High surface magnetic field in red giants as a new signature of planet engulfment?

¹ Geneva Observatory, University of Geneva, Ch. Maillettes 51, 1290 Sauverny, Switzerland
e-mail: giovanni.privitera@unige.ch
² Istituto Ricerche Solari Locarno, Via Patocchi, 6605 Locarno-Monti, Switzerland
³ School of Physics, Trinity College Dublin, The University of Dublin, Ireland
⁴ Department of Theoretical Physics, Universidad Autónoma de Madrid, Módulo 8, 28049 Madrid, Spain
⁵ Penn State Worthington Scranton, Dunmore, PA 18512, USA

Received: 17 June 2016
Accepted: 31 August 2016

Abstract

Context. Red giant stars may engulf planets. This may increase the rotation rate of their convective envelope, which could lead to strong dynamo-triggered magnetic fields.

Aims. We explore the possibility of generating magnetic fields in red giants that have gone through the process of a planet engulfment. We compare them with similar models that evolve without any planets. We discuss the impact of magnetic braking through stellar wind on the evolution of the surface velocity of the parent star.

Methods. By studying rotating stellar models with and without planets and an empirical relation between the Rossby number and the surface magnetic field, we deduced the evolution of the surface magnetic field along the red giant branch. The effects of stellar wind magnetic braking were explored using a relation deduced from magnetohydrodynamics simulations.

Results. The stellar evolution model of a red giant with 1.7 M_⊙ without planet engulfment and with a time-averaged rotation velocity during the main sequence equal to 100 km s^-1 shows a surface magnetic field triggered by convection that is stronger than 10 G only at the base of the red giant branch, that is, for gravities log  g> 3. When a planet engulfment occurs, this magnetic field can also appear at much lower gravities, that is, at much higher luminosities along the red giant branch. The engulfment of a 15 M_J planet typically produces a dynamo-triggered magnetic field stronger than 10 G for gravities between 2.5 and 1.9. We show that for reasonable magnetic braking laws for the wind, the high surface velocity reached after a planet engulfment may be maintained sufficiently long to be observable.

Conclusions. High surface magnetic fields for red giants in the upper part of the red giant branch are a strong indication of a planet engulfment or of an interaction with a companion. Our theory can be tested by observing fast-rotating red giants such as HD 31994, Tyc 0347-00762-1, Tyc 5904-00513-1, and Tyc 6054-01204-1 and by determining whether they show magnetic fields.