Planet-Forming Disk Orbits Twin Suns

Submillimeter Array image of the rotating, gaseous disk surrounding the young twin-star system V4046 Sagittarii (located at the white dot in the image). Note the size of the V4046 Sagittarii disk relative to the orbit of Neptune, shown to scale at the lower right (the filled oval at lower left represents the size of the smallest structures that could be detected in the image). The disk is tipped from our perspective, such that it appears as elliptical rather than circular. The image is color-coded according to the motion of the gas in the system, with blue representing material that is approaching and red representing material that is receding from us. The fastest-moving approaching and receding gas is detected closest to the central binary star system. This disk 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.
Credit: RIT

Astronomers have announced that a sequence of images collected with the Smithsonian’s Submillimeter Array (SMA) radio telescope system clearly reveals the presence of a rotating, molecular disk orbiting the young binary star system V4046 Sagittarii.

The SMA images of V4046 Sagittarii, which were presented by UCLA graduate student David Rodriguez in a press conference at the American Astronomical Society meeting in Pasadena, Calif., provide an unusually vivid snapshot of the process of formation of giant planets, comets, and Pluto-like bodies. The results also confirm that such objects 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 (NY) Institute of Technology, the lead scientist on the study. “We had the first evidence for this rotating disk 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 disk here.”

The other contributors to the SMA study of V4046 Sagittarii led by RIT’s Kastner and UCLA’s Rodriguez are Ben Zuckerman of UCLA and David Wilner of Harvard-Smithsonian Center for Astrophysics. Wilner is one of the world’s experts on radio interferometry, the technique used in this study to form images using the SMA’s multiple radio antennas at Mauna Kea Observatory in Hawaii.

According to Rodriguez, the images clearly demonstrate that the molecular disk orbiting the V4046 Sagittarii binary system extends from within the approximate radius of Neptune’s orbit out to about 10 times that orbit. This region corresponds 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 disk orbiting a young star system,” Wilner says. “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 5 solar diameters.”

This artist’s conception of the V4046 Sagittarii system highlights the disk-shaped molecular gas cloud imaged by the Submillimeter Array.
Credit: David A. Aguilar (CfA)

“This could be the oldest known orbiting protoplanetary molecular disk and 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,” Zuckerman says.

Findings of the SMA imaging study build on previous work published in the December 2008 issue of Astronomy and Astrophysics in which Kastner and his team first suggested that the case of V4046 Sagittarii well illustrates how planets may easily form around certain types of binary stars.

“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, Kastner says. “The SMA images showing an orbiting disk certainly support that idea.”

The evidence for a molecular disk orbiting these twin young suns in the constellation Sagittarius suggested to the scientists that many such binary systems should also host as-yet undetected planets.

“The most successful technique used so far for discovery of extrasolar planets — that of measurement of precision radial velocities – is exceedingly difficult for close binary stars such as V4046 Sagittarii. So these radio observations are probing a new region of discovery space for extrasolar planets,” says Rodriguez.

Artist’s photo concept of a sunset on a planet in a binary star system.
Credit: NASA/JPL-Caltech

“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, which gives us a good shot at imaging any planets that have already formed and are now orbiting the stars,” he continued.

Kastner and collaborators had previously used the 30-meter 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 disks 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,” Kastner says.

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. Young star research by RIT’s Kastner and UCLA’s Zuckerman and Rodriguez is supported by a grant from the NASA Astrophysics Data Analysis program.

An animation of the system is available at: ftp://www.cis.rit.edu/people/kastner/V4046Sgr/v4046sgrMovie.mov.