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

I. Soszyński, A. Olechowska, M. Ratajczak, P. Iwanek, D. M. Skowron, P. Mróz, P. Pietrukowicz, A. Udalski, M. K. Szymański, J. Skowron, M. Gromadzki, R. Poleski, S. Kozłowski, M. Wrona, K. Ulaczyk, and K. Rybicki
The Astrophysical Journal Letters, 911, L22 (arXiv:2103.12748)

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.

Optical (blue points) and infrared (orange and red points) light curves of three LSP variables.

Light curves of three LSP stars from the Large Magellanic Cloud. Top panels show optical I-band observations from the OGLE survey, middle and bottom panels show infrared W1 (3.4 μm) and W2 (4.6 μm) light curves from the NEOWISE-R archive.

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.

Artistic impression of a red giant star obscured by a dusty cloud surrounding a low-mass companion. Author: Matylda Soszyńska.

PLEASE cite the following paper when using the data or referring to these OGLE results:
Soszyński et al., 2021, The Astrophysical Journal Letters, 911, L22 (arXiv:2103.12748)

Any comments about the data and the form of their presentation are welcome as they can improve the future releases of OGLE analysis. Send your messages to this address.

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