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

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 $a=1.26\pm0.05 \text{ au}$ ($P=134.779\pm0.025 \text{ days}$) and thereby confirming it as a close binary. The low orbital inclination $i=19.7\pm1.9^{\circ}$ 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 $e=0.492\pm0.003$ will cause the system to undergo eccentric Roche lobe overflow once the primary expands to $R\simeq50 R_{\odot}$, 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.

Figures (9)

• Figure 1: VLTI/PIONIER data (colored) and best fit binary model (solid black) for the Nunki data in epoch 2017-08-27. The dashed lines show the expected values for a single unresolved star.
• Figure 2: Minimum $\chi^2$ (sum of squared weighted residuals) over the ($P$,$e$,$T_p$) grid as a function of orbital period.
• Figure 3: Astrometric data (blue) labeled by MJD and best fit orbit (black) for Nunki = Sigma Sagittarii. The dashed magenta and orange lines show the line of apsides and the line of nodes respectively.
• Figure 4: Spectral atlas of Nunki. The FEROS spectrum is shown in green and the HERMES spectrum in blue. The red line shows a BSTAR2006 model with $T_{\mathrm{eff}}=19 \text{ kK}$, $\log g=4.0$ and $v \sin i = 160 \text{ km}\text{ s}^{-1}$.
• Figure 5: Continuation of Figure \ref{['fig:plot_spectrum_1']}. Starting at about 5900Å there are strong telluric features.