A Plague of Magnetic Spots Among the Hot Stars of Globular Clusters

Yazan Al Momany, Simone Zaggia, Marco Montalto, David Jones, Henri M. J. Boffin, Santino Cassisi, Christian Moni Bidin, Marco Gullieuszik, Ivo Saviane, Lorenzo Monaco, Elena Mason, Leo Girardi, Valentina D'Orazi, Giampaolo Piotto, Antonino P. Milone, Hitesch Lala, Peter B. Stetson, Yuri Beletsky

Published 2020-06-03Version 1

Six decades and counting, the formation of hot ~20,000-30,000 K Extreme Horizontal Branch (EHB) stars in Galactic Globular Clusters remains one of the most elusive quests in stellar evolutionary theory. Here we report on two discoveries shattering their currently alleged stable luminosity. The first EHB variability is periodic and cannot be ascribed to binary evolution nor pulsation. Instead, we here attribute it to the presence of magnetic spots: superficial chemical inhomogeneities whose projected rotation induces the variability. The second EHB variability is aperiodic and manifests itself on time-scales of years. In two cases, the six-year light curves display superflare events a mammoth several million times more energetic than solar analogs. We advocate a scenario where the two spectacular EHB variability phenomena are different manifestations of diffuse, dynamo-generated, weak magnetic fields. Ubiquitous magnetic fields, therefore, force an admittance into the intricate matrix governing the formation of all EHBs, and traverse to their Galactic field counterparts. The bigger picture is one where our conclusions bridge similar variability/magnetism phenomena in all radiative-enveloped stars: young main-sequence stars, old EHBs and defunct white dwarfs.