Rossiter--McLaughlin models and their effect on estimates of stellar rotation, illustrated using six WASP systems

We present new measurements of the projected spin--orbit angle $lambda$ for six WASP hot Jupiters, four of which are new to the literature (WASP-61, -62, -76, and -78), and two of which are new analyses of previously measured systems using new data (WASP-71, and -79). We use three different models based on two different techniques: radial velocity measurements of the Rossiter--McLaughlin effect, and Doppler tomography. Our comparison of the different models reveals that they produce projected stellar rotation velocities ($v sin I_{rm s}$) measurements often in disagreement with each other and with estimates obtained from spectral line broadening. The Boue model for the Rossiter--McLaughlin effect consistently underestimates the value of $vsin I_{rm s}$ compared to the Hirano model. Although $v sin I_s$ differed, the effect on $lambda$ was small for our sample, with all three methods producing values in agreement with each other. Using Doppler tomography, we find that WASP-61,b ($lambda=4^circ.0^{+17.1}_{-18.4}$), WASP-71,b ($lambda=-1^circ.9^{+7.1}_{-7.5}$), and WASP-78,b ($lambda=-6^circ.4pm5.9$) are aligned. WASP-62,b ($lambda=19^circ.4^{+5.1}_{-4.9}$) is found to be slightly misaligned, while WASP-79,b ($lambda=-95^circ.2^{+0.9}_{-1.0}$) is confirmed to be strongly misaligned and has a retrograde orbit. We explore a range of possibilities for the orbit of WASP-76,b, finding that the orbit is likely to be strongly misaligned in the positive $lambda$ direction.