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

TL;DR

This study measures the obliquity of VHS 1256-1257 b, revealing a large tilt similar to Uranus, and analyzes the system's angular momentum architecture to infer its formation history.

Contribution

It provides the first measurement of the super-Jupiter's obliquity and demonstrates the misalignment of all angular momentum vectors in the system, suggesting a formation via core/fragmentation.

Findings

VHS 1256-1257 b has a large obliquity of 90° ± 25°.

All three angular momentum vectors are misaligned.

The system's characteristics favor a top-down formation scenario.

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 $v \sin{i_p} = 8.7 \pm 0.1 \,\mathrm{km~s^{-1}}$ 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 $i_{\rm p} = 90^\circ \pm 18^\circ$. We refit available astrometry measurements to confirm an orbital inclination of $i_{\rm o} = 23 \substack{+10 \\ -13}^\circ$. 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.