PRESS RELEASE, November 2017
ASTRONOMERS REVEAL NEARBY STARS THAT ARE AMONG THE OLDEST IN OUR GALAXY
Dr. Wei-Chun Jao
Dr. Todd J. Henry
Astronomers in the RECONS group have discovered some of the oldest stars in our Milky Way by pinpointing their locations and velocities through the Galaxy.
Just like humans, stars have a life span: birth, youth, adulthood, senior and death. In this effort, Dr. Wei-Chun Jao, a senior member of the RECONS team focused on old or "senior citizen" stars, also known as cool subdwarfs, that are much older and cooler in temperature than the Sun.
The Milky Way is nearly 14 billion years old, and its oldest stars developed in the early stage of the Galaxy's formation, making them about six to nine billion years old. They're found in the halo, a roughly spherical component of the Galaxy that formed first, in which old stars move in orbits that are highly elongated and tilted. Younger stars in the Milky Way rotate together along the Galaxy's disk in roughly circular orbits, much like horses on a merry-go-round.
In paper #42 of The Solar Neighborhood series by the RECONS team (November 2017 edition of The Astronomical Journal ), Jao and colleagues targeted stars out to a distance of 60 parsecs (200 light years), which is relatively nearby considering the Milky Way is more than 30,000 parsecs (100,000 light years) across. This is farther than the traditional horizon of 25 parsecs (80 light years) for the region of space that is typically referred to as "the solar neighborhood".
"The reason my horizon is more distant is that there are not a lot of senior citizens (old stars) in our solar neighborhood," said Jao. "There are plenty of adult stars in our solar neighborhood, but not a lot of senior citizens, so we have to reach farther away in the Galaxy to find them."
The astronomers first observed the stars over many years with the 0.9 meter telescope at Cerro Tololo Inter-American Observatory (CTIO) in the foothills of the Chilean Andes. They used astrometry to measure the stars' positions and were able to determine the stars' motions across the sky, their distances and whether or not each star had a hidden companion orbiting it.
Results in this paper, combined with previous results on subdwarfs by the team, have increased the known population of old stars within 60 parsecs by 25%. Among the new subdwarfs, the researchers discovered two old binary stars, even though older stars are typically found to be alone, rather than in pairs. Jao says, "It's rare for senior citizens to have companions. Old folks tend to live by themselves. I then used NASA's Hubble Space Telescope to detect both stars in one of the binaries and measured the separation between them, which will allow us to measure their masses." Masses are key to understanding most of what there is to know about a star, including how long it will last.
Jao also outlined two methods to identify these rare old stars. In the first, he used the stars' locations on a fundamental map of stellar astronomy known as the Hertzsprung-Russell (H-R) diagram. This is a classic technique that places the old stars below the sequence of dwarf stars such as the Sun on the H-R diagram, hence the name "subdwarfs."
In the second method, the authors took a careful look at one particular characteristic of known subdwarf stars -- how fast they move across the sky. Jao says, "Every star moves across the sky. They don't stay still. They move in three dimensions, with a few stars moving directly toward or away from us, but most moving tangentially across the sky. We found that if a star has a tangential velocity faster than 200 kilometers per second, it has to be old." So, based on their movements in our Galaxy, astronomers can evaluate whether a star is an old subdwarf or not. In general, the older a star is, the faster it moves because of gravitational interactions with other objects in the Galaxy. The authors applied the tangential velocity cutoff and compared stars in the subdwarf region of the H-R diagram to other existing star databases to identify an additional 29 previously unidentified cool subdwarf candidates.
In 2018, results from the European Space Agency's Gaia mission, which is measuring accurate positions and distances for millions of stars in the Milky Way, will make finding older stars much easier for astronomers. Determining the distances to stars is now very labor intensive and requires a lot of telescope time and patience. Because the Gaia mission will provide a much larger sample size, Jao says the limited sample of subdwarfs will grow, and the rarest of these rare stars -- binary subdwarfs -- will be revealed.
Of course, finding old stars could also lead to the discovery of old planets, and as Jao says, "Maybe we can find some ancient civilizations around these old stars."
This research effort has been supported by the National Science Foundation, via observations made possible by the SMARTS Consortium, and by the Space Telescope Science Institute.