THE CENTER FOR STAR FORMATION STUDIES

D. Hollenbach, K. R. Bell, and G. Laughlin

The Center for Star Formation Studies, a consortium of scientists from the Space Science Division at Ames and the Astronomy Departments of the University of California at Berkeley and Santa Cruz, conducts a coordinated program of theoretical research on star and planet formation. The Center, under the directorship of D. Hollenbach (NASA Ames), supports postdoctoral fellows, senior visitors, and students, meets regularly at Ames to exchange ideas and to present informal seminars on current research, hosts visits of outside scientists, and conducts a week-long workshop on selected aspects of star and planet formation each summer.

In June 2000 the Center worked together with researchers from the Arcetri Observatory (Florence, Italy) to hold an international workshop entitled "High Mass Star Formation: An Origin in Clusters" The weeklong workshop, held in Volterra, Italy, had approximately 175 attendees, and included an invited talk by D. Hollenbach on "Star Formation and the Fluctuating Ultraviolet Field in the Galaxy."

One focus of the NASA Ames portion of the research work in the Center in 2000 involved the effect of ultraviolet radiation from young massive stars on the interstellar medium of a galaxy. The interstellar medium of a galaxy is the gas and dust which lie between the stars. Most of the gas is hydrogen; the dust mass is only about 1% of the gas mass. The gas and dust reside in various components, often characterized by the gas density in the component. The densest and coldest component is the molecular clouds; this component forms stars. .Diffuse clouds are less dense than molecular clouds, they are primarily cold atomic hydrogen. The warm medium consists of neutral and ionized gas at very low density and relatively high temperature. The star formation rate in a galaxy depends on the rate at which molecular clouds can be formed, since only this component forms stars. The molecular clouds are thought to form by the coalescence of diffuse clouds into opaque, self-gravitating clouds. However, high rates of star formation leads to high populations of massive stars which radiate copious ultraviolet flux. The ultraviolet flux in turn heats up the diffuse clouds in the interstellar medium and transforms them into warm medium. Since warm medium is unlikely to form molecular clouds, the lack of diffuse clouds cuts off the supply of molecular clouds in a galaxy which cuts off the star formation rate. This then provides a self-regulation mechanism which controls the rate of star formation in a galaxy.

Another focus of the Ames portion of the Center research in 2000 involved a collaborative theoretical study of the conditions which determine whether a collapsing molecular cloud core of gas and dust gives rise to a single star surrounded by planets or to a binary star stytem. This work focused on the realization that the molecular cloud cores that precede star formation can have equilibrium configurations that are non-axisymmetric (lopsided). An analytical study carried out by the Center reported on the discovery and the properties of a sequence of these unusual egg-shaped equilibrium configurations. The analysis shows that these configurations can collapse in a way that may naturally produce either binary or single stars, depending on the initial degree of distortion.

The theoretical models of the Center have been used to interpret observational data from such NASA facilities as the Infrared Telescope Facility (IRTF), the Infrared Astronomical Observatory (IRAS), the Hubble Space Telescope (HST), and the Infrared Space Observatory (ISO, a European space telescope with NASA collaboration), as well as from numerous ground-based radio and optical telescopes. In addition, they have been used to determine requirements on future missions such as the Stratospheric Observatory for Infrared Astronomy (SOFIA) and theSpace Infrared Telescope Facility (SIRTF).