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

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 $α^1$ (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.

Figures (11)

• Figure 1: I: Alpha Crucis highlighted in the Brazilian flag. II: Smartphone photo of the Southern Cross as viewed from Socorro, SP, Brazil. III: 2MASS H band image of $\alpha$ Cru. IV: VLTI/GRAVITY acquisition camera H band image of $\alpha$ Cru A+B. V/VI: VLTI/GRAVITY interferometric data resolving $\alpha$ Cru Aa+Ab and Ba+Bb.
• Figure 2: Updated multiplicity diagram for the septuple system Alpha Crucis. The main quadruple A+B is the focus of this paper. The Ba+Bb orbit refers to the most likely period solution (see text for details).
• Figure 3: Spectral atlas of Alpha Crucis A. The red line shows a BSTAR2006 model with $T_{\mathrm{eff}}=29 \text{ kK}$, $\log g=4$ and $v \sin i = 84 \text{ km}\text{ s}^{-1}$.
• Figure 4: Continuation of Figure \ref{['fig:plot_spectrum_1']}. Starting at about 5900Å there are strong telluric features.
• Figure 5: Data and best joint fit for the orbit of Alpha Cru Aa+Ab. Top: VLTI/GRAVITY and VLTI/PIONIER astrometric data (blue, labeled by MJD) and best fit orbital solution (red). The black star marks the position of the fixed primary. The dashed magenta line shows the line of apsides and the dashed orange line shows the line of nodes. Bottom: Radial velocity data (blue, green) and best fit orbit (red) for the primary Aa. The dashed black line shows the systemic velocity.