Stellar Disks and Jets

Stellar jets are analogous to giant lawn sprinklers. Whether a sprinkler whirls, pulses or oscillates, it offers insights into how its tiny mechanism works. Likewise stellar jets, billions or trillions of miles long offer some clues to what's happening close into the star at scales of only millions of miles, which are below even Hubble's ability to resolve detail. Hubble's new findings address a number of outstanding questions:

Where Are Jets Made?

Hubble shows that a jet comes from close into a star rather than the surrounding disk of material. Material either at or near the star is heated and blasted into space, where it travels for billions of miles before colliding with interstellar material.

Why Are Jets So Narrow?

The Hubble pictures increase the mystery as to how jets are confined into a thin beam. The pictures tend to rule out the earlier notion that a disk was needed to form a nozzle for collimating the jets, much like a garden hose nozzle squeezes water to a narrow stream. One theoretical possibility is that magnetic fields in the disk might focus the gas into narrow beams, but there is as yet no direct observational evidence that magnetic fields are important.

What Causes a Jet's Beaded Structure?

Hubble is solving the puzzle of a unique beaded structure in the jets, first detected from the ground but never fully understood.

"Before the Hubble observations the emission knots were a mystery", said Jeff Hester. "Many astronomers thought that the knots were the result of interactions of the jet with the gas that the jet is passing through, while others thought that the knots were due to 'sputtering' of the central engine. We now know that the knots are the result of sputtering". Hester bases this conclusion on Hubble images which show the beads are real clumps of gas plowing through space like a string of motor boats. Competing theories, now disproved by Hubble, suggested a hydrodynamic effect such as shock-diamond patterns seen in the exhaust of a jet fighter.

What Do Jets Tell Us about Star Birth?

"The jet's clumpy structure is like a stockbroker's ticker tape; they represent a recorded history of events that occurred close to the star", said Jon Morse. "The spacing of the clumps in the jet reveals that variations are occurring on several time scales close to the star where the jet originates. Like a "put-put" motor, variations every 20 to 30 years create the strings of blobs we see", Morse concluded. "However, every few hundred years or so, a large amplitude variation generates a 'whopper' of a knot, which evolves into one of the major bow-shaped shock waves". Other Hubble views by Chris Burrows reveal new blobs may be ejected every few months. "If the circumstellar disk drives the jet then the clumpiness of the jet provides an indirect measure of irregularities in the disk".

Why Are Jets "Kinky"?

The Hubble pictures also show clear evidence that jets have unusual kinks along their path of motion. This might be evidence for a stellar companion or planetary system that pulls on the central star, causing it to wobble, which in turn causes the jet to change directions, like shaking a garden hose. The jet blast clears out material around the star, and perhaps determines how much gas finally collapses onto the star.

Star Formation:

A star forms through the gravitational collapse of a vast cloud of interstellar hydrogen. According to theory, and confirmed by previous Hubble pictures, a dusty disk forms around the newborn star. As material falls onto the star, some of it can be heated and ejected along the star's spin axis as opposing jets. These jets of hot gas blaze for a relatively short period of the star's life, less than 100,000 years. However, that brief activity can predestine the star's evolution, since the final mass of a star determines its longevity, temperature, and ultimate fate. The jet might carry away a significant fraction of the material falling in toward the star, and, like a hose's water stream plowing into sand, sweeps out a cavity around the star that prevents additional gas from falling onto the circumstellar disk.

Historical Background:

In the early 1950's, American astronomer George Herbig and Mexican astronomer Guillermo Haro independently catalogued several enigmatic "clots" of nebulosity near stars near the Orion nebula that have since been called Herbig-Haro objects. It is only in the last 20 years, however, that the true nature of these objects, and their role in the star formation process, has been revealed. Careful study showed that many of the Herbig-Haro objects represent portions of high-speed jets streaming away from nascent stars. Now there are nearly 300 Herbig-Haro objects identified by astronomers around the world, and the list is growing as new technologies and techniques are developed to probe the dusty depths of nearby stellar nurseries.

From Stellar Disks and Jets (HubbleSite - NewsCenter - Background, June 6, 1995)

Updated: 6 June '95

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