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

Confirming Nunki as the closest core collapse progenitor candidate to the Sun
Idel Waisberg, Boaz Katz
arXiv:2603.17011v1 Announce Type: new
Abstract: 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 $6.5+6.3 M_{odot}$ 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 textit{Nunki}, finding $a=1.26pm0.05 text{ au}$ ($P=134.779pm0.025 text{ days}$) and thereby confirming it as a close binary. The low orbital inclination $i=19.7pm1.9^{circ}$ 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 $e=0.492pm0.003$ will cause the system to undergo eccentric Roche lobe overflow once the primary expands to $Rsimeq50 R_{odot}$, so that a merger into a $M gtrsim 10 M_{odot}$ star is a possible outcome. Therefore, we conclude that textit{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 textit{Spica} and textit{Bellatrix} both at $d approx 77 text{ pc}$. Furthermore, we also report on a VLTI/GRAVITY observation of textit{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 textit{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.arXiv:2603.17011v1 Announce Type: new
Abstract: 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 $6.5+6.3 M_{odot}$ 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 textit{Nunki}, finding $a=1.26pm0.05 text{ au}$ ($P=134.779pm0.025 text{ days}$) and thereby confirming it as a close binary. The low orbital inclination $i=19.7pm1.9^{circ}$ 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 $e=0.492pm0.003$ will cause the system to undergo eccentric Roche lobe overflow once the primary expands to $Rsimeq50 R_{odot}$, so that a merger into a $M gtrsim 10 M_{odot}$ star is a possible outcome. Therefore, we conclude that textit{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 textit{Spica} and textit{Bellatrix} both at $d approx 77 text{ pc}$. Furthermore, we also report on a VLTI/GRAVITY observation of textit{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 textit{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.