Search for magnetic fields in seven slowly rotating A stars

Abstract: A small fraction of A-type stars host strong fossil magnetic fields. Identifying them relies on indirect indicators such as chemical peculiarities, slow rotation, and rotational modulation, which are also shared by non-magnetic stars. Assessing their reliability is essential for future surveys. We magnetically characterise seven slowly rotating, chemically peculiar A-type stars selected through two complementary strategies. Three were selected from abundance analysis combined with potential intrinsically slow rotation, and four from a spectral depression at 5200 Angstrom combined with rotational modulation in TESS photometry. We obtained high-resolution spectropolarimetry, applied least squares deconvolution, and measured longitudinal field strengths for detections. For non-detections, we derived upper limits on polar field strength and evaluated the critical fields of Zahn and Spruit. We detect magnetic fields in all four stars selected via the spectral depression and photometric criterion, with longitudinal field strengths of approximately 145-2900 G, while none of the three stars selected via abundance and slow rotation shows a Zeeman signature. Combined upper limits for two non-detections lie below or near the magnetic desert boundary, but only one is also below the critical fields required for rigid rotation. The non-detections are most naturally interpreted as Am stars. Among the detections, HD 63843 stands out as a magnetic $δ$ Scuti pulsator, promising for magneto-asteroseismology. In conclusion, slow rotation combined with chemical peculiarity alone does not reliably trace large scale magnetic fields, whereas adding the 5200 Angstrom depression and stable rotational modulation in space photometry appears to select magnetic A-type stars with high efficiency, providing a practical guideline for future spectropolarimetric surveys.