Leaning Sideways: VHS 1256-1257 b is a Super-Jupiter with a Uranus-like Obliquity

Abstract

We constrain the angular momentum architecture of VHS J125601.92-125723.9, a 140 $\pm$ 20 Myr old hierarchical triple system composed of a low-mass binary and a widely-separated planetary-mass companion VHS 1256 b. VHS 1256 b has been a prime target for multiple characterization efforts, revealing the highest measured substellar photometric variability to date and the presence of silicate clouds and disequilibrium chemistry. Here we add a key piece to the characterization of this super-Jupiter on a Tatooine-like orbit; we measure its spin-axis tilt relative to its orbit, i.e. the obliquity of VHS 1256 b. We accomplish this by combining three measurements. We find a projected rotation rate $v\sin i_p=8.7\pm 0.1\mathrm{km}{\mathrm{s}}^{-1}$ for VHS 1256 b using near-IR high-resolution spectra from Gemini/IGRINS. Combining this with a published photometric rotation period indicates that the companion is viewed edge-on, with a line-of-sight spin axis inclination of $i_{\mathrm{p}}=90^{\circ }\pm 18^{\circ }$. We refit available astrometry measurements to confirm an orbital inclination of $i_{\mathrm{o}}=23{\begin{array}{c}+10\\ -13\end{array}}^{\circ }$. Taken together, VHS 1256 b has a large planetary obliquity of $\psi =90^{\circ }\pm 25^{\circ }$. In total, we have three measured angular momentum vectors for the system: the binary orbit normal, companion orbit normal, and companion spin axis. All three are misaligned with respect to each other. Although VHS 1256 b is tilted like Uranus, their origins are distinct. We rule out planet-like scenarios including collisions and spin-orbit resonances, and suggest that top-down formation via core/filament fragmentation is promising.

arXiv commentaccepted to AJ. 16 pages, 9 figures

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