Upgrading Alpha Crucis to a seven star system. Discovery of Bb and orbital misalignment
Idel Waisberg, Boaz Katz
arXiv:2603.11194v1 Announce Type: new
Abstract: 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 $M_{Aa}=17.2pm1.2 M_{odot}$ and $M_{Ab}=6.8pm0.3 M_{odot}$. 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 $137pm5^{circ}$, 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 $Msimeq52 M_{odot}$. 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.arXiv:2603.11194v1 Announce Type: new
Abstract: 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 $M_{Aa}=17.2pm1.2 M_{odot}$ and $M_{Ab}=6.8pm0.3 M_{odot}$. 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 $137pm5^{circ}$, 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 $Msimeq52 M_{odot}$. 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.