Dusty Old Star Offers Window To Our Future, Astronomers Report

The results are based on mid-infrared observations made with theGemini 8-meter Frederick C. Gillett Telescope (Gemini North) onHawaii's Mauna Kea. The Gemini observations reveal a surprisingly highabundance of dust orbiting an ancient stellar ember named GD 362.

"This is not an easy one to explain," said Eric Becklin, UCLAastronomer and principle investigator for the Gemini observations. "Ourbest guess is that something similar to an asteroid or possibly even aplanet around this long-dead star is being ground up and pulverized tofeed the star with dust. The parallel to our own solar system'seventual demise is chilling."

"We now have a window to the future of our own planetarysystem," said Benjamin Zuckerman, UCLA professor of physics andastronomy, member of NASA's Astrobiology Institute, and a co-author onthe Gemini-based paper. "For perhaps the first time, we have a glimpseinto how planetary systems like our own might behave billions of yearsfrom now."

"The reason why this is so interesting is that this particularwhite dwarf has by far the most metals in its atmosphere of any knownwhite dwarf," Zuckerman added. "This white dwarf is as rich in calcium,magnesium and iron as our own sun, and you would expect none of theseheavier elements. This is a complete surprise. While we have made asubstantial advance, significant mysteries remain."

The research team includes scientists from UCLA, CarnegieInstitution and Gemini Observatory. The results are scheduled forpublication in an upcoming issue of the Astrophysical Journal. Theresults will be published concurrently with complementary near-infraredobservations made by a University of Texas team led by Mukremin Kilicat the NASA Infrared Telescope Facility, also on Mauna Kea.

"We have confirmed beyond any doubt that dust never doessleep!" quips Gemini Observatory's Inseok Song, a co-author of thepaper. "This dust should only exist for hundreds of years before it isswept into the star by gravity and vaporized by high temperatures inthe star's atmosphere. Something is keeping this star well stocked withdust for us to detect it this long after the star's death."

"There are just precious few scenarios that can explain so muchdust around an ancient star like this," said UCLA professor of physicsand astronomy Michael Jura, who led the effort to model the dustenvironment around the star. "We estimate that GD 362 has been coolingnow for as long as five billion years since the star's death-throesbegan and in that time any dust should have been entirely eliminated."

Jura likens the disk to the familiar rings of Saturn and thinksthat the dust around GD 362 could be the consequence of the relativelyrecent gravitational destruction of a large "parent body" that got tooclose to the dead star.

GD 362 is a white dwarf star. It represents the end-state ofstellar evolution for stars like the sun and more massive stars likethis one's progenitor, which had an original mass about seven times thesun's. After undergoing nuclear reactions for millions of years, GD362's core ran out of fuel and could no longer create enough heat tocounterbalance the inward push of gravity. After a short period ofinstability and mass loss, the star collapsed into a white-hot corpse.The remains are cooling slowly over many billions of years as the dyingember makes its slow journey into oblivion.

Based on its cooling rate, astronomers estimate that betweentwo billion to five billion years have passed since the death of GD362.

"This long time frame would explain why there is no sign of ashell of glowing gas known as a planetary nebula from the expulsion ofmaterial as the star died," said team member and Gemini astronomer JayFarihi.

During its thermonuclear decline, GD 362 went through anextensive period of mass loss, going from a mass of about seven timesthat of the sun to a smaller, one-solar-mass shadow of its former self.

Although about one-quarter of all white dwarfs contain elementsheaver than hydrogen in their atmospheres, only one other white dwarfis known to contain dust. The other dusty white dwarf, designatedG29-38, has about 100 times less dust density than GD 362.

The Gemini observations were made with the MICHELLEmid-infrared spectrograph on the Gemini North telescope on Mauna Kea,Hawaii.

"These data are phenomenal," said Alycia Weinberger of theCarnegie Institution. "Observing this star was a thrill! We were ableto find the remnants of a planetary system around this star onlybecause of Gemini's tremendous sensitivity in the mid-infrared. Usuallyyou need a spacecraft to do this well."

The Gemini mid-infrared observations were unique in theirability to confirm the properties of the dust responsible for the"infrared excess" around GD 362. The complementary Infrared TelescopeFacility near-infrared observations and paper by the University ofTexas team provided key constraints on the environment around the star.

University of Texas astronomer and co-author Ted von Hippeldescribes how the Infrared Telescope Facility (IRTF) observationscomplement the Gemini results: "The IRTF spectrum rules out thepossibility that this star could be a brown dwarf as the source of the'infrared excess,'" von Hippel said. "The combination of the two datasets provides a convincing case for a dust disk around GD 362."

The Gemini observations included time made available as part of an exchange of instrument time with the W.M. Keck Observatory.