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How stellar death leads to twin celestial jets

It takes two The mystery behind how a spherical star can produce aspherical nebula may have been solved, according to a new study.

"Most stars in the universe die this way," says the study's lead author, Professor Eric Blackman of the University of Rochester in New York.

"Astronomers are interested in how stars are born and how they die, and this gets to the mystery of how they die."

When Sun-like stars run out of hydrogen they become known as an Asymptotic Giant Branch (AGB) star. It is during this stage that a star's outer envelope is 'puffed off', becoming a planetary nebula.

The name planetary nebulae was originally coined by astronomer William Herschel, who observed them in the 1780s, and thought they were newly forming planets.

"Although the name's persisted, we now know they're the end states of low mass stars," says Blackman.

But unlike the stars that form them, not all planetary nebula are spherical.

"We have been trying to come up with a better understanding of what happens to change these spherical AGB stars into aspherical nebulae, with two jets shooting out in opposite directions," says Blackman.

Existing models suggest aspherical planetary nebulae are produced by a binary star; an AGB star and another star orbiting close together.

The aspherical shape was thought to be caused by material being shredded off one star and forming an accretion disk swirling around the other.

According to Blackman, the material heats up and is focused by magnetic fields to form jets that shoot out perpendicular to the disk, producing the spectacular nebulae.

However, narrowing down the fine details of this process has been difficult because the cores of the AGBs — where the disks form — are too small to be directly resolved by telescopes.

Blackman and his student, Scott Lucchini used observational data, combined with theoretical models to refine these theories. Their findings appear in the Monthly Notices of the Royal Astronomical Society.

"We looked at what the published information on specific cases reveals about the strength of these out flows, their momentum, and their energy," says Blackman.

"We compared this observational data to theoretical models to determine how different levels of strength and accretion, would affect the outflow power for different interacting binaries."

Close together

Blackman and Lucchini found only very high rates of material flowing onto the accretion disk can produce these jets.

This can only occur when the stars are so close, that a high flow rate of material from the AGB is pulled into an accretion disk around the companion star.

Alternatively, it could occur if the companion star or a large planet enters the 'atmosphere' of the AGB star.

Blackman says this would generate also a high flow rate of material to fall onto the accretion disk around either the companion star or the AGB stellar core.

"Both of these scenarios could provide enough energy and momentum to produce the jets that have been observed."