Abstract
ABSTRACT GG Carinae (GG Car) is a binary whose primary component is a B[e] supergiant. Using photometric data from the Transiting Exoplanet Survey Satellite (TESS), All Sky Automated Survey (ASAS), Optical Monitoring Camera (OMC), and All Sky Automated Survey for Supernovae (ASAS-SN), and spectroscopic data from the Global Jet Watch to study visible He i, Fe ii, and Si ii emission lines, we investigate the short-period variations that are exhibited in GG Car. We find a hitherto neglected periodicity of 1.583156 ± 0.0002 d that is present in both its photometry and the radial velocities of its emission lines, alongside variability at the well-established ∼31-d orbital period. We find that the amplitudes of the shorter period variations in both photometry and some of the emission lines are modulated by the orbital phase of the binary, such that the short-period variations have largest amplitudes when the binary is at periastron. There are no significant changes in the phases of the short-period variations over the orbital period. We investigate potential causes of the 1.583-d variability, and find that the observed period agrees well with the expected period of the l = 2 f-mode of the primary given its mass and radius. We propose that the primary is periodically pulled out of hydrostatic equilibrium by the quadrupolar tidal forces when the components are near periastron in the binary’s eccentric orbit (e = 0.5) and the primary almost fills its Roche lobe. This causes an oscillation at the l = 2 f-mode frequency that is damped as the distance between the components increases.
Highlights
B[e] supergiants (B[e]SGs) are a class of rare stars which are not predicted by any stellar evolution models
We have shown that the B[e]SG binary GG Car is significantly variable in both photometry and spectroscopy at 1.583156±0.0002 days, and we have studied this variability in detail for the first time
We have shown that the short-period variability cannot be caused by the rotation of the B[e]SG primary, the presence of a hidden third body, or intrinsic variability of the secondary’s flux
Summary
B[e] supergiants (B[e]SGs) are a class of rare stars which are not predicted by any stellar evolution models They are characterized by hybrid spectra of hot stars with infrared excess, strong emission in Hydrogen Balmer and Helium lines, strong permitted and forbidden emission lines from a number of elements, wide absorption lines in the ultraviolet (UV) spectrum, and significant infrared excesses. The structure of this paper is as follows: Section 2 introduces the V-band and TESS photometry, and the Global Jet Watch spectroscopy of GG Car; Section 3 studies the variability of the system’s photometry and emission lines’ radial velocities; Section 3.3 investigates the relationship between the amplitude of the short-period variability in the system and the orbital phase of the binary; Section 4 presents our discussions; and Section 5 presents our conclusions. This led them to state that one of the GG Car components is a variable, but no further analysis was undertaken and this periodicity has been neglected since that publication. Krtičková & Krtička (2018) were unable to determine a clear UV lightcurve of GG Car over the orbital period, presumably due to variability in the system; they conclude that the binary component that is brightest in the UV
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