A conclusive non-detection of magnetic field in the Am star

Context. The A-type metallic-line (Am) stars are typically considered to be non-magnetic or to possess very weak sub-G magnetic fields. This view has been repeatedly challenged in the literature; most commonly for the bright hot Am star o Peg. Several studies claim to have detected 1–2 kG field of unknown topology in this object, possibly indicating a new process of magnetic-field generation in intermediate-mass stars. Aims. In this study, we revisit the evidence of a strong magnetic field in o Peg using new high-resolution spectropolarimetric observations and advanced spectral fitting techniques. Methods. We estimated the mean magnetic field strength in o Peg from the high-precision CRyogenic InfraRed Echelle Spectrograph (CRIRES+) measurement of near-infrared (NIR) sulphur lines. We modelled this observation with a polarised radiative transfer code, including treatment of the departures from local thermodynamic equilibrium. In addition, we used the least-squares deconvolution multi-line technique to derive longitudinal field measurements from archival optical spectropolarimetric observations of this star. Results. Our analysis of the NIR S <ns0:sc>I</ns0:sc> lines reveals no evidence of Zeeman broadening, ruling out magnetic field with a strength exceeding 260 G. This null result is compatible with the relative intensification of Fe <ns0:sc>II</ns0:sc> lines in the optical spectrum, taking into account blending and uncertain atomic parameters of the relevant diagnostic transitions. Longitudinal field measurements on three different nights also yield null results with a precision of 2 G. Conclusions. This study refutes the claims of kG-strength dipolar or tangled magnetic field in o Peg. This star therefore appears to be non-magnetic, with surface magnetic field characteristics no different from those of other Am stars.