Graduate student Artemis Theodoridis is using her latest catalog of red giant stars to prove to the asteroseismic community that TESS data is worth paying attention to.
Indiana Jones might take the title for most exciting archeologic specialty, but galactic archeologists have got to be a close runner-up. By determining ages and interior compositions of stars, this physics sub-specialty pieces together the structure and evolution of our Milky Way galaxy. To put galactic history in chronological order, archeologists need accurate ages for hundreds of thousands - or even millions - of stars in any given section of the galaxy.
Graduate student Artemis Theodoridis has spent her research career working to aid this effort. Theodoridis is an asteroseismologist. She studies stellar oscillations, which tells her about a star's internal structure and mass. With enough data, she is also able to determine a star's age. Theodoridis studies stars towards the end of their lifecycles, or in the 'red giant branch' of their evolution (depicted in the straight lines of the MESA model above). Here, a star is exhausting all the hydrogen in its core. According to Theodoridis, this stage of a star's life is where the fun actually begins.
In her first year of graduate school, Theodoridis has just published the largest single catalog of red giant star ages in our galaxy - by an order of magnitude. Her work uses custom stellar evolution models to identify ages for 132,794 oscillating red giants.
As impressive as her result is how Theodoridis sourced her data. She's bet her undergraduate research career on a survey, TESS, widely dismissed in favor of the reigning asteroseismic gold-standard from the Kepler missions.
As an undergraduate studying at the University of Florida, Theodoridis and her professor Dr. Jamie Tayar took a big gamble. They decided Theodoridis should comb through NASA's Transiting Exoplanet Survey Satellite (TESS) data to determine if any of it could be used for asteroseismic research. The prevailing sentiment in the community was no. TESS was an all-sky survey. It had impressive scope, indeed enough to revolutionize available archeological data, but its shallow depth led many to distrust the accuracy of its seismic data offhand. NASA's Kepler missions, surveying small slices of sky to impressive depth, were considered the gold standard of asteroseismic data.
At the time, Theodoridis was equally prepared to write a paper on how TESS data wasn't precise enough for future asteroseismic studies. To her delight, Theodoridis published work in 2023 proving the opposite; Where there was overlap, seismic data of oscillating red giant stars from TESS was calibrated to the Kepler data within a 5% discrepancy for about 90% of the red giants. In her field, this is an outstanding agreement.
TESS was kind of an underdog of a mission. An all-sky survey wasn't considered as trustworthy as the in-depth Kepler mission, which is definitely the 'golden child' of our field. Proving the validity of TESS was exciting! Especially when we never expected such a good agreement between the two.
- Artemis Theodoridis, Graduate Student
Now that Theodoridis had verified the accuracy of the largest seismic sample of red giant stars in our galaxy, it was time to put it to use. Using a custom MESA program, she created 621 stellar evolution models that could predict the lifecycle of a star given its general mass and other measurable factors. Her collaborator, Leslie Morales, used a program called Kiauhoku to place every identified star at a specific point along a singular MESA track. Each point along a track corresponds to a specific stellar age.
Using this method, the researchers were able to infer ages for 132,794 red giant stars from the original 158,000 that the TESS mission identified (Hon et al. 2021). Their calculated stellar ages have an average uncertainty of 23%. In similar catalogs, average uncertainties often hover around 80%. Seismic data allows for the most accurate calculation of a star's mass, and mass allows for more accurate age estimates. Previous catalogs have calculated stellar age based on effective temperature, which can be a very finicky parameter in red giant stars.
Theodoridis is less than a year into her graduate studies, and is already providing data at scale for broader galactic archeology efforts. She plans to continue working with red giant stars, and is especially excited about the next TESS data release. As her studies evolve, she hopes to find new connections between stellar evolution and dark matter. Regardless of the next research topic, Theodoridis will be holding her second-most valuable discovery close to heart; "Whenever you give yourself a deadline, multiply it by three. Minimum."
You can read Artemis Theodoridis' new publication here.