JILA astrophysicists investigate new stars, nearby stars, cool stars, and dying stars. Peter Conti focuses on the nature and evolution of the hottest and most luminous stars in the Milky Way and other galaxies. He collaborates with astronomers around the world to study the birth of some of our galaxy's youngest massive stars, many of which are still embedded in their natal dust shrouds. He also studies starburst galaxies, which exhibit massive bursts of star formation that can alter galactic structure and create a super wind that spews large quantities of energy and mass into the intergalactic medium.

Accretion disk

Phil Armitage is interested in theories that explain the causes of star formation within massive accretion disks orbiting black holes and the coalescence of planetary systems from disks of gas and dust orbiting new stars. He is currently evaluating the "gravitational collapse" theory, which posits that gravitational interactions in an accretion disk cause clumps of gas to form. These clumps then attract more matter to them, eventually forming stars or planets. Many scientists believe this mechanism is more likely to occur in the outer regions of a supermassive black hole than in a planetary accretion disk. If so, the gravitational collapse theory may explain why there are often a multitude of stars near black holes.

The successful creation of stars and planets depends on how efficiently an orbiting accretion disk can cool. If the disk cools rapidly during an orbital period, some gas will collapse, initiating star or planet formation. However, if it cools more slowly, the disk begins to form spiral arms that direct much of its mass into the central black hole or star, making it larger. Armitage looks at the role of stress, i.e., friction between different parts of the disk, in forming clumps. He's discovered that disk stress increases dramatically the faster an accretion disk cools. The greater the stress the more likely it is that gravitational collapse will occur.

Juri Toomre and the astrophysical fluid dynamics group study the internal workings of stars like the Sun. They focus on the interaction of turbulent convection and differential rotation to build magnetic fields by dynamo action. The Sun is unique since flows well below its surface can now be probed using helioseismology. By observing sound waves in the Sun, Toomre's group has discovered large-scale meandering motions much like jet streams coexisting with intense convection in the Sun's convection zone. These findings inspire and guide major supercomputer simulations of convection and magnetism aimed at understanding how this zone of turbulent plasma is able to build the strong magnetic fields that are seen to erupt through the solar surface. The group's models can now largely explain the Sun's differential rotation with strong shear layers thought to be crucial elements in the operation of the solar dynamo.

Simulation of the Sun's surface

Toomre has collaborated for more than a decade with investigators around the world in the study of the vibrational modes of the Sun, which are analogous to the vibrational modes of the Earth that can be excited by large earthquakes. These studies have shown that (1) the upper one-third of the Sun consists of a convective layer that experiences differential rotation, which depends both on depth and distance from the solar equator and (2) weather patterns consisting of large-scale flows such as jets, tornadoes, and strong winds exist below the Sun's surface. Toomre and colleague Nic Brummel have created supercomputer simulations that led to the discovery of bi-directional magnetic pumping of compressible solar gases through the turbulent convection zone down to the tachocline. The tachocline is an area of rapid change between the convection zone and radiative zone, which is the inner two-thirds of the Sun. The simulations revealed how the interaction of the turbulent convection zone and the tachocline leads to the formation of the magnetic pumps and other structures. These structures likely play a role in the Sun's consistent, but variable, rotation rate and surface features such as the 22-year cycle of sunspots.

Once they're satisfied they understand solar structure and dynamics, the scientists plan to conduct similar research on other stars. They are already using their insight into the Sun's turbulent flows to model large-scale motions in Earth's atmosphere and oceans and Jupiter's atmosphere.

Jeffrey Linsky and his group investigate the role of stellar winds and interstellar gas clouds in forming astrospheres and hydrogen walls around stars. Astrospheres, which are analogs of the heliosphere, are produced when the stellar wind interacts with interstellar gas flow. Hydrogen walls are compressed regions of hot hydrogen atoms surrounding some stars. Linsky and his group investigate the structure and motions of warm interstellar gas clouds to understand how these clouds interact with stellar winds to form astrospheres and hydrogen walls.

Linsky's group also studies the coronae (hot ionized plasmas in the outer atmospheres of most stars), chromospheres (regions between photospheres and coronae), stellar winds (analogs of the solar wind), and circumstellar envelopes of stars in the Milky Way Galaxy. They are comparing these structures among different sizes and types of stars. They also want to know how such stellar regions are affected inside binary star systems.

Recently, Linsky detected stellar winds for 13 nearby cool stars. (Our Sun is a cool star.) Using computer modeling, he was able to deduce the relationship between the strength of a stellar wind and the age of a star and the X-ray luminosity of its corona. Understanding the evolution of stellar winds with time allowed him to deduce that a strong solar wind, which appeared about 700 million years after the Sun began to shine, literally blew away the dense atmosphere of the planet Mars.

Supernova 1987A

Richard McCray has focused for nearly 20 years on the death of a single star. He has combined data analysis with theoretical simulations of SN1987A, the brightest supernova visible from Earth since 1604. It occurred when a blue supergiant star exploded in the Large Magellanic Cloud, a galaxy located 160,000 light years from Earth. Thanks to the Hubble Space Telescope and the Chandra and XMM-Newton observatories, this is the first time in history astronomers have been able to witness the evolution of a supernova unfolding in real time.

McCray has studied the composition, temperature, and density of the supernova's debris and investigated its circumstellar ring system. He and his colleagues correctly predicted that the ring system would become a thousand times brighter in 1997 when struck by the blast wave from the stellar explosion. Since then, hot spots have appeared in the ring system as more parts of it have been clobbered by the supernova blast. McCray predicts that by 2007, the hot spots will merge as the entire ring system lights up as it is entirely overtaken by the supernova's shockwave.

McCray's observations led him to conclude that the giant star lost most of its outer layers about 20 thousand years before it exploded. Then a high-speed wind blowing off the star's surface carved out a cavity in the cloud. Currently, the supernova's shockwave is interacting with dense fingers of gas that protrude inward from the edge of the circumstellar gas cloud. As the shockwave rides further into the cloud, it will continue to illuminate the star's past.

Mitch Begelman's group recently described one mechanism that can trigger a supernova. Gamma-ray jets produced deep within massive stars can blow the star apart as they emerge through its surface. The jets are created by a complex interaction of a black hole, an accretion disk, and very strong magnetic fields that come into being when a massive star depletes its hydrogen fuel and falls into itself. The black hole produces two jets that travel away from it in opposite directions toward the poles of the star. The pencil-thin jets travel near the speed of light through the star's outer layers to reach the surface. Once they emerge, they spread out so rapidly, the expansion propagates back toward the black hole and blows up the star.