Evolution of stellar magnetic activity: Probing planet engulfment by red giants

IRAP, Université de Toulouse, CNRS, CNES, UPS, 14 Avenue Edouard Belin, 31400 Toulouse, France

^★ Corresponding author: charlotte.gehan@irap.omp.eu

Received: 27 May 2025
Accepted: 27 October 2025

Abstract

It has been observed that the fraction of low-mass (LM) stars (M ≤ 1.5 M_⊙) showing photospheric activity in their light curve is larger on the horizontal branch (HB) than during the prior, red giant branch (RGB) phase, while the opposite trend has been observed for intermediate-mass (IM) stars (M > 1.5 M_⊙). One hypothesis is that LM red giants (RGs) engulf more planets than IM RGs, which results in a faster surface rotation and a higher magnetic activity. This hypothesis is based on the fact that LM stars reach a maximum radius at the RGB tip that is much larger than that achieved for IM stars, making them more likely to engulf planets. However, we need to study the evolution of the active star fraction along the RGB to firmly check this hypothesis. I used independent indicators tracing the activity level in the chromosphere based on the Ca II H&K, Hα, Mg I, and infrared Ca II spectral lines from LAMOST data for ∼3000 RGs whose evolutionary stage has been determined by asteroseismology with the Kepler mission. I found that the fraction of active stars shows different trends for LM and IM stars along the RGB, decreasing for IM stars, but unexpectedly increasing for LM stars. Such an increase cannot be explained by models of single-star evolution and it is consistent with the fact that LM stars are more likely than IM stars to engulf planets. Indeed, the data show that IM main sequence stars exhibit a dearth of planets, which is consistent with predictions from planet formation theory. In addition, I observe that the fraction of active stars tends to increase for both LM and IM stars on the HB, which stands in partial contrast with previous findings. Finally, I discovered that the IM RGB star KIC 9780154 might have engulfed one or more planet(s) as its surface rotation from photometry is twice faster than its envelope rotation from asteroseismology. Characterizing planet engulfment by RGs provides insights into the evolution and fate of most planetary systems, since ∼97% of stars pass through the RG evolution stage.