08 September 2005

Texas astronomers, others find dead stars collecting dust

MAUNA KEA, Hawaii — Two independent teams of astronomers using telescopes on Mauna Kea, Hawaii have glimpsed dusty debris around an essentially dead star where gravity and radiation should have long ago removed any sign of dust. The discovery might provide insights into our own solar system's demise billions of years from now.

The observations, made with the NASA Infrared Telescope Facility (IRTF) and the Gemini 8-meter Frederick C. Gillett Telescope, reveal a surprisingly high abundance of dust orbiting an ancient stellar ember named GD 362.

GD 362 is a white dwarf star. It represents the end-state of stellar evolution for almost all stars, including the Sun and more massive stars like this one's progenitor, which had an original mass about seven times the Sun's. After undergoing nuclear reactions for millions of years, GD 362's core ran out of fuel and could no longer create enough heat to counterbalance the inward push of gravity. After a short period of instability and mass loss, the star collapsed into a white-hot corpse. The remains will cool slowly over many billions of years as the dying ember makes its final, slow journey into oblivion.

In its former life, GD 362 may have been home to a planetary system. The dust disk may be evidence of that.

According to University of Texas graduate student Mukremin Kilic, who led the team making the IRTF observations, “The best explanation for the disk around GD 362 is that a planet or asteroid-like object was tidally disrupted by the white dwarf and ground up to tiny particles that ended up in a debris disk around the star. It is likely that we are witnessing the destruction of a planetary system and that a similar fate may await our own planetary system in about five billion years.”

These results are exploring new ground in the search for planetary systems. “This is only the second white dwarf star known to be surrounded by a debris disk,” Kilic said. The other is called G29-38.

“Both of these stars' atmospheres are continuously polluted by metals - that is, heavy chemical elements - almost surely accreted from the disk,” Kilic said. “If the accretion from a debris disk can explain the amounts of heavy elements we find in white dwarfs, it would mean that metal-rich white dwarfs - and this is fully 25% of all white dwarfs - may have debris disks, and therefore planetary systems, around them,” he said, concluding, “Planetary systems may be more numerous than we thought.”

The IRTF team also includes Ted von Hippel and Don Winget from The University of Texas and Sandy Leggett of the Joint Astronomy Centre. The Gemini team is led by Eric Becklin of UCLA, and includes researchers from the Carnegie Institution and Gemini Observatory.

The IRTF and Gemini data are “beautifully complementary,” Texas' von Hippel said. “Both data sets support the idea of a dust disk around GD 362, but they do so based on different evidence.”
He explained that the Gemini data provide “measurements at longer wavelengths that are more sensitive to the dust temperature,” while the IRTF results provide higher-resolution spectroscopy in the near-infrared, thus “excluding an orbiting brown dwarf as the source” of the excess infrared light from the white dwarf.

The teams will publish back-to-back papers in an upcoming issue of Astrophysical Journal Letters.

The IRTF is a 3-meter telescope, optimized for infrared observations, operated and managed for NASA by the University of Hawaii Institute for Astronomy. Gemini is an international partnership managed by the Association of Universities for Research in Astronomy under a cooperative agreement with the National Science Foundation.

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NOTE: A related press release and high resolution images are available from Gemini Observatory.