IllinoisPaul Ricker



In this section

Binary stars


Sirius A and B
Sirius A and B (MacDonald Obs.)
At least half of the visible points of light in the sky that a naked-eye observer would identify as "stars" are, in fact, binary stars -- pairs of stars that orbit each other. If the stars are close enough together, their mutual gravitational pull can deform them from a spherical shape. If one of the stars is more massive than the other, as is usually the case, their differing evolutionary timescales can lead to very interesting behavior -- including the transfer of matter from one star to the other. If the star receiving matter is a compact object (a white dwarf, neutron star, or black hole), a variety of energetic phenomena, such as novae, X-ray bursts, and supernovae, can occur.

University of Illinois graduate student Kuo-Chuan Pan works with me on these projects.

Common-envelope evolution

In order for two stars in a binary system to interact strongly with each other, they must form a "close binary" -- ie. their separation must be a small multiple of the larger star's radius. Most binary systems do not form in this configuration, but they can evolve into close binaries by passing through what is known as the common-envelope phase. During the common-envelope phase, one of the stars in a binary system has evolved through consumption of its hydrogen fuel into a red giant. Red giants have large, distended atmospheres, so even though the stars may have been well-separated when they were smaller, now that one of them is a red giant some of the material in the red giant may be close enough to the companion star to fall towards the companion. This situation is called Roche-lobe overflow. Some of the overflowing material accretes onto the companion, while some of it is kicked out of the system by the companion's gravitational field. The tidal interaction between the red-giant envelope and the companion star causes the companion to spiral inward, orbiting within the red-giant (now "common") envelope, ejecting more and more of the envelope material.

Common-envelope simulation
The common-envelope phase lasts for a very short period of time in the lives of the stars -- the stars themselves may spend billions of years fusing hydrogen on the main sequence, but once the common-envelope phase begins it lasts only a few months at most. Nevertheless, its effects are profound. There are two possible outcomes: one in which the companion's inspiral continues until the two stars fuse into a single giant, rapidly spinning star, and one in which the inspiral stops at some intermediate radius, leaving a close binary system. A binary system may pass through more than one common-envelope phase as the companion star reaches the end of its main-sequence lifetime and becomes a red giant.

Together with Ronald Taam at Northwestern University, our group is performing computer simulations of the hydrodynamics of the common-envelope phase to identify the physical mechanisms that drive energy, mass, and angular momentum loss from such systems and to determine which binary star systems will end up as merged, rapidly spinning stars and which ones will become close binaries. Our results thus far suggest that hydrodynamic drag is far less important in driving the inspiral than gravitational tidal drag due to the extended common envelope. We have also found that a common analytical prescription used to estimate the effects of common-envelope evolution on populations of binary star systems dramatically overestimates the amount of accretion onto the companion. We continue to work toward a better understanding of the population effects of the common-envelope phase by simulating the common-envelope evolution of binary systems with different masses, evolutionary states, and initial orbital separations.

Type Ia supernovae

The most widely accepted model for Type Ia supernovae holds that they occur when a white dwarf star in a close binary system accretes sufficient material from its giant companion to exceed a stability threshold called the Chandrasekhar mass (equal to about 1.4 times the Sun's mass). Once it exceeds this threshold, density and temperature fluctuations in its core cause runaway fusion reactions involving carbon and oxygen to blow apart the star. The entire problem considering steady accretion onto the white dwarf, ignition and propagation of the nuclear flame, detonation, and interaction of the ejected white dwarf material with its surroundings is a very challenging computational problem because of the wide range of length and time scales involved. However, we can study each step separately from the others to gain insights into the working of the entire process.

Our group is using a simulation setup based on our common-envelope work to study the question of how a Type Ia supernova affects the non-white-dwarf companion star and the surrounding interstellar medium. In particular, we are interested in learning how much of the companion star is stripped away or ionized by the explosion, as well as how much of the nuclear fusion products of the supernova are deposited onto the companion. These simulations will enable us to determine the properties of post-Type Ia systems and allow us to connect their observable characteristics to the specific type of progenitor binary system responsible for the supernova.