A TENTATIVE DETECTION OF A STARSPOT DURING CONSECUTIVE TRANSITS OF AN EXTRASOLAR PLANET FROM THE GROUND: NO EVIDENCE OF A DOUBLE TRANSITING PLANET SYSTEM AROUND TrES-1

There have been numerous reports of anomalies during transits of the planet TrES-1b. Recently, Rabus and coworkers' analysis of Hubble Space Telescope observations lead them to claim brightening anomalies during transit might be caused by either a second transiting planet or a cool starspot. Observations of two consecutive transits are presented here from the University of Arizona's 61 inch Kuiper Telescope on 2008 May 12 and May 15 UT. A 5.4 ± 1.7 mmag (0.54% ± 0.17%) brightening anomaly was detected during the first half of the transit on May 12 and again in the second half of the transit on May 15. We argue that the significance of these spot events are 3.2 and 2.9 σ for May 12 and May 15, respectively, and we estimate that each of these have a probability ⩾90% of not being systematic red-noise peaks. Therefore, we conclude that this is a tentative detection of a r ⩾6R⊕ starspot rotating on the surface of the star. We suggest that all evidence to date suggest TrES-1 has a spotty surface, and there is no need to introduce a second transiting planet in this system to explain these anomalies. Assuming that the spin axis of the star and orbital axis of the planet are aligned (and in the plane of the sky) suggests a stellar rotational period of 40.2 ± 0.1 days. Introducing the λ = 30° ± 21° inclination of the stellar spin axis with respect to the planetary orbital axis of Narita et al. adds much more uncertainty, and we are only able to constrain the rotational period of the star to 40.2+22.9−14.6 days, which is consistent with the previously observed Pobs = 33.2+22.3−14.3 day period. We note that this technique could be applied to other transiting systems for which starspots exist on the star in the transit path of the planet in order to constrain the rotation rate of the star.