Obliquity Constraints on the Planetary-mass Companion HD 106906 b. (arXiv:2108.13437v1 [astro-ph.EP])

Obliquity Constraints on the Planetary-mass Companion HD 106906 b. (arXiv:2108.13437v1 [astro-ph.EP])
<a href="http://arxiv.org/find/astro-ph/1/au:+Bryan_M/0/1/0/all/0/1">Marta L. Bryan</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Chiang_E/0/1/0/all/0/1">Eugene Chiang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Morley_C/0/1/0/all/0/1">Caroline V. Morley</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Mace_G/0/1/0/all/0/1">Gregory N. Mace</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bowler_B/0/1/0/all/0/1">Brendan P. Bowler</a>

We constrain the angular momentum architecture of HD 106906, a 13 $pm$ 2 Myr
old system in the ScoCen complex composed of a compact central binary, a widely
separated planetary-mass tertiary HD 106906 b, and a debris disk nested between
the binary and tertiary orbital planes. We measure the orientations of three
vectors: the companion spin axis, companion orbit normal, and disk normal.
Using near-IR high-resolution spectra from Gemini/IGRINS, we obtain a projected
rotational velocity of $vsin{i_p}$ = 9.5 $pm$ 0.2 km/s for HD 106906 b. This
measurement together with a published photometric rotation period implies the
companion is viewed nearly pole-on, with a line-of-sight spin axis inclination
of $i_p$ = 14 $pm$ 4 degrees or 166 $pm$ 4 degrees. By contrast, the debris
disk is known to be viewed nearly edge-on. The likely misalignment of all three
vectors suggests HD 106906 b formed by gravitational instability in a turbulent
environment, either in a disk or cloud setting.

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