ELM Survey Discovers 98 Double White Dwarfs

Scientists at the Center for Astrophysics | Harvard & Smithsonian (CfA) have completed the Extremely Low Mass — also known as ELM — spectroscopic study of white dwarf stars in the Sloan Digital Sky Survey (SDSS).
“We targeted candidate low mass white dwarf stars and found that they are all ultra-compact binaries. It makes sense,” said Dr. Warren Brown, astronomer at CfA and lead author on the survey. “The stars we studied lost so much of their mass during their evolution that they ended up as a low mass white dwarf.”

White dwarf stars are the remnant core of a star, what is left over after the star has burned through its nuclear fuel. The stars catalogued in the ELM survey do not follow the traditional “rules” for the creation of white dwarfs.

“The universe isn’t old enough to make such low mass white dwarfs on their own, and yet, here they are. That’s because they have companions in close orbits. The universe can’t make a low mass white dwarf unless it’s part of a compact binary,” said Brown. “The completed survey now represents more than half of the known detached double white dwarf binaries. This is a substantive piece of work that offers models for future studies and discoveries.”

The ELM Survey is just the beginning, said Dr. Mukremin Kilic, from the University of Oklahoma, and co-author on the survey. Pulling data from the SDSS and Gaia, paired with follow-ups using the 6.5-m MMT at the Fred Lawrence Whipple Observatory in Amado, Arizona, the survey team was able to collect a well-defined sample of existing binary white dwarf stars. “The models estimate there’s an order of a hundred million white dwarf binaries in our galaxy,” said Kilic. “We’ve found and confirmed 100 of them. Our observations can anchor the models for future surveys, and allow us to observe a specific subset of white dwarfs and cut through the population.”

The clean and complete data set also acts as a precursor to future gravitational wave studies. The LISA (Laser Interferometer Space Antenna) gravitational wave observatory — planned for launch in 2034 — will detect MHz gravity-wave sources, and is expected to detect hundreds of thousands of binary white dwarf stars. “There are things you can do if you have sources with both light and gravity waves,” said Brown. “With light we can measure temperature, distance, velocity, but we don’t measure mass directly; gravity-wave measurements measure mass.”

As new technology and new methodologies approach reality, scientists are keen to see what the future holds for the stars in the ELM survey. “The traditional response to these binaries was to call them supernova progenitors. Someday they will merge together and become something else, and it’s unclear what,” said Brown. “If there’s one thing we know for certain, it is that the stars we’ve listed in the survey will be great sources for the LISA mission and for future white dwarf star and gravitational wave studies; they are gravity wave sources, they are the signature multi-messenger systems of the future.”

Reference:
“The ELM Survey. VIII. 98 Double White Dwarf Binaries,” Warren R. Brown et al., 2020 Jan. 24, Astrophysical Journal [https://iopscience.iop.org/article/10.3847/1538-4357/ab63cd, preprint: https://arxiv.org/abs/2002.00064].