Institute for Astronomy Home
IFA Home Page   |    Search   |    Other Editions    No. 32 - 2009 
  All Articles  


SMA Uncovers Progenitors of Planetary Systems, Massive Stars


The SMA, a joint project of the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics in Taiwan, detects light at wavelengths where the coldest objects in the cosmos glow brightest. By employing a technology called interferometry, in which signals from two or more small antennas are combined, the SMA produces images of unparalleled resolution at these wavelengths. (Jonathan Weintroub, SMA.)


The Submillimeter Array (SMA) on Mauna Kea consists of eight movable antennas, each six meters in diameter. They are used to detect radiation with wavelengths from 0.3 to 1.7 millimeters (0.01 to 0.07 inches), which is on the short end of the radio spectrum. The main source of this radiation is cold interstellar material--gas, dust, and small rocks. Unlike telescopes that detect radiation in the shorter visible and infrared wavelengths, the SMA allows scientists to look into dense interstellar clouds and to witness the birth of stars and planetary systems.

IfA scientists have used the SMA to make pioneering discoveries. Doctoral student Rita Mann and her faculty advisor, Jonathan Williams, found a binary star-disk system in which each star is surrounded by the kind of dust disk that is frequently the precursor of a planetary system. In a separate project, SMA Postdoctoral Fellow Jonathan Swift found a massive, quiescent object in a dark cloud that is probably the direct progenitor of a massive star or stars.


Left: SMA image of the binary system. The mass of the disk on the left is 70 times the mass of Jupiter, while the one on the right is 20 Jupiter masses. University of Hawaii. Right: The optical image taken by the Hubble Space Telescope shows the shadow of the large disk, but the smaller disk is obscured in the glare of the brighter star. Courtesy Nathan Smith, University of California at Berkeley.

The binary system discovered by Mann and Williams stands out as the first known example of two optically visible stars, each surrounded by a disk with enough mass to form a planetary system like our own. It lies 1,300 light-years from Earth, in the famous Orion Nebula, the kind of rich cluster of stars that is a common birth environment for most stars in our Milky Way galaxy, including our Sun.

A binary star system consists of two stars bound together by gravity that orbit a common center of gravity. Most stars form as binaries, and if both stars are hospitable to planet formation, it increases the likelihood that scientists will discover Earth-like planets.

One of the disks was discovered in an image taken with the Hubble Space Telescope, but the other disk was hidden in the glare of the star. Hubble saw only the disk shadow, so the amount of material and its capability for planet formation was unknown until the IfA team made the SMA observations. "The SMA was able to image the binary system at almost the same level of detail as the Hubble Space Telescope, but in the extreme infrared, where we can see the glow from the dust, rather than its shadow," explained Mann.

The two stars would take 4,500 years to complete one orbit around their common center. Both stars are only about a third the mass of our Sun and are much cooler and redder in color. Viewed from a potential future planet, the stellar neighbor would appear as an intense point in the night sky, about one thousand times brighter than the brightest star in our night sky, Sirius. Planets around the other star would be visible only through telescopes, but they would be within reach of spacecraft from a civilization with the same level of technology as ours.

The larger disk is the most massive found in the Orion Nebula so far. The discovery of this massive disk and the binary disk system improve our understanding of how common planet formation is in our Galaxy and place our solar system in context.

mid-infrared image of the infrared dark cloud
A color composite mid-infrared image of the infrared dark cloud overlaid with gray contours that trace the mass in the dark cloud at low resolution. The orange contours represent the emission detected by the SMA that is dominated by the massive, cold core near the center of mass of the cloud. The Oort cloud of comet nuclei is the outermost part of our solar system. Its radius is about 100,000 times the distance from the Sun to Earth. J. Swift; NASA/JPL-Caltech; E. Churchwell, Univ. of Wisconsin; James Clerk Maxwell Telescope/Joint Astronomy Centre.

Swift's discovery may be the first time that scientists have been able to see such a region before massive stars form. Located 23,000 light-years from us, it has a mass 120 times that of the Sun contained within a volume smaller than the Oort cloud of comets orbiting at the edge of our solar system. Its temperature is less than 18 degrees above absolute zero (-273 degrees C or -460 degrees F). Such a large amount of cold dense gas is likely to evolve into one or more massive stars.

Massive stars--those with a mass of more than eight times that of the Sun--are much rarer than Sun-like stars. However, they produce disproportionately more radiation, causing them to lead short, spectacular lives. The rarity of massive stars and their propensity to quickly destroy the environments from which they form has posed a serious challenge to understanding their formation.

"The SMA is a unique instrument in a superb location that facilitates our ability to map the conditions preceding the formation of massive stars with high resolution. Perhaps the most exciting thing is that we now know that massive and dense cores with no sign of star formation activity do exist," said Swift, noting that further study is necessary.