line will yield vital information, complementary to that obtained from higher frequency observations of molecules and dust. This research project will enhance our understanding of molecular clouds and how new stars are formed by providing important information on key processes in dense regions, including the grain surface H2 formation and the cosmic ray heating rate, by giving important information on the lifetime of molecular clouds; in particular the time delay between the onset of atomic to molecular conversion and the formation of new stars, by developing a new technique for measuring the magnetic field in dense regions, with much enhanced sensitivity, and by offering the capability of producing maps of line of sight magnetic field strength in a reasonable time.
Molecular hydrogen is the most abundant molecule in dense, well shielded regions of the interstellar medium, but is extremely difficult to trace directly. It now appears possible, that by careful measurements of the atomic hydrogen associated with dense molecular cloud cores, we can learn much about the transition from atomic to molecular form and thus of the evolution of dense regions, starting from being nearly atomic and ending up as largely molecular. The HI/H2 ratio can serve as a chronometer for the evolution of interstellar clouds and measure their evolution towards the phase in which they are the sites of star formation. The magnetic field and its interaction with matter are critical for a number of processes that occur in the interstellar medium, including star and planet formation. This importance has motivated many researchers, but there is very little information about the initial conditions in terms of magnetic field strength, for core evolution and collapse. It is plausible that the present work will prove so effective a method of Zeeman determination of the magnetic field, that there will for the first time be maps of the magnitude of the (line of sight) value of the magnetic field in dense clouds. These data will be a strong motivation for continued theoretical studies on a range of magnetohydrodynamic (MHD) topics, including studies of MHD turbulence and its decay in dense clouds and the relationship of the magnetic field to disks, jets, and molecular outflows. The new results will address questions closely tied to many important issues in astronomy including the initial mass function and the chemical evolution of galaxies.
The topic of star formation is clearly of wide interest Groups of senior citizens, university retirees, and students at purely undergraduate institutions all see molecular clouds and star formation as topics to which they can relate. It is anticipated that in the course of this research program, Dr. Goldsmith. will continue to give talks to these groups. By connecting the issue of the chemistry in molecular clouds to that in the primitive solar system, and by linking the magnetic field we are studying to that in protostellar disks such as that from which our solar system formed, this work will have an impact on a broad audience. ***
