Do stars remember their past?

Stars in their "young" years, from newborn to adolescent, pose a major scientific challenge. Because of their complexity, modeling the star formation process in theoretical models is extremely challenging. Observing a star's oscillations is one of the few ways to find out more about a star's age, structure, or formation.

Konstanze Zwintz said, "In contrast to the exploration of the Earth's interior with the help of seismology, we can also shed light on the internal structure of stars based on their oscillations, and thus also on the age of the stars."

Zwintz is considered a pioneer in the field of young asteroid seismology and is the head of the "Stellar Evolution and Asteroid Seismology" research group at the Institute of Astrophysics and Particle Physics at the University of Innsbruck.

The study of stellar oscillations has developed considerably in recent years due to the increased precision of space telescopes such as TESS, Kepler, and James Webb. These advances have also shed new light on decades-old theories of stellar evolution.

As long as stars have not yet converted hydrogen to helium in their cores, they are called "children". At this stage, they are in the pre-main sequence; after ignition, they reach adulthood and enter the main sequence.

"So far, studies of stars have focused on adult stars - like our Sun," said Thomas Steindl, a member of the Constance Zwentz research group and first author of the study. Little attention has been paid to the evolution of the pre-main sequence because this stage is very disordered and difficult to model. It's only in recent years that technological advances have allowed us to look more closely at the budding phase of stars - and thus at that moment when stars start to fuse hydrogen into helium."

In their current study, the two Innsbruck researchers have now proposed a model that can be used to realistically depict the earliest stages of stellar life - well before they become adults. The model is said to be based on the open-source stellar evolution program MESA (Module for Experimental Stellar Astrophysics).

Inspired by a talk given by University of Vienna astronomer Eduard Vorobyov at a conference in 2019, Thomas Steindl spent months refining methods that use this stellar evolution code to recreate the chaotic stages of early star formation and then predict their specific oscillations.

"Our data show that stars on the pre-main sequence took a very chaotic course in their evolution. Despite its complexity, we can now use it in our new theoretical model," Steindl said.

As a result, the astronomer showed that the way stars form can affect oscillatory behavior even after fusion is ignited in the main sequence. "Infancy has an effect on the pulsations in the later stages of the star. This sounds very simple but strongly questionable. Classical theory assumes that the time before ignition does not matter at all. This is incorrect. The equivalent of a musical instrument is that even small differences in composition can lead to significant changes in timbre. Our modern models therefore better describe the oscillations of real stars."

Konstanze Zwintz is pleased with this discovery and very optimistic about the future. "About 20 years ago, when I first saw the oscillations of a young star in front of me on the screen I was already sure that one day I would be able to demonstrate the significance of early stellar evolution for 'adult' stars. Thanks to the great work of Thomas Steindl, we have now succeeded. This is a Eureka moment for our research group and a milestone for a better understanding of the steps of stellar growth."