Signature of spin-down stalling in stellar magnetic activity

The case of the open cluster NGC 6811

¹ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, PT4150-762 Porto, Portugal
² Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre 687, PT4169-007 Porto, Portugal
³ Instituto de Astrofísica de Canarias (IAC), E-38205 La Laguna, Tenerife, Spain
⁴ Universidad de La Laguna (ULL), Departamento de Astrofísica, E-38206 La Laguna, Tenerife, Spain
⁵ INAF – Osservatorio Astrofisico di Catania, Via S. Sofia, 78, 95123 Catania, Italy
⁶ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France

^⋆ Corresponding author: Angela.Santos@astro.up.pt

Received: 4 February 2025
Accepted: 21 March 2025

Abstract

Context. Stellar rotation and magnetic activity have a complex evolution that reveals multiple regimes. One of the related transitions that is seen in the rotation distribution for main-sequence (MS) solar-like stars has been attributed to core-envelope coupling and the consequent angular-momentum transfer between a fast core and a slow envelope. This feature is known as spin-down stalling and is related to the intermediate-rotation gap seen in field stars.

Aims. Beyond this rotation signature, we search for evidence of it in stellar magnetic activity.

Methods. We investigated the magnetic activity of the 1 Gyr old NGC 6811, a Kepler-field cluster, and Kepler MS stars of different ages. The magnetic activity was measured through the photometric magnetic activity proxy, S​_ph. To characterize the evolution of the magnetic activity for the Kepler sample, we split it according to the relative rotation and computed the respective activity sequences.

Results. We found the signature of core-envelope coupling in the magnetic activity of NGC 6811 and in the Kepler MS sample. In NGC 6811, we found enhanced magnetic activity for a range of effective temperatures that remained for significant timescales. In the Kepler sample, the magnetic activity sequences pile up in two distinct regions: (1) at high activity levels that coincide with stars near the stalling mentioned above, where a behavior inversion is observed (slowly rotating stars have higher activity levels than fast-rotating stars, which is opposite to the overall behavior); and (2) at low activity levels corresponding to slow rotators close to the detection limit, potentially facing a weakening of the magnetic braking.

Conclusions. These results support the recent proposition that the strong shear experienced by stars during the core-envelope coupling phase can cause enhanced activity. This study helps us to shed light on the interplay between rotation, magnetic activity, and their evolution.