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

Gehan, Charlotte

Publisher
Zenodo
DOI
10.5281/zenodo.17243890
Repository
zenodo.org
Language
en

Abstract

It has been observed that the fraction of low-mass (LM) red giants (RGs) showing photospheric activity in their light curve is larger on the horizontal branch (HB) than during the previous red giant branch (RGB) phase, while the opposite trend has been observed for intermediate-mass (IM) stars. One hypothesis is that LM RGs (M ≤ 1.6 M⊙) 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 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. We use independent indicators tracing the activity level in the chromosphere based on LAMOST spectral lines for ~ 3000 RGs whose evolutionary stage has been identified by asteroseismology with the Kepler mission. We find that the fraction of active LM stars decreases along the RGB more weakly compared to that of IM stars. This is not explained by models of single-star evolution and is consistent with the hypothesis that LM stars engulf more planets than IM stars. The studied RGB stars are not evolved enough to reach radii that differ significantly. Instead, our results show that LM stars host, and therefore engulf, more planets than IM stars, as predicted by the theory of planet formation. 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.

Full text

Charlotte Gehan Evolution of stellar magnetic activity: probing planet engulfment by red giants Courtesy: NSF’S NOIRLab Context ★Red giant branch (shell-H burning) versus horizontal branch (core-He burning) → fraction of magnetically active stars (photometric modulation due to active regions co-rotating with the photosphere, Gaulme+ 2020): ○decreases for intermediate-mass stars → expected; ○increases for low-mass stars → potential signature of planet engulfment (e.g. Aurière+ 2015, Privitera+ 2016). 1 6 % 14.1 % 3.2 % 2.9 % 8.5 % 4.9 % 57.4 % 25.5 % Gaulme+ (2020) RGB HB Context ★Red giant branch (shell-H burning) versus horizontal branch (core-He burning) → fraction of magnetically active stars (photometric modulation due to active regions co-rotating with the photosphere, Gaulme+ 2020): ○decreases for intermediate-mass stars → expected; ○increases for low-mass stars → potential signature of planet engulfment (e.g. Aurière+ 2015, Privitera+ 2016). 1 6 % 14.1 % 3.2 % 2.9 % 8.5 % 4.9 % 57.4 % 25.5 % Gaulme+ (2020) RGB HB Decrease Context ★Red giant branch (shell-H burning) versus horizontal branch (core-He burning) → fraction of magnetically active stars (photometric modulation due to active regions co-rotating with the photosphere, Gaulme+ 2020): ○decreases for intermediate-mass stars → expected; ○increases for low-mass stars → potential signature of planet engulfment (e.g. Aurière+ 2015, Privitera+ 2016). 1 6 % 14.1 % 3.2 % 2.9 % 8.5 % 4.9 % 57.4 % 25.5 % Increase Gaulme+ (2020) RGB HB Decrease Context ★Red giant branch (shell-H burning) versus horizontal branch (core-He burning) → fraction of magnetically active stars (photometric modulation due to active regions co-rotating with the photosphere, Gaulme+ 2020): ○decreases for intermediate-mass stars → expected; ○increases for low-mass stars → potential signature of planet engulfment (e.g. Aurière+ 2015, Privitera+ 2016). 1 Mercury Venus Earth Gaulme+ (2020) → Need to check the evolution of the active star fraction along the red giant branch. 6 % 14.1 % 3.2 % 2.9 % 8.5 % 4.9 % 57.4 % 25.5 % Increase Gaulme+ (2020) RGB HB Decrease Context ★Photometric detection of photospheric spots → subject to biases towards large and/or long-living spots (Rackham+ 2018, Basri & Shah 2020). 2 → Need for independent activity indicators, regardless of whether or not spots are detected. Impact of spots lifetime Rackham+ (2018) Impact of spots size Chromospheric magnetic activity ★Spectroscopic S-index → proxy of the strength of surface magnetic fields (Wilson 1978, Duncan+ 1991). ★Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST DR7, e. g. Liu+ 2015) → low resolution (R = 1600) but: ○millions of stellar spectra for cool stars; ○include many red giants observed by Kepler → knowledge of their evolutionary status (RGB/HB) through asteroseismology. 3 Non thermal emission CaII H&K lines Gehan+ (2022, 2024) (e-) Properties of stellar evolution 4 Gaulme+ (2020) Studied sample Properties of stellar evolution ★Stellar radius → increases at same rate along the RGB for different masses. ★ Mass loss → negligible (< 2% on the part of the RGB considered). 4 → We expect the surface rotation and hence magnetic activity to evolve similarly for stars with different masses, in the absence of planet engulfment. (Paxton+ 2011, 2013, 2015, 2018, 2019; Jermyn+ 2023) Gaulme+ (2020) Studied sample Red giant branch Evolution Gehan (submitted) Discussion ★Tidal interactions with a stellar companion? ○single stars → not identified as binaries by Gaia (DR3). ○fraction of multiple systems → increases with stellar mass (e. g. Whitworth & Lomax 2015, Moe & Di Stefano 2017). 8 Moe & Di Stefano (2017) 3 Single Binary Triple Quadruple → Not compatible with the observational indication that low-mass red giants engulf more companions. Conclusions 9 ★Evolution of the active star fraction along the red giant branch → compatible with the hypothesis that low-mass stars engulf more planets than intermediate-mass stars. ★Evolution of the active star fraction from the red giant branch to the horizontal branch → not necessarily inconsistent with more planets engulfed by low-mass stars since mass loss comes into play. ★Prospects → more than 10 000 red giants expected with PLATO-SCV program. Additional slides Context ∼ 400 subgiant & red giant host stars ∼ 7400 planet host stars (data from the Extrasolar Planets Encyclopaedia on May 26, 2025) Known planet-host stars Gaulme+ (2020) Red giant branch Horizontal branch Properties of stellar evolution Properties of stellar evolution Discussion Studied sample Other activity indicators Other activity indicators MgI Gehan+ (2024)