Discovery of new magnetic δ Scuti stars and impact of magnetism on pulsation excitation

Abstract

Context. At this time, the list of known magnetic δ Scuti stars is extremely limited, with only a handful of well-studied examples. Aims. We seek to expand this list, by retrieving targets from a variety of sources and demonstrating that they present simultaneously a surface magnetic field signature and δ Scuti pulsations. Methods. We obtained archival and new spectropolarimetric datasets for a variety of known δ Scuti stars and analysed them using the Least Squares Deconvolution method to generate mean Stokes I and V profiles for each target, from which we can determine longitudinal magnetic field measurements. Additionally, we assessed photometric data from the TESS satellite to discern frequency peaks consistent with δ Scuti pulsations in known magnetic stars, and to identify magnetic candidates via rotational modulation. Results. We present a compiled list of all the confirmed magnetic δ Scuti stars discovered to date, containing 13 stars. The majority of this sample lies outside the usual δ Scuti instability strip in the H-R diagram, though we do not observe any specific correlations between magnetic field strength and various stellar parameters. This indicates that strong global magnetic fields play a fundamental role in shaping interior structure and processes. Magnetic fields thus must be included in realistic stellar models in order to more accurately predict structure and evolution. Conclusions. This work constitutes the largest database to date of strongly magnetic δ Scuti stars, one that will continue to grow over time with subsequent studies.

Figures (12)

• Figure 1: Calculated $B_l$ values with respect to phase, using the newly determined rotation frequency $f_{\rm rot} = 0.178085$ d$^{-1}$ for HD 41641. Green dashed line represents the dipolar field model fit to the $B_l$ values.
• Figure 2: Representation of the 13 confirmed magnetic $\delta$ Sct stars known to date, categorised by magnetic field characterisation (colour) and field strength (symbol size). Orange lines correspond to cesam2k20 evolutionary tracks (Manchon et al. 2025 subm.) for the mass range typical of $\delta$ Scuti stars with no above core overshooting ($\alpha = 0$) and no magnetism included. Blue lines are the same, except with overshooting included following the Claret2016 prescription ($\alpha = \alpha_{CT}$). The approximate start and end of the main sequence are represented with green and magenta dashed lines respectively for the two types of tracks. The red and blue dashed lines correspond to the observationally constrained red- and blue-edge of the $\delta$ Sct instability region. Black points are non-magnetic reference stars from murphy2019 classified as $\delta$ Sct stars.
• Figure 3: Variation of the 2 $M_\odot$ star cesam2k20 models presented in Fig. \ref{['fig:HRD']} based on a variety of criteria, including: i) metallicity, ii) above core convective overshooting, iii) mean surface rotation velocity during the MS, and iv) radiative acceleration (see Sect. \ref{['sec:params']} for more details). For the metallicity case, we have elected to also include the 1.9 $M_\odot$ star cesam2k20 model as comparison.
• Figure 4: Comparison plots between the polar magnetic field strength and other stellar parameters for the stars in our sample.
• Figure 5: Lomb-Scargle periodograms for the stars in our sample, arranged in descending order with respect to the calculated polar field strength. Note that the rotational modulation has been removed to emphasise the pulsational signals.