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.