Telescope unveils a stellar deception

A heavenly masquerade may shed light on the nature of astrophysical jets—the beams of material spewed by a wide variety of celestial objects.

Top: Image of Hen 2-90, some 8,000 light-years away, shows pair of directed jets. X-shaped streaks are artifacts. Bottom: The object’s disk (arrow), seen edge on. NASA, Sahai, Nyman

Astronomers came upon the stellar puzzle while observing a dust-shrouded body called Hen 2-90 with the Hubble Space Telescope. The object had been classified as the remains of a dying, lightweight star. The new images reveal, however, that the object exhibits behavior unlike that of any corpse. Hen 2-90 has a pair of oppositely directed jets that spout clumps of gas.

Newborn stars possess jets, but Hen 2-90 is too hot and luminous to qualify as a youngster, notes Raghvendra Sahai of NASA’s Jet Propulsion Laboratory in Pasadena, Calif. Instead, he and Lars-Åke Nyman of the European Southern Observatory in Santiago, Chile, and the Onsala Space Observatory in Sweden propose that the body consists of two aging stars hidden behind a lane of dust.

One member of the unseen duo may be an elderly, bloated star known as a red giant. The outer layers of such a star are easily captured by the tug of a companion star. The gas then forms a swirling disk around the companion. This second star is likely to be a recently formed white dwarf, the compact remnant of a sunlike star, Sahai and Nyman suggest in the Aug. 1 Astrophysical Journal Letters.

The jets may derive from gas piling onto the disk and creating periodic instabilities, the researchers suggest. Other systems also produce jets when one star rips matter from another.

Sahai says that other Hubble images showing a dark disk around Hen 2-90 lend credence to his team’s hypothesis. This lopsided disk is too large to be the swirling gas around just the white dwarf, but it indicates that material surrounding Hen 2-90 doesn’t have a spherical distribution.

That’s what astronomers would expect if one star were dumping matter onto a companion. Spectra of Hen 2-90 could reveal the chemistry of the hidden duo, Sahai adds. “This is one of the most beautiful, best behaved, and most collimated bipolar jets ejected by a stellar source of any sort,” comments John F. Bally of the University of Colorado at Boulder. Noting that jets are common in objects ranging from newborn stars to galactic cores, the new find “may well prove to be a Rosetta stone for understanding the launch and collimation of stellar jets,” he says

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