Grand Theft Astro

Magnetic fields exist in many stars, including the Sun. Yet they remain too poorly understood to be reliably included in stellar evolution models, limiting their accuracy. Now, a new study by researchers at the Institute of Science and Technology Austria (ISTA), published in Astronomy & Astrophysics, shows why a star’s “criminal” evolutionary history may be crucial for correctly interpreting its internal magnetic fields.

To determine stellar ages, researchers use theoretical stellar evolution models that need to be as accurate as possible—and that means also taking magnetic fields into account. Yet it is often unclear how strong these fields are, where exactly they are located, or how they are shaped. Their complex geometry is difficult to determine with the available data.

A new study by Assistant Professor Lisa Bugnet’s research group at the Institute of Science and Technology Austria (ISTA), in collaboration with the Universidad de La Laguna in Tenerife and Université Paris Cité, now shows that, in some old Suns, signals may point to a ”criminal” past in which they may have “stolen” mass from a companion star. Such an event would reshape the relationship between the star’s core and envelope structure, leaving it far more susceptible to the effects of magnetic fields when compared to a “normal” single star.

The case of the missing mode

We can’t look directly inside stars: they are too far away and opaque. But researchers can use highly precise telescopes to measure tiny, regular changes in their brightness. These changes are caused by vibrations or oscillations. Every star has its own pattern of oscillations, depending on its internal structure.

This branch of astrophysics is called asteroseismology. It is like seismology on Earth: while seismic waves reveal information about our planet’s interior, stellar oscillations give researchers insights into the interior of stars.

Of particular interest to Nicolas Muntean, a PhD student in the Bugnet group, are red giants —Sun-like stars in a late phase of their lives—and so-called dipole mixed modes. The latter are oscillations that probe regions close to the stellar core. In some red giants, these dipole mixed modes are barely visible. Researchers refer to this as dipole-mode suppression.

“A specific oscillation mode is missing from the spectrum or is only barely visible—as if it had been cut out or suppressed,” Muntean explains. “Recent research suggests that strong magnetic fields inside the star located near its core region are responsible: they appear to cause this particular mode to disappear from the spectrum.”

The problem is that, according to existing models, the magnetic fields needed to explain this suppression in some stars would have to be extremely strong. How such fields could form remain unclear.

Merger mystery

One possible explanation is that a red giant may have “stolen” mass from a companion star during its evolution or may have merged with another star. Such an event would alter the relationship between its core and envelope structure, making the star much more sensitive to magnetic fields compared to stars with identical mass but an ordinary evolutionary history. As a result, a considerably weaker magnetic field could already be enough to suppress dipole modes and explain the same observations.

“This was exactly the hypothesis we wanted to investigate theoretically,” says Muntean. “For this, we needed stellar models that could simulate such mass gain and allow us to analyze the resulting differences in the oscillation signatures.”

Using the open-source stellar-evolution code MESA, Muntean simulated models of stars with and without a mass-gain event. He then compared how sensitive these models were to magnetic fields.

“We saw a major difference—the stars that had gained mass were much more sensitive to magnetic fields.”

Having demonstrated this possibility theoretically, Muntean began looking for suitable candidates in the Universe. Are there stars already known in scientific literature that may have formed through a merger? Are there stars whose seismic parameters suggest that they could be merger products?

The answer is yes. Red giants that already showed evidence of strong magnetic fields could be described using the new theoretical model, which assumes a “Grand Theft Astro” took place—an earlier stellar merger or a mass gain.

 “The idea is that our previous interpretation may be biased because we assume that a star evolved on its own,” adds Bugnet. “If we instead suppose that it gained mass, the observed oscillations become more sensitive to a magnetic field. This provides an alternative scenario for explaining the same observed signals.”

Looking beyond the evidence

Magnetic fields are among the important elements that have so far been included in stellar models only in a very simplified and not particularly realistic manner. The new study is one step towards more realistic models of stellar evolution.

It suggests that many stars in which magnetic fields are observed—particularly larger red giants—may in fact have gained mass from another star. To reliably determine magnetic fields inside stars, researchers should therefore also take a star’s life history into account.