Open Clusters Like
Orion Have Low Fertility Rate
(8 July 2008) A detailed survey of
stars in the Orion Nebula has found that fewer than 10 percent have enough
surrounding dust to make Jupiter-sized planets, according to a report by
astronomers at the University of California, Berkeley, the California Institute
of Technology (Caltech) and the Harvard-Smithsonian Center for
Astrophysics.
Because stars like the sun probably formed in hot
open clusters like Orion, the finding suggests that sun-like stars have a low
probability of forming planets, or at least, planets the size of Jupiter or
larger.
"We think that most stars in the galaxy are formed in dense,
Orion-like regions, so this implies that systems like ours may be the exception
rather than the rule," said lead author Joshua Eisner, a Miller postdoctoral
fellow at UC Berkeley. This is consistent with the results of current planet
searches, which are finding that only about 6 percent of stars surveyed have
planets the size of Jupiter or larger.
The study by Eisner, Caltech
astronomer John M. Carpenter and their colleagues will appear in the August 10
print edition of The Astrophysical Journal.
The Orion Nebula is a
brilliant cluster only a million years old and glowing with the light of newly
formed stars like a jewel in the sword of the hunter Orion. The cluster is also
very dense, Eisner said, with 1,000 stars packed into a region several light
years on a side. For comparison, in the neighbourhood of the sun, there's only
one star within that volume of space.
Four billion years ago, however,
the sun may have been in a dense, open cluster like Orion. Because open
clusters like Orion eventually become gravitationally unbound, they disperse
over the course of billions of years, and as a result, the sun's birth
neighbours are long gone.
Studying star clusters like the Orion Nebula
Cluster "helps our understanding of the typical mode of star and planet
formation," Eisner said.
The new findings come from some of the first
observations of a radio telescope array jointly operated by UC Berkeley,
Caltech, the University of Maryland and the University of Illinois and located
at Cedar Flat in eastern California's Inyo Mountains near the city of Bishop.
The Combined Array for Research in Millimetre Astronomy (CARMA) was created in
2004 by relocating the nine 6-metre telescopes of the
Berkeley-Illinois-Maryland Association (BIMA) array from Hat Creek, Calif., and
the six 10-metre telescopes of Caltech's Owens Valley Radio Observatory (OVRO)
millimetre-wave array to Cedar Flat. The 15-dish array conducted its first
observations in 2006.
The CARMA array observes at millimetre
wavelengths, which is ideal for piercing the clouds of dust and gas surrounding
young stars to see their dense, dusty disks. Eisner and his colleagues also
used the Submillimetre Array (SMA) atop Mauna Kea in Hawaii for this study. The
combination of CARMA and the SMA enabled the sensitivity and high image quality
needed to observe the dusty disks in Orion.
The astronomers'
observations of Orion's central region of more than 250 known stars showed that
only about 10 percent emit 1.3-millimetre wavelength radiation typically
emitted by a warm disk of dust. Even fewer - less than 8 percent of stars
surveyed - were judged to have dust disks with masses greater than
one-hundredth the mass of the sun, a mass thought to be the lower limit for
formation of Jupiter-sized planets. The average mass of a protoplanetary disk
in the region was only one-thousandth of a solar mass, the researchers
calculated.
Eisner noted that previous surveys he and Carpenter have
conducted of other young, open clusters that are older or younger than Orion
show an evolutionary trend in the average masses of disks in the different
regions. Older clusters tend to show less dust, perhaps because much of it has
already gathered into planets.
Previous surveys of another
lower-density, star-forming region - the Taurus cluster - showed that more than
20 percent of its stars have enough mass to form planets. The difference is
probably related to the tightly packed, hot stars of the Orion cluster, said
Carpenter, a senior research astronomer and deputy director of
OVRO.
"Somehow, the Orion cluster environment is not conducive to
forming high mass disks or having them survive long, presumably due to the
ionisation field from the hot, massive OB stars, which you might expect would
photoevaporate dust and lead to small disk masses," he said.
Many of the
stars in Orion imaged by CARMA had been photographed earlier by the Hubble
Space Telescope and were dubbed proplyds, short for protoplanetary disks. While
Hubble saw the dust disks silhouetted against the star, CARMA directly detected
emissions from the dust itself.
"CARMA is an ideal instrument for this
type of study, with its 15 telescopes providing the fine resolution needed to
resolve protoplanetary disks so that we can determine their structure and
measure their masses more precisely," Eisner said.
Carpenter noted that
future improvements to the CARMA array could allow detection of even smaller
disks capable of giving rise to sub-Jupiter planets. To detect even smaller
disks able to form large Earth-like planets, or super-Earths, will require a
more extensive array, such as the Atacama Large Millimetre Array (ALMA) now
being built in Chile.
Coauthors on the journal paper are Richard L.
Plambeck of UC Berkeley, graduate student Stuart A. Corder of Caltech and
Chunhua Qi and David Wilner of the Harvard Smithsonian Center for Astrophysics,
which operates the Submillimetre Array.
(source: University of
California Berkeley)
