Measurement of the Rossiter–McLaughlin Effect in the Transiting Exoplanetary System TrES-1

We report a measurement of the Rossiter–McLaughlin effect in the transiting extrasolar planetary system TrES-1, via simultaneous spectroscopic and photometric observations with the Subaru and MAGNUM telescopes. By modeling the radial velocity anomaly that was observed during a transit, we determine the sky-projected angle between the stellar spin axis and the planetary orbital axis to be λ = 30∘± 21∘. This is the third case for which λ has been measured in a transiting exoplanetary system, and the first demonstration that such measurements are possible for relatively faint host stars (V ~ 12, as compared to V ~ 8 for the other systems). We also derive a time of mid-transit, constraints on the eccentricity of the TrES-1b orbit ($e =$ 0.048±0.025), and upper limits on the mass of the Trojan companions ( 14 M⊕) at the 3σ level.