PREDNASAJUCI / LECTURER :

Ján Budaj

(1) Astronomical Institute, Slovak Academy of Sciences,
059 60 Tatranská Lomnica

NAZOV / TITLE :

An enigmatic RCB star

ABSTRAKT / ABSTRACT :

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. We test the above hypothesis and search for
eclipses of RCB stars that could be due to a companion. We exploit old archival photometric data
from Harvard photographic plates (DASCH) as well as modern era photometric surveys such as Gaia,
ASAS-SN, ZTF, and others to search for eclipses, and conduct follow-up spectroscopic observations.
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 the eclipse. Light curve simulations support the idea that the eclipses
can 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, 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. Future research is
encouraged to confirm or refute the binary nature of the star and search for other RCB binaries.