Measuring a helium white dwarf

Binary stars are extremely common in our galaxy, as are white dwarf stars. White dwarfs made of pure helium, on the other hand, are rare. Astrophysicists at UC Santa Barbara have identified an eclipsing binary system containing one ‘normal’ white dwarf and one helium white dwarf. The discovery enabled them to accurately measure a helium white dwarf for the first time.

White dwarfs are formed when stars below a certain size limit exhaust their fuel supplies. The end result is usually a dense core made mainly of carbon and oxygen. Helium white dwarfs, while identified over 20 years ago, are less well understood. For example, while assumed to be larger and hotter than ordinary white dwarfs, those predictions had never been confirmed experimentally. Lars Bildsten and his students at UCSB, in collaboration with Steve Howell, an astronomer at the National Optical Astronomy Observatory (NOAO) in Tucson, Arizona have now done exactly that.

The scientists had been studying a helium white dwarf designated NLTT 11748. At regular intervals, the star appeared to dim in intensity. The scientists realized that a binary partner was eclipsing the star. By carefully timing the passage of the normal star as it eclipsed the helium dwarf, the cosmologists were able to accurately measure the size of the helium white dwarf.

As predicted, NLTT 11748 is considerably larger in volume, and gives off 30 times more light than its normal white dwarf partner. Yet, the helium white dwarf has a mass just 10 to 20% of that of our sun, in contrast to its partner star, which has a mass closer to 70% of our sun.

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.

Credit: Steve Howell/Pete Marenfeld/NOAO

As usual, astronomers like to take the long view. Bildsten says:

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.