Magnetic Fields or Overstable Convective Modes in HR 7495: Exploring the Underlying Causes of the Spike in the 'Hump & Spike' Features

TL;DR

This study investigates the cause of the 'hump & spike' feature in HR 7495, providing evidence that stellar spots and magnetic activity, rather than overstable convective modes, are responsible for the observed spectral characteristics.

Contribution

The paper offers observational evidence favoring stellar spots over OsC modes as the cause of the 'hump & spike' feature in HR 7495, including analysis of photometry, spectroscopy, and modeling.

Findings

Light curves align with rotational modulation by spots.

No significant magnetic fields detected, suggesting complex magnetic activity.

Model comparisons disfavor pulsations as the cause of the spike.

Abstract

More than 200 A- and F-type stars observed with Kepler exhibit a distinctive 'hump & spike' feature in their Fourier spectra. The hump is commonly interpreted as unresolved Rossby modes, while the spike has been linked to rotational modulation. Two competing interpretations exist for the spike: magnetic phenomena, such as stellar spots, or Overstable Convective (OsC) modes resonantly exciting low-frequency g modes within the stellar envelope. We analysed photometric data from Kepler and TESS for HR 7495, the brightest 'hump & spike' star (V=5.06), covering 4.5 years and four seasons, respectively. Additionally, radial velocity measurements and spectropolarimetric data were used 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. The phase-folded light curves show that the spike characteristics of HR 7495 align more closely with rotational modulation by stellar spots than with OsC modes. No significant magnetic fields were detected, limiting the field's possible amplitude and geometry. This supports the hypothesis of a subsurface convective layer operating a dynamo, producing low-amplitude, complex magnetic fields. The variability patterns suggest multiple evolving spots. A comparison of contemporaneously observed light and RV data with modelled OsC modes reveals a 0.5 phase offset, strongly disfavouring pulsations as the cause of the spike. 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 & spike' stars, harbour an undetected weak magnetic field, likely driven by a dynamo mechanism.