White dwarfs shed light on dark matter

Posted: March 23, 2001

  White dwarf
A white dwarf, WD 0346, first reported by Nigel C. Hambly and Simon T. Hodgkin in 1997 and subsequently identified as a blue cool white dwarf, an unusual beast that turns bluish as it cools below about 4,000 Kelvin. The cross-hairs follow the star over the course of 43 years as it exhibits proper motion among background stars around the southern pole of our galaxy. Credit: Nigel C. Hambly, University of Edinburgh/Simon T. Hodgkin, Cambridge University. From Hambly et al. (ApJ Letters, 1997) and hodgkin et al. (Nature, 2000).
Some of the invisible "dark matter" that makes up most of the mass of our galaxy may be in the form of previously undetected white dwarf stars, astronomers reported this week.

In a paper published in Friday's issue of the journal Science, a team of astronomers in the U.S. and Britain reported that over three dozen cool white dwarfs -- which despite their name are actually more blue than white -- recently discovered in the vicinity of the Earth could represent a larger population of stars that may account for a significant fraction of our galaxy's mass.

The mystery of dark matter dates back to the 1930s, when astronomer Fritz Zwicky measured rotation rates of galaxies that were much higher than expected given their estimated masses. The only way to explain those high rotation rates, he concluded, was that the galaxy was much more massive than estimated because of the existence of unseen, or dark, matter.

Zwicky's observations were verified in the decades to come by other astronomers, who not only found that galaxies were spinning faster than expected, but that the rotation rates in the outer portions of spiral galaxies remained constant, and did not drop off as expected. This could only be explained if the galaxies were embedded in a much larger halo of dark matter. Astronomers now estimate that 95 percent of the mass of galaxies like the Milky Way is in the form of dark matter.

While astronomers were able to quantify the amount of dark matter, they have been unable to determine its composition. Some believe that a large fraction of the mass -- at least two-thirds -- consists of exotic, heavy particles that have yet to be observed: such particles are collectively known as weakly interacting massive particles, or WIMPs. The rest is in the form of baryonic, or ordinary, matter. In contrast to WIMPs, baryonic matter is thought to be aggregated into bodies called massive compact halo objects: MACHOs, in a deliberate twist of nomenclature.

While WIMPs by their nature would be difficult to detect, groups of astonomers have made efforts to discover MACHOs. While too dim to be directly observed, astronomers hoped to indirectly detect them by looking for changes in brightness of more distant stars when a MACHO passed between it and the Earth. Astronomers were able to observe several such "microlensing" events, giving them evidence that not only MACHOs exist, but that they have masses similar to white dwarfs, a class of faint stars that represent the end stage of the lives of most stars.

"These indirect observations indicated that white dwarfs may make up a substantial fraction of the dark matter, but the results could have been, and were, interpreted differently by some astronomers," explained Ben Oppenheimer of the University of California Berkeley, lead author of the Science paper. "They knew compact objects were in the halo, and they could measure the masses, but they never saw the objects themselves."

In an effort to conclusively tie MACHOs to white dwarfs, Oppenheimer scoured photographic plates of the sky around the south galactic pole taken over the last 30 years, looking for any objects that could be white dwarfs. He identified 92 candidate white dwarfs in the images, all relatively nearby because of their motion in the images. He, along with Nigel Hambly and Andrew Digby of the Institute for Astronomy of the University of Edinburgh, took spectra of as many of those stars as possible using a four-meter telescope in Chile.

Of the 92 objects, Oppenheimer and his colleagues obtained spectra of 69. Of those, 38 had spectra which matches those of white dwarfs. Moreover, all but four of those objects were actually bluish in color: evidence that the stars are particularly old and cold. Astronomers had theorized that as white dwarfs cool, they would form compressed atmospheres of molecular hydrogen that would filter out red and infrared light, giving the stars a blue appearance.

All 38 of the white dwarfs are located within 450 light years of the Earth. Extrapolating out through the entire galaxy, Oppenheimer and his co-authors conclude that white dwarfs are the most likely candidates for the MACHOs previously detected, and may make up to 35 percent of the dark matter mass. "We've found a previously undetected population of stars in the galactic halo that represents a fraction of the baryonic dark matter in the galaxy," Oppenheimer concluded.

While the focus of the paper is on the potential for white dwarfs to explain the dark matter in the galaxy, the research also plays a role in understanding stellar evolution. "These cool white dwarfs are the fossils of the early population of halo stars," explained Vanderbilt University astronomer Didier Saumon, a co-author on the paper. "There is much to learn about how galaxies form, and about how stars form in the process, from studying this population of white dwarfs."

"This research is not just about white dwarfs and dark matter," said Saumon. "This will really branch out into many different areas of astrophysics. There's the dark matter. There's the history of the galaxy. All thatıs tied to this discovery."