Figure 1: Artist’s impression of an exoplanet in a binary star system.
WASP-94A is a F8V star with 1.29 ± 0.10 times the mass and 1.36 ± 0.13 times the radius of the Sun, and its effective surface temperature is 6170 ± 80 K. It hosts a transiting hot-Jupiter, identified as WASP-94Ab. WASP-94Ab has 0.445 ± 0.026 times the mass and 1.72 ± 0.06 times the radius of Jupiter, and its orbital period is 3.95 days. The mass of WASP-94Ab was determined using the radial velocity method which measures how much the planet’s host star wobbles due to gravitational perturbations from the planet itself. Additionally, the Rossiter-McLaughlin effect is clearly observable each time WASP-94Ab transits its host star.
The Rossiter-McLaughlin effect occurs when a planet transits across the face of its host star. Since the star is rotating, half of its observable hemisphere will be rotating towards the observer (i.e. approaching quadrant) and the other half of its observable hemisphere will be rotating away from the observer (i.e. receding quadrant). Light from the star is blue-shifted on the approaching quadrant and red-shifted on the receding quadrant. Since the approaching and receding quadrants are symmetrical, a net redshift is generated when the planet is in front of the approaching quadrant and a net blueshift is generated when the planet is in front of the receding quadrant.
A net redshift to blueshift change indicates the planet is in a prograde orbit (i.e. planet orbits in the same direction as the star’s spin); while a net blueshift to redshift change indicates the planet is in a retrograde orbit (i.e. planet orbits in the opposite direction to the star’s spin). Measuring the Rossiter-McLaughlin effect allows the spin-orbit angle (i.e. angle of the planet’s orbital plane with respect to the spin axis of its host star) of WASP-94Ab to be determined and the measurements indicate that WASP-94Ab is in a retrograde orbit.
WASP-94B is a F9V star with 1.24 ± 0.09 times the mass and 1.35 ± 0.12 times the radius of the Sun, and its effective surface temperature is 6040 ± 90 K. It hosts a non-transiting hot-Jupiter identified as WASP-94Bb. WASP-94Bb was detected using the radial velocity method. The amplitude of the radial velocity measurements indicates that WASP-94Bb has a mass of at least 0.617 ± 0.028 times the mass of Jupiter, and the periodicity of the radial velocity measurements show that the orbital period of WASP-94Bb is 2.008 days.
Figure 2: Top: radial velocity measurements indicating the presence of WASP-94Ab. Middle: residuals of the best-fit curve to the radial velocity measurements. Bottom: zoom-in on the radial velocities measurements taken during the transit over-plotted with the best solution for the Rossiter-McLaughlin effect. M. Neveu-VanMalle et al. (2014)
Figure 3: Top: radial velocity measurements indicating the presence of WASP-94Bb. Bottom: residuals of the best-fit curve to the radial velocity measurements. M. Neveu-VanMalle et al. (2014)
M. Neveu-VanMalle et al. (2014), “WASP-94 A and B planets: hot-Jupiter cousins in a twin-star system”, arXiv:1409.7566 [astro-ph.EP]