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Solving the Mystery of Varying Red Giants

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illustration of a large red star surrounded by spherical shells of mass
Artist's illustration of a red giant star expelling mass at the end of its life. A new study addresses the mystery of why many evolved stars exhibit periodic variation on long timescales. [JAXA]

Scientists have long puzzled over slow and regular variations in the brightness of many evolved giant stars. Now, clues from newly analyzed infrared observations may finally have solved this mystery.

Unexplained Changes

Series of illustrations of a star at different stages of life, from main sequence to red giant to planetary nebula.

Artist’s illustration of the life stages of a star like the Sun, which eventually will age into a cool red giant star and then eject its outer layers to form a planetary nebula. [ESO/S. Steinhöfel]

As stars like our Sun age, they inflate to hundreds of times their main-sequence size, engulfing the orbits of their inner planets and becoming luminous red giants. In the final stages of their lives, they eventually become variable stars, exhibiting changes in their brightnesses that can be caused by intrinsic pulsations, motions of convection cells in their stellar envelopes, and even the presence of circumstellar dust.

Most of this variability of red giants is reasonably well understood, but there’s one type that has remained mysterious: so-called long secondary periods.

Common Evolution

In addition to showing ordinary variation from pulsations, long-secondary-period red giants show regular dips in their optical light curves that occur on timescales an order of magnitude longer than the pulsations — typically several months to several years.

Evolved stars with long secondary periods are surprisingly common: at least a third of luminous asymptotic giant branch stars and supergiants show these long-period variations. Yet despite their ubiquity, long secondary periods have remained unexplained for decades. Are these variations intrinsic to the aging star? Or are they caused by some external factor?

Three light curves for a sample long-secondary-period variable star.

An example set of light curves from a long-secondary-period variable star shows the primary eclipses in all three light curves. Secondary eclipses appear only in the two infrared bands (W1 and W2, the orange and red data), not in the optical band (I, the blue data). [Adapted from Soszyński et al. 2021]

Now, new research from a team of scientists led by Igor Soszyński (University of Warsaw, Poland) has used infrared observations to identify the likely culprit: dust-shrouded binary companions.

Optical vs. Infrared

Soszyński and collaborators gathered optical observations of a sample of 16,000 known long-secondary-period variable stars in the Milky Way and the nearby Magellanic Clouds. For roughly 700 of these stars, the authors then obtained corresponding light curves at two infrared wavebands from the NEOWISE-R mission.

A striking feature was immediately evident when comparing the optical and infrared observations of these variables: where the optical light curves showed a single long-period dip in brightness, the infrared light curves of about half of the stars also showed a second dip that appeared exactly out of phase with the primary dip.

Former Planet, Now Dusty Shade

What does this mean? Soszyński and collaborators argue that these second dips confirm that the long-period variability is caused by eclipses from a binary companion.

Illustration of an orbiting planet trailing a cloud of dust.

Artist’s illustration of an orbiting planet trailing a comet-like tail of dust. A dust-shrouded brown-dwarf companion is the best explanation for long-period variability seen in the light curves of many evolved stars. [Maciej Szyszko]

In the authors’ explanation, a close-in planet accumulates mass from the expanding envelope of its red-giant host, eventually growing to brown-dwarf size. As this substellar companion — shrouded in a comet-like, extended cloud of dust — passes between us and the red giant, we observe long-period eclipses in the star’s optical and infrared light. When the cloud and companion then pass behind the star, their primarily infrared emission briefly vanishes, causing the secondary eclipse observed at infrared wavelengths only.

This solution to the decades-old mystery of long secondary period variability in red giants opens a door to new discoveries. By studying the shape of the eclipses in the variable stars’ light curves, there’s plenty more we can now hope to learn about how stars like the Sun evolve alongside their planets.

Citation

“Binarity as the Origin of Long Secondary Periods in Red Giant Stars,” I. Soszyński et al 2021 ApJL 911 L22. doi:10.3847/2041-8213/abf3c9