The circumbinary rings of GG Carinae: indications of disc eccentricity growth in the B[e] supergiant's atomic emission lines

Abstract

B[e] supergiants have unusual circumstellar environments which may include thin, concentric rings displaying atomic line emission. GG Carinae is a B[e] supergiant binary which exhibits such a geometry in its circumbinary environment. We study atomic emission lines arising from GG Carinae's circumbinary disc in FEROS spectra collected between 1998 and 2015. We find that semi-forbidden Fe II] and permitted Ca II emission are formed in the same thin circumbinary ring previously reported to have forbidden [O I] and [Ca II] emission. We find that there are two circumbinary rings orbiting with projected velocities of 84.6 ± 1.0 km s −1 and 27.3 ± 0.6 km s −1 . Deprojecting these velocities from the line-of-sight, and using updated binary masses presented by Porter et al. (2021a), we find that the radii of the circumbinary rings are 2.8 +0.9 −1.1 AU and 27 +9 −10 AU for the inner ring and outer ring respectively. We find evidence of subtle dynamical change in the inner circumbinary ring over the 17 years spanned by the data, manifesting in variability in the ratio of the intensity of the blueshifted peak to the redshifted peak of its emission lines and the central velocity becoming more blueshifted. We perform smoothed-particle hydrodynamic simulations of the system which suggest that these observed changes are consistent with pumping of the eccentricity of a radially thin circumbinary ring by the inner binary. We find a systemic velocity of the GG Carinae system of −23.2 ± 0.4 km s −1 .