Two stars caught fusing into one

Astronomers observe a merger for the first time

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MATCH MADE IN HEAVEN Contact binary stars like the system in this computer simulation merge over time, forming what will eventually settle down to be a single star. Patrick Motl

For stellar astronomers, “the two shall become one flesh” just took on a whole new meaning.

Scientists have directly observed for the first time the merger of two closely orbiting stars. Experts have suggested for decades that such stars — which whirl so close to each other that their outer layers actually touch — should ultimately commingle. The new work, by Romuald Tylenda of the Nicolaus Copernicus Astronomical Center in Toruń, Poland and collaborators, catches the stars in the act.

The researchers’ claim of catching the stars in the act is “not just plausible; it’s compelling,” says Robert Williams of the Space Telescope Science Institute in Baltimore, who was not involved in the study. The results, to appear in an upcoming Astronomy & Astrophysics, add to previous work by Williams and colleagues to understand the nature of the star pair, called V1309 Scorpii.

V1309 Sco was discovered in 2008 when it erupted in a bright flare. Astronomers have proposed various explanations for the burst since then without reaching a consensus.

The new work hinges on a piece of good luck: Tylenda realized that the Optical Gravitational Lensing Experiment, a Warsaw University Observatory project hunting for dark matter since the mid-1990s, had been pointing its telescope at V1309 Sco’s region of the sky for years. Trolling through more than 2,000 observations taken from 2002 to 2010, he and his colleagues found light variations that suggest V1309 Sco was originally a contact binary star, a just-touching pair of stars circling each other about every 1.4 days. Over time, this periodic variation shortened as the stars’ outer layers combined to cocoon both orbs in a single envelope.

At that point the object got brighter, its light doubling every 19 days until late August 2008, when it brightened by a factor of 300 over 10 days. V1309 Sco’s final burst occurred that month when the stars’ cores finally merged and energy from their combined spins erupted outward. It became 10,000 times brighter than its original luminosity and more than 30,000 times brighter than the sun, then quickly faded away over the course of a few months to roughly its original brightness.

The best explanation for these variations is the merger of a contact binary system, Tylenda and his colleagues assert.

While the resulting object should be a star — albeit one with a weird gut structure and a quick spin — the material thrown off during coalescence still largely blocks V1309 Sco, so astronomers can’t see what it looks like. Astronomers have requested time on the Hubble Space Telescope to observe the object, says Williams, but it may take years for the disk to dissipate, notes Stefan Kimeswenger of the University of Innsbruck in Austria.

Kimeswenger fully agrees with the conclusion that V1309 Sco was a contact binary. But he is skeptical of the authors’ suggestion that a merger scenario could explain flares of a larger class of objects called V838 Mon-type eruptions, named after an object sighted in 2002 (SN 10/14/06, p. 248). “They all had outbursts of unknown type and they moved quickly to a red cold shell,” he says. But with different burst energies and chemical compositions, “that’s all they have in common.”

While Tylenda agrees that V838 Mon itself was “almost certainly not” a contact binary, he does think that stellar mergers of different types could explain these eruptions.

“The objects share the same crucial characteristics, that is they become extremely cool (for stars) at the end of the eruption,” he says. “This indicates that the energy source of the eruption quickly faded during the eruption. This is just what is expected to happen when the violent merger processes are over.”

A MATCH MADE IN THE HEAVENS from Science News on Vimeo.

Over time, binary stars will lose orbital energy and merge, like in this simulation of the flow of matter between two low-mass stars. The stars in V1309 would have taken hundreds of orbits to merge and had more complex interiors than the stars depicted here.

Credit: Patrick Motl

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