Magnetic fields or overstable convective modes in HR 7495

Exploring the underlying causes of the spike in the ‘hump and spike’ features^⋆

¹ DTU Space, Technical University of Denmark, Elektrovej 327, Kgs., Lyngby 2800, Denmark
² Center for Computational Astrophysics, Flatiron Institute, 162 5th Avenue, New York, NY 10010, USA
³ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
⁴ Département de Physique et d’Astronomie, Université de Moncton, Moncton, NB E1A 3E9, Canada
⁵ Astronomical Institute, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
⁶ Sydney Informatics Hub, Core Research Facilities, University of Sydney, NSW, 2006, Australia

^⋆⋆ Corresponding author; antoci@space.dtu.dk

Received: 7 May 2024
Accepted: 16 February 2025

Abstract

Context. More than 200 A- and F-type stars observed with Kepler exhibit a distinctive hump and spike feature in their Fourier spectra. The hump is commonly interpreted as unresolved Rossby modes, while the spike has been linked to rotational modulation. This feature has led these stars to be referred to as ‘hump and spike’ stars. However, two competing interpretations exist for the spike.

Aims. This study aims to determine whether the observed spike in these stars is caused by magnetic phenomena, such as spots on the stellar surface, or by overstable convective (OsC) modes resonantly exciting low-frequency g modes within the star’s envelope.

Methods. We analysed photometric data from Kepler and TESS covering 4.5 years and four seasons, respectively, for HR 7495, which is the brightest hump and spike star with a visual magnitude of 5.06. Additionally, we incorporated radial velocity measurements and spectropolarimetric data from three different epochs in order to investigate magnetic fields and surface features. Furthermore, we analysed model-based artificial light and radial velocity curves to examine the influence of OsC modes on the phase-folded light curves.

Results. Our analysis of the phase-folded light curves indicates that the spike characteristics of HR 7495 align more closely with surface rotational modulation by stellar spots than with OsC modes. No significant magnetic fields were detected, which constrains the possible amplitude and geometry of the field. This is consistent with the hypothesis of a subsurface convective layer operating a dynamo, resulting in low-amplitude magnetic fields with potentially complex geometries. The variability patterns suggest multiple evolving spots. Comparing the observed light and radial velocity data with modelled OsC modes revealed a phase offset of 0.5, thus strongly disfavouring pulsations as the cause of the spike.

Conclusions. While the evolutionary stage of HR 7495 does not entirely preclude the possibility of OsC modes, the observational data overwhelmingly support the stellar spots hypothesis. Our analysis, combined with previous literature, suggests that if not all A- and F-type, at least the hump and spike stars harbour an undetected weak magnetic field that is likely driven by a dynamo mechanism.