2026/03/11 by Idel Waisberg, Boaz Katz · 2 voices
Physics and Astronomy · #astro-ph.SR #astro-ph.HE
Alpha Crucis is the closest very high multiplicity massive star to the Sun. At its heart is the 4" ↔ 430 au binary α1 (A) + α2 (B) Cru, which combined make up the 13th visually brightest star in the night sky. Here we make use of archival VLTI data of α Cru A+B in order to study its multiplicity and orbital architecture. The data spatially resolved the close (6 mas) companion in α Cru A (a known spectroscopic binary) and revealed that α Cru B is also a close (17 mas) binary, which upgrades α 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 MAa=17.2±1.2 M\odot and MAb=6.8±0.3 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 ± 5∘ or 137±5∘, pointing to a dynamical formation scenario for the system. The photometric masses MBa=12.4 M\odot and MBb=9.8 M\odot together with the less massive wide component α Cru Ca+Cb+D yield a total mass M≃52 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.