Rotational modulation in A and F stars: magnetic stellar spots or convective core rotation?

Abstract

The Kepler mission revealed a plethora of stellar variability in the light curves of many stars, some associated with magnetic activity or stellar oscillations. In this work, we analyse the periodic signal in 162 intermediate-mass stars, interpreted as Rossby modes and rotational modulation - the so-called hump & spike feature. We investigate whether the rotational modulation (spike) is due to stellar spots caused by magnetic fields or due to Overstable Convective (OsC) modes resonantly exciting g modes, with frequencies corresponding to the convective core rotation rate. Assuming that the spikes are created by magnetic spots at the stellar surface, we recover the amplitudes of the magnetic fields, which are in good agreement with theoretical predictions. Our data show a clear anti-correlation between the spike amplitudes and stellar mass and possibly a correlation with stellar age, consistent with the dynamo-generated magnetic fields theory in (sub)-surface convective layers. Investigating the harmonic behaviour, we find that for 125 stars neither of the two possible explanations can be excluded. While our results suggest that the dynamo-generated magnetic field scenario is more likely to explain the spike feature, we assess further work is needed to distinguish between the two scenarios. One method for ruling out one of the two explanations is to directly observe magnetic fields in hump & spike stars. Another would be to impose additional constraints through detailed modelling of our stars, regarding the rotation requirement in the OsC mode scenario or the presence of a convective-core (stellar age).

Kepler main-sequence solar-like stars: surface rotation and magnetic-activity evolution

“…Once processed, Kepler data constrained surface rotation periods and photometric magnetic activity for several tens of thousands of solar-like stars from the main-sequence to the red-giant phase (e.g., García et al, 2014a;McQuillan et al, 2014;Ceillier et al, 2017;Santos et al, 2019;2021). While the focus of this review is the main-sequence (MS) solar-like stars, it is worth noting that magnetic fields and activity are also found in earlier spectral types (e.g., Balona, 2015;Balona, 2019;Mathys, 2017;Henriksen et al, 2023).…”

Pulsation of chemically peculiar stars

“…With an amplitude of up to 200 ppm, observing this variability is reserved for high-precision (space) photometry. However, the interpretation of the corresponding light curves as a manifestation of surface spots is still debatable (Henriksen et al 2023).…”

Unresolved Rossby and gravity modes in 214 A and F stars showing rotational modulation

“…Furthermore, Saio et al (2018a) argued that r modes could be mechanically excited by the disruption of the rotational flow on the stellar surface caused by stellar spots. This phenomenon would give rise to a specific periodic signal, hump and spike, found in hundreds of early-type stars (e.g., Balona 2013;Balona et al 2015;Trust et al 2020;Henriksen et al 2023), where the hump represents unresolved r modes and the spike represents the spots that are co-rotating with the stellar surface. Henriksen et al (2023) showed that the spike, if due to stellar spots, could be translated into an estimate of the strength of the magnetic field, as the spike amplitudes are anti-correlated with the stellar mass, which agrees with the theoretical predictions from Cantiello & Braithwaite (2019).…”

“…This phenomenon would give rise to a specific periodic signal, hump and spike, found in hundreds of early-type stars (e.g., Balona 2013;Balona et al 2015;Trust et al 2020;Henriksen et al 2023), where the hump represents unresolved r modes and the spike represents the spots that are co-rotating with the stellar surface. Henriksen et al (2023) showed that the spike, if due to stellar spots, could be translated into an estimate of the strength of the magnetic field, as the spike amplitudes are anti-correlated with the stellar mass, which agrees with the theoretical predictions from Cantiello & Braithwaite (2019). Furthermore, it appears that the lifetimes of the spike are short, as expected from magnetic features of subsurface dynamo generate fields.…”

“…On the other hand, the rotational modulation that is translated into the spike could also be attributed to overstable convective (OsC) modes that resonantly excite low-frequency g modes in the stellar envelope (Lee & Saio 2020;Lee 2021). While the harmonic signature of around 20 per cent of the hump and spike stars studied in Henriksen et al 2023, indicate a signal consistent with stellar spots, for the larger part of the stars in the sample, the OsC modes cannot be ruled as the cause of spike feature. This calls for additional analysis to understand the phenomenon behind the spike feature, for example, direct magnetic field measurements.…”