Planet-forming disc found orbiting twin suns

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 11 June, 2009

Images collected with the Smithsonian’s Submillimeter Array (SMA) radio telescope system reveal the presence of a molecular disc orbiting a young binary star system.

The new images of binary system V4046 Sagittarii, presented by UCLA graduate student David Rodriguez at the American Astronomical Society meeting this week, provide a unique and vivid snapshot of the planet-forming process. The results also confirm that gas giants and rocky bodies may just as easily form around double stars as around single stars like our Sun.

“It’s a case of seeing is believing,” says Joel Kastner of Rochester Institute of Technology, the lead scientist on the study. “We had the first evidence for this rotating disc in radio telescope observations of V4046 Sagittarii that we made last summer. But at that point, all we had were molecular spectra, and there are different ways to interpret the spectra. Once we saw the image data from the SMA, there was no doubt that we have a rotating disc here.”

SMA image of rotating, gaseous disc around twin-star system V4046 Sagitarii (white dot). Blue represents material that is approaching and red represents material that is receding from us. The disc gas likely represents the raw material out of which Pluto-like bodies, comets, and perhaps gas giant planets will form (or might already have formed) around the double star. The filled oval at lower left represents the size of the smallest structures that could be detected in the image).

The molecular disc around V4046 Sagittarii extends from within the approximate radius of Neptune’s orbit out to about 10 times that orbit, corresponding to the zone where the Solar System’s giant planets, as well as its Pluto-like Kuiper Belt objects, may have formed.

“We believe that V4046 Sagittarii provides one of the clearest
examples yet discovered of a Keplerian, planet-forming disc orbiting a young star system,” says David Wilner of Harvard-Smithsonian Center for Astrophysics. “This particular system is made that much more remarkable by the fact that it consists of a pair of roughly solar-mass stars that are approximately 12 million years old and are separated by a mere five solar diameters.”

Ben Zuckerman of UCLA comments that the system could be the oldest known orbiting protoplanetary molecular disc. “It shows that, at least for some stars, formation of Jovian-mass planets may continue well after the few million years, which astronomers have deduced is characteristic of the formation time for most such planets.”

The Submillimeter Array is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics and is funded by the Smithsonian Institution and the Academia Sinica.

Kastner and his team first suggested V4046 Sagittarii as a potential hunting ground for planets in a paper published in Astronomy and Astrophysics in December last year. “We thought the molecular gas around these two stars almost literally represented ‘smoking gun’ evidence of recent or possibly ongoing ‘giant,’ Jupiter-like planet formation around the binary star system,” says Kastner. “The SMA images showing an orbiting disc certainly support that idea.”
Such binary systems could host currently undetected planets, and at a distance of only 240 light years from the Solar System, the V4046 Sagittarii binary is at least two times closer to Earth than almost all known planet-forming star systems, presenting a good opportunity for imaging.

Kastner and colleagues had previously used the 30 metre radio
telescope operated by the Institut de Radio Astronomie Millimetrique (IRAM) to study radio molecular spectra emitted from the vicinity of the twin stars. The scientists used these data to identify the raw materials for planet formation around V4046 Sagittarii – circumstellar carbon monoxide and hydrogen cyanide – in the noxious circumstellar molecular gas cloud. “In this case the stars are so close together, and the profile of the gas – in terms of the types of molecules that are there – is so much like the types of gaseous discs that we see around single stars, that we now have a direct link between planets forming around single stars and planets forming around double stars,” says Kastner.