Query : 2016A&A...593L..15P

2016A&A...593L..15P - Astronomy and Astrophysics, volume 593, L15-15 (2016/9-1)

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


Abstract (from CDS):

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 1.7M without planet engulfment and with a time-averaged rotation velocity during the main sequence equal to 100km/s shows a surface magnetic field triggered by convection that is stronger than 10G only at the base of the red giant branch, that is, for gravities logg>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 15MJ planet typically produces a dynamo-triggered magnetic field stronger than 10G 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.

Abstract Copyright: © ESO, 2016

Journal keyword(s): planet-star interactions - stars: magnetic field - stars: rotation

CDS comments: Tyc 6054-01204-1 not in SIMBAD.

Simbad objects: 6

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Number of rows : 6
N Identifier Otype ICRS (J2000)
ICRS (J2000)
Mag U Mag B Mag V Mag R Mag I Sp type #ref
1850 - 2022
1 V* OP And RS* 01 36 27.2005869192 +48 43 22.000439676   7.47 6.22     K1III: 97 0
2 HD 232862 ** 03 57 19.967736 +50 51 18.56412   10.47 9.58 9.36   K2V 27 0
3 BD-17 992 RB? 04 56 02.4519335208 -17 08 23.896205484   10.71 9.51     K7 6 0
4 HD 31994 * 04 59 49.4194416624 -08 58 04.581528312   9.80 9.25     F3/5IV 7 0
5 V* V1192 Ori RS* 05 00 08.2195942104 +03 17 12.024920724   8.75 7.503   6.223 K1/2III 82 0
6 TYC 347-762-1 RB? 15 07 42.1301935008 +05 53 01.469388804   10.97 9.77     ~ 5 0

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