Heavy Metals

The Milky Way galaxy is home to approximately 100 billion stars. Timothy Beers only needs one hundred — one hundred of the rarest stars known to exist. But the Notre Dame Chair in Astrophysics has a plan to do it, and so far, the future is looking bright.

It started on a seemingly ordinary day in August, 143 million light-years away from Earth, when a flash of gamma rays followed by a series of gravitational waves were detected by the advanced LIGO (Laser Interferometer Gravitational-Wave Observatory) facility in the U.S. and the Virgo facility in Europe. The instruments had captured the collision of two neutron stars; it was the first time a cosmic event had been viewed in both gravitational waves and light waves. It was one of, if not the most significant discovery in modern astrophysics.

The two high-density neutron stars had been circling each other for a long time (think billions of years), edging closer and closer in orbit until finally crashing into one another. The collision caused neutron-heavy metals to form and scatter through the universe, like a cosmic fireworks display.

The event may help prove one of the longest-held theories in astrophysics. The nuclear reactions that take place at the time of a neutron star collision are believed to be responsible for the presence of heavy metals here on Earth. We use these materials all the time in everyday life. The platinum used in catalytic converters and dental work, the uranium fueling nuclear power plants and the gold found in jewelry cases all over the world — all are made of “star stuff.”

Astrophysicists call it the rapid-neutron capture process (r-process). To understand the nature of the r-process definitively, they need to compare the frequencies of a sample of older r-process-enhanced stars with the predicted frequencies of newer stars arising from a collision.

Scientists now have the latter, thanks to the neutron star merger observed in August. Now they need a sample of the former — 100 to be exact — but the phrase “needle in a haystack” may understate how daunting a challenge that can be.

That’s where Beers comes in.

Taking inventory of the universe isn’t easy. Astronomers at universities all over the world compete for research time on large telescopes, like the 6.5-meter Magellan telescopes at the Las Campanas Observatory in Chile, or the 8.4-meter scopes at the Large Binocular Telescope Observatory in Arizona.

Getting time depends on the nature of their research and what kind of funding they have available. Inclement weather limits the work researchers can actually do.

For every 100 painstakingly pre-selected stars Beers and his team observe, only three might be r-process-enhanced stars. They could be anywhere in the galaxy. Astronomers currently only know of 25 to 30. “We’re not finding them fast enough,” Beers said.

To speed up the process, he devised a way to use each of those challenges to his advantage.

Beers and his team are working to identify the brightest low-metallicity stars in the Milky Way’s halo, in the search for r-process-enhanced stars among them. Looking for the brightest stars allows the team to make use of modest, moderate-aperture-size telescopes such as the 3.5-meter telescope at the Apache Point Observatory in New Mexico and the 2.7-meter Harlan J. Smith Telescope at the McDonald Observatory in Texas.

“These low-metallicity stars are in the halo of the galaxy around us,” Beers said. “No one would give us a significant amount of time on any telescope to look through our long lists of targets. It’s impractical to do it with large telescopes when the numbers of expected detections are so small.”

In order to assemble the lists of several thousand bright metal-poor stars, Beers provided “bad weather” projects to large telescopes such as the Gemini 8-meter observatories in Chile and Hawaii, for when weather caused delays on other research projects. This approach has led to Beers’ project being awarded thousands of hours of precious time.

Beers said he uses the smaller telescopes to do the heavy lifting, obtaining moderate-quality high-resolution spectroscopy, or “snapshots” as he calls them, of hundreds of candidates during the course of a single run.

“All we need are spectra that are good enough to tell us which stars are r-process-enhanced,” he said. Once he and his team identify objects with detectable characteristics of r-process elements, they turn to bigger scopes, like the 6.5-meter Magellan, for help.

Those telescopes provide higher-quality spectra, or “portraits,” of those stars for a more conclusive determination.

The team has become so adept at its method of identifying characteristics and leveraging relationships at various observatories that it has dramatically increased the rate of discovery of r-process-enhanced stars. When Beers began his survey, astronomers were finding r-process-enhanced stars at a rate of one per year. Beers’ team found 15 stars in just the first year and a half of a planned four-year effort.

The project has also resulted in the discovery of two r-process-enhanced stars with detectable uranium — the heaviest natural element — two of only six on record. Stars with measurable amounts of uranium can be used to estimate the age of the universe.

Beers hopes to secure additional funding to complete his search for r-process-enhanced stars, which he calls EXPRESS (Exploration of R-Process-Enhanced Stars Survey). The survey would make observations of 2,500 stars Beers and Notre Dame Research Assistant Professor Vinicius Placco have selected from previous studies of bright metal-poor candidates at medium resolution during bad-weather observations and other sources.

The results could answer one of the most widely sought after in questions in the field of astrophysics.