White dwarfs discovered in eclipsing binary system

A pair of white dwarf stars have been discovered in an eclipsing binary system, offering astronomers the first chance to directly measure the radius of a rare white dwarf composed of pure helium.

White dwarfs are the extremely dense remnants of stars like our Sun, crammed into a volume equivalent to the size of the Earth. They are thought to mark the final evolutionary state of stars with masses not quite large enough to end in a supernova explosion – some 97 percent of stars in our Galaxy. When the star has exhausted its nuclear fuel it is typically left with a core of carbon and oxygen, although a rarer breed associated with binary systems are often left with helium cores.

In this artist conception of the unique binary star NLTT 11748, the larger but less massive helium white dwarf star is partially eclipsed by the smaller but more massive normal white dwarf, which is about the size of the Earth. Image: Steve Howell/Pete Marenfeld/NOAO.

University of California Santa Barbara (UCSB) physics graduate Justin Steinfadt was studying the star NLTT 11748 – one of the few very low-mass, helium core white dwarfs – with the Faulkes Telescope North, when he noted that in a few of the consecutive image frames, which were taken at a rate of one exposure per minute, the star appeared fainter.

"Our initial reason for looking at this star was the hope that it would be a pulsating star," Steinfadt tells Astronomy Now. "Observationally, these pulsations manifest themselves as small variations (less than one percent fluctuations) in brightness, getting brighter and fainter over a period of a few minutes. Theoretically, the behavior of these pulsations tell a lot about the interior structure of the stars."

But it turned out that the astronomers had observed three-minute eclipses of a binary star system twice during the 5.6-hour orbit. The observations were promptly followed up with the ten-metre Keck telescope located on Mauna Kea in Hawaii. Using the Keck, the astronomers were able to measure the changing Doppler shift of the star as it orbited its faint, but more massive companion. These observations led to the confirmation that stars end their lives in a variety of ways. "The formation of such a binary system containing an extremely low mass helium white dwarf has to be the result of interactions and mass loss between the two original stars," says Steve Howell of the National Optical Astronomy Observatory, one of Steinfadt's supervisors.

The helium-core white dwarf in the newly discovered system has a mass of just 15 percent that of the Sun. Theory predicts that these stars burn hotter and are larger than ordinary white dwarfs, but until now their size had never been measured. The other star in the binary is also a white dwarf, albeit a more ordinary one, composed of mostly carbon and oxygen with about 70 percent of the mass of the sun. This star is more massive but also much smaller than the other white dwarf, and the light it gives off is 30 times fainter than that of its partner star in the binary.

"The helium white dwarf has a radius of about 0.04 solar radii (4.4 times Earth's radius) and the carbon/oxygen white dwarf has a radius of about 0.01 solar radii (1.1 times Earth's radius)," confirms Steinfadt, who adds: "The discovery was a completely serendipitous one; there are several dozen systems known that have He-core white dwarfs in them, compared to the thousands of known carbon/oxygen-core white dwarfs, but of the systems with He-core white dwarfs, ours is the only one to show eclipses."

Lars Bildsten, a professor at UCSB's Kavli Institute for Theoretical Physics, comments: "A particularly intriguing possibility to ponder is what will happen in 6 to 10 billion years. This binary is emitting gravitational waves at a rate that will force the two white dwarfs to make contact. What happens then is anybody's guess."