How Stars Can Be Used as 'Probes' for Galactic Archaeology

Astronomers are combining different types of stellar data to better understand our cosmic backyard.

The Milky Way. Image: Carter Roberts

Archaeologists sort through the artifacts and remnants of ancient societies to understand the history of human culture. Galactic archaeologists do more or less the same thing, but on a cosmic scale—they sift through the stellar fossil record to unravel the mysteries of our galaxy’s lifetime.

Now, scientists with the Strömgren survey for Asteroseismology and Galactic Archaeology (SAGA) have developed a more precise way of determining the ages of stars, which heretofore has been lacking. It’s an insight that will help astronomers pinpoint exactly when big events happened in the Milky Way.

Thanks to space-based missions like NASA’s Kepler Space Telescope and the European Space Agency’s Convection, Rotation, and Planetary Transits mission (CoRoT), astronomers have gathered oscillation frequency data on hundreds of main sequence stars and thousands of giant stars. And a team led by Dr. Luca Casagrande from the Research School of Astronomy and Astrophysics is putting this wealth of information to good by combining it with classical stellar data.

According to the team's latest work, to be published in The Astrophysical Journal, asteroseismologic and classic stellar data is combined to assess a stripe of the Kepler field. The result is a new diagnostic tool that should tell astronomers more detailed information about nearby stars.

Wide-field Strömgren photometry gathers light from stars using a series of four-colour medium-band filters with a known sensitivity to radiation. It’s a type of photometry typically used to gather reliable, classical stellar parameters. This means things like a star’s metallicity (the portion of the star that isn’t made of hydrogen or helium gas) as well as its temperature and angular diameter. But there’s more to learn about a star, and that’s where asteroseismology comes in.

Asteroseismology is the study of a pulsating star’s internal structure as interpreted by its frequency spectrum. It’s sort of akin to how geologists can use seismic waves to study the internal structure of our home planet. Asteroseismology allows astronomers to determine stellar properties that would be otherwise impossible to understand, namely a star’s radius, mass, distance, and age.

"I was working on the Stroemgren technique which uses the colour of stars to work out stellar properties, while my friend, Dr. Victor Silva Aguirre, was working on data from the Kepler Satellite which measures the pulsations of stars; effectively their sound," Dr. Casagrande said in a release. "We realised that combining the two measurement types would give much more precise stellar parameters, including the possibility of deriving good ages for stars."

By combining asteroseismology from Kepler with Strömgren photometry obtained using the Wide Field Camera on the 8.2-foot Isaac Newton Telescope on La Palma, SAGA provides the data astronomers need to explain and constrain the development and structure of our galaxy. The goal of the method is to use stars as "probes" of galactic archaeology "for the purpose of understanding the formation and evolution of stellar populations in the Milky Way."

"We might also uncover evidence for violent events in the past, such as collisions with other galaxies," Dr. Casagrande said.

As the authors note, studying stars outside of our galaxy is difficult, writing that "the study of a sizeable number of individual stars in the Milky Way enables us to directly access different phases of its formation and evolution, in a fashion which is still hardly achievable for external galaxies."

But the team's new system could help. "While the ultimate purpose is to use these observations to provide homogeneous and reliable stellar parameters for both candidate planet host and seismic stars, the geometry chosen
for our observations already enables Galactic studies using stars with detected oscillations," the authors write. In essence, they're combining different types of stellar data to better understand our cosmic backyard, with the hope that such work will help astronomers better understand what's farther afield.