Confirming Nunki as the closest core collapse progenitor candidate to the Sun

We have recently suggested that Nunki=Sigma Sagittarii is the closest core collapse progenitor candidate to the Sun based on a VLTI/GRAVITY observation that unveiled it as a $6.5+6.3M_{\odot }$ binary at a projected separation of 0.60 au. Here we combine this observation with three VLTI/PIONIER archival and one previous MAPPIT observation to solve for the orbit of $\mathit{\text{Nunki}}$, finding $a=1.26\pm 0.05\text{ au}$ ($P=134.779\pm 0.025\text{ days}$) and thereby confirming it as a close binary. The low orbital inclination $i=19.7\pm {1.9}^{\circ }$ coupled with the high projected rotational velocity $v\sin i\simeq 160\text{ km}{\text{ s}}^{-1}$ and the absence of a decretion disk are a strong hint for spin-orbit misalignment. The significant eccentricity $e=0.492\pm 0.003$ will cause the system to undergo eccentric Roche lobe overflow once the primary expands to $R\simeq 50R_{\odot }$, so that a merger into a $M\gtrsim 10M_{\odot }$ star is a possible outcome. Therefore, we conclude that $\mathit{\text{Nunki}}$ at a distance $d\approx 69\text{ pc}$ can indeed be considered the closest core collapse progenitor candidate to the Sun as it is closer than $\mathit{\text{Spica}}$ and $\mathit{\text{Bellatrix}}$ both at $d\approx 77\text{ pc}$. Furthermore, we also report on a VLTI/GRAVITY observation of $\mathit{\text{Bellatrix}}$ that shows that it does not have any close companion with a K band flux ratio higher than 1%; in particular, it is not a close equal mass binary as previously suspected. Two archival spectra of $\mathit{\text{Nunki}}$ illustrate how equal-mass binaries with rapidly rotating components can easily hide to become virtually spectroscopically undetectable when the radial velocity separation is several times smaller than the individual line widths.