First discovery of a magnetic field in a main sequence delta Scuti star: the Kepler star HD188774

TL;DR

This study reports the first detection of a magnetic field in a main sequence delta Scuti star, HD 188774, revealing it is not a hybrid pulsator but exhibits magnetic surface spots affecting its light and velocity curves.

Contribution

The paper presents the first evidence of magnetism in a main sequence delta Scuti star, challenging previous classifications of hybrid pulsators based on Kepler data.

Findings

Detected magnetic field in HD 188774 using spectropolarimetry.

Low frequencies are likely caused by rotational modulation of magnetic surface spots.

Reclassification of HD 188774 as a magnetic delta Scuti star rather than a hybrid.

Abstract

The Kepler space mission provided a wealth of {\delta} Sct-{\gamma} Dor hybrid candidates. While some may be genuine hybrids, others might be misclassified due to the presence of a binary companion or to rotational modulation caused by magnetism and related surface inhomogeneities. In particular, the Kepler {\delta} Sct-{\gamma} Dor hybrid candidate HD 188774 shows a few low frequencies in its light and radial velocity curves, whose origin is unclear. In this work, we check for the presence of a magnetic field in HD 188774. We obtained two spectropolarimetric measurements with ESPaDOnS at CFHT. The data were analysed with the least squares deconvolution method. We detected a clear magnetic signature in the Stokes V LSD profiles. The origin of the low frequencies detected in HD 188774 is therefore most probably the rotational modulation of surface spots possibly related to the presence of a magnetic field. Consequently, HD 188774 is not a genuine hybrid {\delta} Sct-{\gamma} Dor star, but the first known magnetic main sequence {\delta} Sct star. This makes it a prime target for future asteroseismic and spot modelling. This result casts new light on the interpretation of the Kepler results for other {\delta} Sct-{\gamma} Dor hybrid candidates.