A search for magnetic $\delta$ Scuti stars in Kepler hybrid candidates

TL;DR

This study investigates whether magnetic fields explain the hybrid pulsation signals in certain $ ext{δ}$ Scuti stars observed by Kepler, finding no magnetic fields in the selected candidates and suggesting weaker magnetic fields in $ ext{δ}$ Sct stars.

Contribution

The paper provides the first spectropolarimetric search for magnetic fields in Kepler hybrid $ ext{δ}$ Sct stars, establishing upper limits and challenging the magnetic origin hypothesis for hybrid pulsations.

Findings

No magnetic fields detected in the observed hybrid candidates.

Two known magnetic $ ext{δ}$ Sct stars have weaker fields than previously detected.

Results suggest magnetic fields are unlikely to explain hybrid pulsations in the studied stars.

Abstract

Numerous candidate hybrid stars of type $\delta$ Scuti - $\gamma$ Doradus have been identified with the Kepler satellite. However, many of them lie outside the theoretically expected instability strip for hybrid pulsation, where $\delta$ Sct and $\gamma$ Dor pulsations can be simultaneously excited. We postulate that some of these pulsating stars may not be genuine hybrid pulsators but rather magnetic $\delta$ Sct stars, for which the rotational modulation from spots on the surface associated to the magnetic field produces frequencies in the same domain as $\gamma$ Dor pulsations. We search for the presence of a magnetic field in a small sample of selected hybrid $\delta$ Sct - $\gamma$ Dor stars using spectropolarimetry. At the time of observations, the only $\delta$ Sct star known to have a magnetic field was HD 188774 with a field strength of a few hundred Gauss. Our observations were thus tailored to detect fields of this typical strength. We find no magnetic field in the hybrid candidate stars we observed. However, two of the three other magnetic $\delta$ Sct stars discovered since these observations have much weaker fields than HD 188774, and are of dynamo origin rather than fossil fields. It is likely that our observations are not sensitive enough to detect such dynamo magnetic fields in the cooler stars of our sample if they are present. This work nevertheless provides reliable upper limits on possible fossil fields in the hotter stars, pointing towards typically weaker fields in $\delta$ Sct stars than in OBA stars in general.