The slow spin of the young substellar companion GQ Lupi b and its orbital configuration

The spin of a planet or brown dwarf is related to the accretion process, and therefore studying spin can help promote our understanding of the formation of such objects. We present the projected rotational velocity of the young substellar companion GQ Lupi b, along with its barycentric radial velocity. The directly imaged exoplanet or brown dwarf companion joins a small but growing ensemble of wide-orbit, substellar companions with a spin measurement. The GQ Lupi system was observed at high spectral resolution (R ∼100 000), and in the analysis we made use of both spectral and spatial filtering to separate the signal of the companion from that of the host star. We detect both CO (S/N = 11.6) and H₂O (S/N = 7.7) in the atmosphere of GQ Lupi b by cross-correlating with model spectra, and we find it to be a slow rotator with a projected rotational velocity of 5.3^+0.9_-1.0 km s^-1. The slow rotation is most likely due to its young age of 5 Myr, as it is still in the process of accreting material and angular momentum. We measure the barycentric radial velocity of GQ Lupi b to be 2.0 ± 0.4 km s^-1, and discuss the allowed orbital configurations and their implications for formation scenarios for GQ Lupi b.