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 λ 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_s) measurements often in disagreement with each other and with estimates obtained from spectral line broadening. The Boué model for the Rossiter-McLaughlin effect consistently underestimates the value of v sin I_s compared to the Hirano model. Although v sin I_s differed, the effect on λ 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 (λ =4.0°^{+17.1}_{-18.4}), WASP-71 b (λ =-1.9°^{+7.1}_{-7.5}), and WASP-78 b (λ = -6.4° ± 5.9) are aligned. WASP-62 b (λ =19.4°^{+5.1}_{ -4.9}) is found to be slightly misaligned, while WASP-79 b (λ =-95.2°^{+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 λ direction.