The incidence of magnetism in blue and yellow straggler stars

Context. Our understanding of the generation of magnetic fields in intermediate-mass and massive OBA stars remains limited. Some theories have proposed that their magnetic fields could be a result of strong binary interactions, including stellar mergers. Blue straggler stars, which lie well beyond the main sequence turn-off point on the colour-magnitude diagram of stellar clusters, are widely theorised to be merger products or interacting binaries and therefore can be considered excellent test targets to get insights into the origin of magnetic fields in stars with radiative envelopes.Aims. We search for the presence of magnetic fields in a sample of blue and yellow straggler stars listed in the Gaia DR2-based catalogue of blue straggler stars in open clusters.Methods. We measured the mean longitudinal magnetic field from high-resolution HARPSpol spectra of five blue straggler and three yellow straggler stars using the least-squares deconvolution technique.Results. We present the first observational evidence that blue straggler and yellow straggler stars possess magnetic fields of the order of a hundred to a few hundred Gauss. The targets in our sample belong to open clusters of very different ages and metallicities, but we do not detect any relationship between the presence or strength of the detected magnetic field and the cluster characteristics. For the first time, using high-resolution spectropolarimetric observations, a definite detection of a magnetic field is achieved in a Be-shell star, HD 61954. The two yellow straggler stars, HD 62329 and HD 65032, appear to be members in binary systems, whereas the blue straggler star HD 62775 is possibly a triple system. HD 62329 and HD 65032 exhibit in their spectra weak Nd III 6145 lines, which are usually prominent in magnetic Ap and Bp stars. Our observations provide crucial information necessary for testing predictions of existing theories and place strong constraints on the origin of magnetic fields in stars with radiative envelopes.