White dwarfs are the remnants of stars like the Sun. They also provide some of the best means to measure large distances in the Universe if they explode as "type Ia" supernovae. All of those explosions occur in binary systems consisting either of two white dwarfs or a white dwarf paired with an ordinary star. To understand the whole process, astronomers need to identify progenitor systems before they explode: binaries with one or more white dwarf.

A particularly interesting example was recently identified and described in a Science paper by Ethan Kruse and Eric Agol. In this system, a white dwarf is locked in mutual orbit with a Sun-like star. The orientation of the binary relative to Earth means the two bodies periodically eclipse each other. When the white dwarf passes in front of its companion, gravitational lensing—the focusing of light by a massive body—magnifies the star's light very slightly. This is the first such "self-lensing" system containing a white dwarf, and should allow researchers to better understand understand the behavior of white dwarfs in binaries.

Lensing

Gravitational lensing is best known in very large, massive objects: galaxies or galaxy clusters that focus the light from more distant objects, often producing multiple images. However, thanks to general relativity, we know that lensing is a property of any massive object. The first observational test of the theory was lensing of starlight during a total solar eclipse in 1919, when Arthur Eddington measured the deflection of starlight around the momentarily hidden Sun.



When one star passes in front of another (from our point of view), the gravity of the foreground star magnifies the light of the background object very slightly. This effect is very small, and so it is known as gravitational microlensing (or just microlensing) to distinguish it from the more dramatic form described in the sidebar. Microlensing can be used in some cases to detect exoplanets orbiting around the star in the foreground: the planet provides a tiny extra boost, beyond that provided by its host star, to the light of the star in the background.

A special case of microlensing is self-lensing, which occurs when both the foreground and background objects are part of the same binary system. During each orbit, one star passes in front of the other, momentarily magnifying its light even as it produces an eclipse. (For exoplanets, the effect of self-lensing is far too small to be seen.) The problem: in most binaries, the eclipse blocks far more light than the microlensing amplifies, meaning astronomers can't measure the effect of general relativity.

The situation is slightly better when one of the objects is a white dwarf. That's because white dwarfs have the mass of a star, but it's compressed into an object roughly Earth-sized. Thus, they produce a smaller eclipse and bend light slightly more. Even so, self-lensing is hard to detect.

In the binary system known as KOI-3278, microlensing from the white dwarf boosts the light of its companion by 0.1 percent. (KOI stands for "Kepler object of interest", meaning it was discovered using the Kepler telescope and identified as a possible exoplanet system.) Each eclipse lasts about five hours, and each orbit takes about 88 days, coincidentally the same as Mercury's orbit around the Sun.

The companion star is a yellow star almost identical to the Sun, having nearly the same mass. Using microlensing data and the orbital properties, the researchers determined the white dwarf mass is about 63 percent that of the Sun, a fairly typical number. Even more importantly, they measured the white dwarf's radius to be about 1.1 percent of that of the Sun—a very difficult measurement to perform, since that's nearly the same as the size of Earth.

KOI-3278 is nowhere close to becoming a white dwarf supernova. Like our Sun, the star in the binary is in the main part of its life; eventually it will also burn out and leave a second white dwarf behind, but not for a long time. However, this system is a great example of what should be a fairly common class of binary. Even though Kepler is done with its primary mission, analysis of its existing data could turn up hundreds of other similar white dwarf/Sun-like star binaries. That would enable astronomers to test many aspects of our models of white dwarf structure, as well as provide data on supernova progenitors.

Science, 2014. DOI: 10.1126/science.1251999 (About DOIs).