The Center for Star Formation Studies D. Hollenbach, K. R. Bell, G.Laughlin

The Center for Star Formation Studies, a high temperature. The star formation rate in a consortium of scientists from the Space Sci- galaxy depends on the rate at which molecular ence Division at Ames and the Astronomy clouds can be formed, because only this Departments of the University of California at component forms stars. The molecular clouds Berkeley and Santa Cruz, conducts a coordi- are thought to form by the coalescence of nated program of theoretical research on star diffuse clouds into opaque, self-gravitating and planet formation. Under the directorship of clouds. However, high rates of star formation D. Hollenbach (Ames), the Center supports lead to high populations of massive stars that postdoctoral fellows, senior visitors, and radiate copious ultraviolet flux. The ultraviolet students; meets regularly at Ames to exchange flux in turn heats up the diffuse clouds in the ideas and to present informal seminars on interstellar medium and transforms them into a current research; hosts visits of outside scien- warm medium. Because a warm medium is tists; and conducts a week-long workshop on unlikely to form molecular clouds, the lack of selected aspects of star and planet formation diffuse clouds cuts off the supply of molecular each summer. clouds in a galaxy, thereby cutting off the star formation rate. This scenario then provides a In June 2000, the Center worked together with self-regulation mechanism that controls the rate researchers from the Arcetri Observatory of star formation in a galaxy. (Florence, Italy) to hold an international workshop entitled “High Mass Star Formation: Another focus of the Ames portion of the An Origin in Clusters.” The week-long Center research in FYOO involved a collabora- workshop, held in Volterra, Italy, had approxi- tive theoretical study of the conditions that mately 175 attendees, and included an invited determine whether a collapsing molecular talk by D. Hollenbach on ”Star Formation and cloud core of gas and dust gives rise to a single the Fluctuating Ultraviolet Field in the star surrounded by planets or to a binary star Galaxy.” system. This work focused on the realization that the molecular cloud cores that precede star One focus of the NASA Ames portion of the formation can have equilibrium configurations research work in the Center in FYOO involved that are nonaxisymmetric (lopsided). An the effect of ultraviolet radiation from young analytical study carried out by the Center massive stars on the interstellar medium of a reported on the discovery and the properties of galaxy. The interstellar medium of a galaxy is a sequence of these unusual egg-shaped the gas and dust that lie between the stars. equilibrium configurations. The analysis Most of the gas is hydrogen; the dust mass is shows that these configurations can collapse in only about 1% of the gas mass. The gas and a way that may naturally produce either binary dust reside in various components, often or single stars, depending on the initial degree characterized by the gas density in the compo- of distortion. nent. The densest and coldest component is the molecular clouds; this component forms stars. The theoretical models of the Center have been Diffuse clouds are less dense than molecular used to interpret observational data from such clouds; they are primarily cold atomic hydro- NASA facilities as the Infrared Telescope gen. The warm medium consists of neutral and Facility (IRTF), the Infrared Astronomical ionized gas at very low density and relatively Observatory (IRAS), the Hubble Space Telescope (HST),and the Infrared Space Infrared Astronomy (SOFIA) and the Space Observatory (ISO. a European space telescope Infrared Telescope Facility (SIRTF). Lvith NASA collaboration ). as well as from numerous ground-based radio and optical Point of Contact: D.Hollenbach I 64 telescopes. In addition. they have been used to (650) 604-4 [email protected].nasa.gov determine requirements for future missions such as the Stratospheric Observatory for

The Formation and Dynamics of Planetary Systems Gregory Laughlin

Progress was made in FYOO in numerous areas amplifier in allowing spiral waves to grow'. bearing on the overall problem of the formation This in turn gives us a clearer theoretical aild evolution of planetary systems. Specific picture of the very earliest stages of star and topics of research have ranged from the earliest planet formation. stages of star formation through the long-term fate of the Earth. and they are described in four A second line of inquiry has developed a way to constrain the conditions under which our peer-reviewed research papers. own solar system formed. The outer giant In the present-day solar system. the sun con- planets in our solar system all have nearly tains 99.94 of the mass. whereas the planets coplanar. circular orbits. This orderly configu- contain the bulk of the system angular momen- ration indicates that the sun and the planets tum. The clouds of gas and dust that collapse to ha\-e always existed in relati\;e isolation. If form star-planet systems. however. are essen- another stellar system had passed within tially in uniform rotation. One of the major several hundred astronomical units of the sun. unsol\:ed puzzles in the theory of star and gravitational perturbations would have scat- planet formation thus involves the detailed tered the outer planets (particularly Neptune) mechanism by which mass is transported into highly eccentric. inclined orbits. An inward onto the protostar while angular extensive set of Monte Carlo star-planet momentum is simultaneously pushed outward. scattering calculations has shown that the solar It is believed that spiral gravitational instabili- system likely formed in an aggregate contain- ties play a key role in eliciting angular momen- ing fewer than 1500 stars. and thus was not tum transport. but a full description of how born in a dense stellar cluster (resembling. say, spirals grow and operate on a global scale (that the Trapezium region in Orion). Primitive is. throughout the entire protoplanetary disk) is meteorites. however. contain daughter products not understood. Considerable theoretical of extinct radioactive elements that have half- progress was made in this area by performing a lives of one million years or less. In order to stability analysis of idealized singular isother- explain the presence of such short-lived mal disks. This research. carried out and isotopes in meteorites, it has been proposed published in collaboration with researchers at that either 1 ) the presolar nebula \vas enriched the University of California at Berkeley. by a nearby supernova explosion. or alternately Arcetri (Italy).and UNAM (Mexico). has that (2) x-ray flares associated with the nascent clearly explained the role of the corotation sun ivere able to create radioactive atoms via

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