Upgrading Alpha Crucis to a seven star system. Discovery of Bb and orbital misalignment
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| Formato: | Preprint |
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2026
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| _version_ | 1866910049625964544 |
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| author | Waisberg, Idel Katz, Boaz |
| author_facet | Waisberg, Idel Katz, Boaz |
| contents | Alpha Crucis is the closest very high multiplicity massive star to the Sun. At its heart is the $4" \leftrightarrow 430 \text{ au}$ binary $α^1$ (A) + $α^2$ (B) Cru, which combined make up the 13th visually brightest star in the night sky. Here we make use of archival VLTI data of $α$ 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 $M_{Aa}=17.2\pm1.2 M_{\odot}$ 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 $α$ 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. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2603_11194 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | Upgrading Alpha Crucis to a seven star system. Discovery of Bb and orbital misalignment Waisberg, Idel Katz, Boaz Solar and Stellar Astrophysics High Energy Astrophysical Phenomena Alpha Crucis is the closest very high multiplicity massive star to the Sun. At its heart is the $4" \leftrightarrow 430 \text{ au}$ binary $α^1$ (A) + $α^2$ (B) Cru, which combined make up the 13th visually brightest star in the night sky. Here we make use of archival VLTI data of $α$ 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 $M_{Aa}=17.2\pm1.2 M_{\odot}$ 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 $α$ 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. |
| title | Upgrading Alpha Crucis to a seven star system. Discovery of Bb and orbital misalignment |
| topic | Solar and Stellar Astrophysics High Energy Astrophysical Phenomena |
| url | https://arxiv.org/abs/2603.11194 |