MOBSTER -- IV. Detection of a new magnetic B-type star from follow-up spectropolarimetric observations of photometrically selected candidates

Abstract

In this paper, we present results from the spectropolarimetric follow-up of photometrically selected candidate magnetic B stars from the MOBSTER project. Out of four observed targets, one (HD 38170) is found to host a detectable surface magnetic field, with a maximum longitudinal field measurement of 105$\pm$14 G. This star is chemically peculiar and classified as an ${\alpha }^2$ CVn variable. Its detection validates the use of TESS to perform a photometric selection of magnetic candidates. Furthermore, upper limits on the strength of a putative dipolar magnetic field are derived for the remaining three stars, and we report the discovery of a previously unknown spectroscopic binary system, HD 25709. Finally, we use our non-detections as case studies to further inform the criteria to be used for the selection of a larger sample of stars to be followed up using high-resolution spectropolarimetry.

9 pages, 6 figures, 4 tables, accepted by MNRAS (2021 March 23)

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• Testing the fossil field hypothesis: could strongly magnetised OB stars produce all known magnetars?
• A spectroscopic test of the rotational modulation origin of periodic \emph{Kepler} photometric variability of A-type stars
• Analysis of eight magnetic chemically peculiar stars with rotational modulation
• Spectropolarimetry of Magnetic Chemically Peculiar Stars in the Orion OB1 Association