Upgrading Alpha Crucis to a seven star system. Discovery of Bb and orbital misalignment

Alpha Crucis is the closest very high multiplicity massive star to the Sun. At its heart is the $4" \leftrightarrow 430 \text{ au}$ binary $\alpha^1$ (A) + $\alpha^2$ (B) Cru, which combined make up the 13th visually brightest star in the night sky. Here we make use of archival VLTI data of $\alpha$ Cru A+B in order to study its multiplicity and orbital architecture. The data spatially resolved the close (6 mas) companion in $\alpha$ Cru A (a known spectroscopic binary) and revealed that $\alpha$ Cru B is also a close (17 mas) binary, which upgrades $\alpha$ Cru to a seven star system. By combining the interferometric data with radial velocities, we solve for the full orbit of Aa+Ab and find dynamical masses $M_{Aa}=17.2\pm1.2 M_{\odot}$ and $M_{Ab}=6.8\pm0.3 M_{\odot}$. While the data on Alpha Cru B are not yet sufficient to tightly constrain all orbital parameters, we find that the orbital period is most likely 405 days (with 203 days also a possibility). The orientation of the orbital planes are sufficiently constrained to yield a mutual inclination between Aa+Ab and Ba+Bb of either $50 \pm 5^{\circ}$ or $137\pm5^{\circ}$, pointing to a dynamical formation scenario for the system. The photometric masses $M_{Ba}=12.4 M_{\odot}$ and $M_{Bb}=9.8 M_{\odot}$ together with the less massive wide component $\alpha$ Cru Ca+Cb+D yield a total mass $M\simeq52 M_{\odot}$. At larger distances, the seven-star nature of Alpha Crucis would be arguably very challenging to unveil, suggesting that the companion frequency in massive star surveys may be underestimated.