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DH Tau b is a young planetary-mass companion orbiting at a projectedseparation of 320 AU from its $\sim$2 Myr old host star DH Tau. With anestimated mass of $8-22$ $M_{\rm{Jup}}$ this object straddles thedeuterium-burning limit, and might have formed via core or pebble accretion,disk instability, or molecular cloud fragmentation. To shed light on theformation history of DH Tau b, we obtain the first measurement of rotationalline broadening for this object using high-resolution (R $\sim$25,000)near-infrared spectroscopy from Keck/NIRSPEC. We measure a projected rotationalvelocity ($v$sin$i$) of $9.6\pm0.7$ km/s, corresponding to a rotation rate thatis between 9-15% of DH Tau b's predicted break-up speed. This low rotation rateis in good agreement with scenarios in which magnetic coupling between thecompanion and its circumplanetary disk during the late stages of accretionreduces angular momentum and regulates spin. We compare the rotation rate of DHTau b to published values for other planetary-mass objects with masses between$0.3-20$ $M_{\rm{Jup}}$ and find no evidence of a correlation between mass androtation rate in this mass regime. Finally, we search for evidence ofindividual molecules in DH Tau b's spectrum and find that it is dominated by COand H$_2$O, with no evidence for the presence of CH$_4$. This agrees withexpectations given DH Tau b's relatively high effective temperature ($\sim$2300K).

A Rotation Rate for the Planetary-Mass Companion DH Tau b