Measuring the Rotation Rate of the Planetary-Mass Companion DH Tau b

DH Tau b is a planetary-mass companion orbiting the T Tauri star DH Tau (~0.5 solar masses) with a projected separation of 340 AU. With an age of ~2 Myrs, it is one of the youngest planetary-mass companions discovered to date. DH Tau b straddles the deuterium-burning limit with an estimated mass of 8-22 Jupiter masses, and like many directly imaged companions it is unclear whether it formed like a star (i.e., via turbulent fragmentation) or like a planet (i.e., either disk instability or core accretion). To shed light on DH Tau b's formation history, we obtain the first-ever measurement of rotational line broadening for this object using high-resolution (R ~25,000) near-infrared spectroscopy from Keck/NIRSPEC. We then compare DH Tau b's rotation rate with previously established trends in mass and age for both bound and free-floating planetary-mass objects, and find that DH Tau b reinforces existing trends in rotation rates from this population.