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

Abstract

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 Is) 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 Is compared to the Hirano model. Although v sin Is 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 ($\lambda =4{^{\circ}_{.}}0^{+17.1}_{\,\,\,-18.4}$), WASP-71 b ($\lambda =-1{^{\circ}_{.}}9^{+7.1}_{\,\,\,-7.5}$), and WASP-78 b (λ = −6 $_{.}^{\circ}$4 ± 5.9) are aligned. WASP-62 b ($\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 λ direction.