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

TL;DR

This study demonstrates that long secondary periods in red giant stars are caused by binarity, specifically a dusty cloud orbiting with a substellar companion, often a former planet now a brown dwarf, obscuring the star periodically.

Contribution

It provides the first strong evidence linking LSPs to binarity and dusty clouds, explaining the phenomenon through infrared secondary eclipses in a large stellar sample.

Findings

Approximately half of the studied LSP variables show secondary eclipses.

LSPs are caused by dusty clouds orbiting with a substellar companion.

The companion is likely a former planet that has grown into a brown dwarf.

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.