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 sub-stellar 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 sub-stellar companions with a spin measurement .
The GQ Lupi system was observed at high spectral resolution ( R \sim \num 100000 ) , 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 \ce CO ( S/N=11.6 ) and \ce 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 } \si \km \per \s .
The slow rotation is most likely due to its young age of < \SI { 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 \SI 2.0 ± 0.4 \km \per \s , and discuss the allowed orbital configurations and their implications for formation scenarios for GQ Lupi b .