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 lambda = 30 degrees +/- 21 degrees. This is the third case for which, has been measured in a transiting exoplanetary system, and the first demonstration that such measurements are possible for relatively faint host stars (V similar to 12, as compared to V similar to 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 (less than or similar to 14 M-circle plus) at the 3 sigma level.