RX Gru: a short-period pre-main-sequence eclipsing binary with a distant circumbinary companion

Abstract: We report the discovery of a new short-period pre-main-sequence eclipsing binary, RX Gru, orbited by a distant circumbinary companion. We characterized the system by analysing the photometric observations from the Solaris network, the Transiting Exoplanet Survey Satellite, and the Super Wide Angle Search for Planets survey, combined with the radial velocities from four high-resolution spectrographs: HARPS, FEROS, CHIRON, and HRS. We derived the parameters of the eclipsing components, which are $M_{\rm Aa} = 1.004<sup>{+0.027}<em>{-0.026}\,$M$</em>\odot$, $R_{\rm Aa} = 1.007\pm0.021\,$R$<em>\odot$, and $T</em>{\rm eff,Aa} = 5379\pm289\,$K for the primary, and $M_{\rm Ab} = 0.985<sup>{+0.024}<em>{-0.025}\,$M$</em>\odot$, $R_{\rm Ab} = 1.024\pm0.023\,$R$<em>\odot$, and $T</em>{\rm eff,Ab} = 5322\pm278\,$K for the secondary. We determined the age of the system from the observed parameters using two evolution codes, MESA and Cesam2k20. We obtained an age of $\sim$28$\,$Myr, placing the two stars at the very end of the pre-main-sequence phase. We also derived the minimum mass and orbital period of the tertiary companion, which are found to be $M_{\rm B} = 89.0\pm3.5\,$M$<em>{\rm Jup}$ and $$P</em>{\rm AB} = 23.79 ^{+0.10 }_{-0.25}\,$$yr, respectively. We conclude that RX Gru consists of a tight inner binary composed of two twin components and an outer low-mass companion (a massive brown dwarf or a very low-mass star) in a relatively wide orbit, and we suggest that the system was formed via the dynamical unfolding mechanism coupled with the shared accretion of the circumbinary material by the binary components.