Science

Astronomers Investigate Stellar Interiors – Sky & Telescope

Using data from NASA’s Transiting Exoplanet Survey Satellite (TESS), astronomers have untangled the constantly changing brightness of 60 variable stars. In the process, they have found the means of establishing these stars’ masses, internal structures, ages, and other fundamental properties.

The study, published May 14th in Nature, is the first of its kind for this abundant type of star, known as Delta Scuti stars. According to TESS principal investigator and study coauthor George Ricker (MIT), “The findings . . . have opened up entirely new horizons for better understanding a whole class of stars.”

This artist’s concept of a Delta Scuti star shows a slightly oblate star, due to its rotation. The light curve at bottom shows its brightness fluctuations. The fluctuations look regular at first glance, but a closer look will reveal higher-frequency variations that have been difficult to identify until now.
Chris Boshuizen / Simon Murphy / Tim Bedding

From Starquakes to Interiors

Just as scientists can study Earth’s interior by looking at the propagation of earthquakes, astronomers can probe the interior of a star by precisely identifying and measuring the often-complicated pulsations and vibrational patterns on its surface — a technique known as asteroseismology.

Puzzling this out isn’t easy — in many cases there’s a plethora of possible oscillations. For example, acoustic waves could course through a star in response to changes in gas pressure. Or changes in buoyancy could drive gravity waves. Some oscillations are radial, where the star expands and contracts as though breathing. Others are nonradial, with some parts of the surface moving up as other parts move down.

“In the case of Delta Scuti stars, we didn’t know which oscillations were present,” says Saskia Hekker (Max Planck Institute for Solar System Research, Germany), who was not involved in the new study. “This is essential information if you want to do asteroseismology.”

In particular, the identification of nonradial oscillations yields knowledge about the deep stellar interior, adds Conny Aerts (KU Leuven, Belgium). “That information is needed if you want to determine stellar ages,” she says.

Deciphering Delta Scuti Stars

Delta Scuti stars are young A- or F-type stars that are roughly double the mass of the Sun and much brighter.  Well-known examples (apart from the prototype, Delta Scuti) include Altair, Denebola, and Beta Pictoris. Their brightness typically changes by a few percent over periods of a few hours.

Like Cepheid variable stars, Delta Scuti stars swell and shrink on a regular, well-studied cycle — these dominant pulsations even have their own period-luminosity law. But they also exhibit many additional brightness variations, and these have been more difficult to understand.

An international team led by Timothy Bedding (University of Sydney) has now studied these smaller, higher-frequency variations for some 1,000 Delta Scuti stars using TESS’s precise brightness measurements. In 60 cases, the stars rotated slowly enough that researchers could identify regular patterns and their underlying oscillation modes. Similar results had been achieved before from Kepler data for lower-mass stars like our Sun, but not for ones as complicated as Delta Scuti stars.

According to Hekker, most Delta Scuti stars are older and therefore faster-spinning, so they exhibit much more complicated brightness variations, making it difficult if not impossible to identify the underlying oscillation modes.

Still, the new result is important because many young Delta Scuti stars are members of so-called moving groups — stars that share a common origin and motion through the Milky Way galaxy. If asteroseismologists succeed in precisely determining their ages and other fundamental properties, “we might learn more about the evolution of the Milky Way,” write József Benkö and Margit Paparó (Konkoly Observatory, Hungary) in an opinion piece that accompanied the study in Nature.

“We are currently in the renaissance of stellar astrophysics, because asteroseismology has turned the study of stellar interiors into an observational science,” Aerts says. “Stay tuned!”

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