Detection of extragalactic magnetic massive stars

Abstract

Studies of the magnetic characteristics of massive stars have recently received significant attention because they are progenitors of highly magnetised compact objects. Stars initially more massive than about 8M_sun leave behind neutron stars and black holes by the end of their evolution. The merging of binary compact remnant systems produces astrophysical transients detectable by gravitational wave observatories. Studies of magnetic fields in massive stars with low metallicities are of particular interest because they provide important information on the role of magnetic fields in the star formation of the early Universe. While several detections of massive Galactic magnetic stars have been reported in the last few decades, the impact of a low-metallicity environment on the occurrence and strength of stellar magnetic fields has not yet been explored. Because of the similarity between Of?p stars in the Magellanic Clouds (MCs) and Galactic magnetic Of?p stars, which possess globally organised magnetic fields, we searched for magnetic fields in Of?p stars in the MCs. Additionally, we observed the massive contact binary Cl* NGC 346 SSN7 in the Small Magellanic Cloud to test the theoretical scenario that the origin of magnetic fields involves a merger event or a common envelope evolution. We obtained and analysed measurements of the magnetic field in four massive Of?p stars in the MCs and the binary Cl* NGC 346 SSN7 using the ESO/VLT FORS2 spectrograph in spectropolarimetric mode. We detected kilogauss-scale magnetic fields in two Of?p-type stars and in the contact binary Cl* NGC 346 SSN7. These results suggest that the impact of low metallicity on the occurrence and strength of magnetic fields in massive stars is low. However, because the explored stellar sample is very small, additional observations of massive stars in the MCs are necessary.

7 pages, 2 tables, 8 figures, published as a letter in A&A

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