WISE J005128.08+645651.7: An enigmatic R Coronae Borealis star

Abstract

Context. R Coronae Borealis (RCB) stars are hydrogen-deficient and carbon-rich (HdC) supergiants that likely originate from a merger of two white dwarfs. As such they are not expected to be members of close binary systems, in agreement with current observations. Aims. We test the above hypothesis and search for eclipses of RCB stars that could be due to a companion. Methods. We exploited old archival photometric data from Harvard photographic plates (DASCH) as well as modern-era photometric surveys such as Gaia, ASAS-SN, and ZTF to search for eclipses, and conducted follow-up spectroscopic observations. Results. We found one RCB star (WISE J005128.08+645651.7) that features two 3.5 yr long dimming events. We argue that these may be eclipses separated by an orbital period of 66 yr and that the star is an eclipsing binary candidate. The next eclipse is predicted for 2080. If confirmed, this would be the first RCB star in a binary system, the second-longest orbital period of any known eclipsing binary, and the longest duration of an eclipse. Light curve simulations support the idea that the eclipses could be caused by a dusty disk around the companion. The star shows unusually large pulsations. Our follow-up spectroscopy indicates a stellar temperature of about 11000 K, the presence of sharp emission lines (including forbidden lines) during the normal brightness, and extremely strong diffuse interstellar bands. The star is likely surrounded by a hotter chromosphere and an extended low-density nebula. We calculated atmosphere models of the star. Models feature a density inversion and the derived chemical composition is compatible with some cool extreme He stars (EHe). The star constitutes a bridge between the classical RCB, hot RCB, and EHe stars. Conclusions. Future research is encouraged to confirm or refute the binary nature of the star and search for other RCB binaries.

recently published in A&A

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