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Published April 20, 2021 | Published
Journal Article Open

Binarity as the Origin of Long Secondary Periods in Red Giant Stars

Abstract

Long secondary periods (LSPs), observed in a third of pulsating red giant stars, are the only unexplained type of large-amplitude stellar variability known at this time. Here we show that this phenomenon is a manifestation of a substellar or stellar companion orbiting the red giant star. Our investigation is based on a sample of about 16,000 well-defined LSP variables detected in the long-term OGLE photometric database of the Milky Way and Magellanic Clouds, combined with the mid-infrared data extracted from the NEOWISE-R archive. From this collection, we selected about 700 objects with stable, large-amplitude, well-sampled infrared light curves and found that about half of them exhibit secondary eclipses, thus presenting an important piece of evidence that the physical mechanism responsible for LSPs is binarity. Namely, the LSP light changes are due to the presence of a dusty cloud orbiting the red giant together with the companion and obscuring the star once per orbit. The secondary eclipses, visible only in the infrared wavelength, occur when the cloud is hidden behind the giant. In this scenario, the low-mass companion is a former planet that has accreted a significant amount of mass from the envelope of its host star and grown into a brown dwarf.

Additional Information

© 2021. The American Astronomical Society. Received 2021 February 24; revised 2021 March 23; accepted 2021 March 31; published 2021 April 16. We thank the anonymous referee for constructive comments that helped us to improve the manuscript. We are grateful to Andrzej Niedzielski for helpful suggestions that improved the paper. This work has been supported by the National Science Centre, Poland, grant MAESTRO No. 2016/22/A/ST9/00009. The OGLE project has received funding from the Polish National Science Centre grant MAESTRO No. 2014/14/A/ST9/00121. This publication makes use of data products from the Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE), which is a joint project of the Jet Propulsion Laboratory/California Institute of Technology and the University of Arizona. NEOWISE is funded by the National Aeronautics and Space Administration.

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October 3, 2023
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