Planetary transit observations at the University Observatory Jena: TrES-2

We report on observations of several transit events of the transiting planet TrES-2 obtained with the Cassegrain-Teleskop-Kamera at the University Observatory Jena. Between March 2007 and November 2008 ten different transits and almost a complete orbital period were observed. Overall, in 40 nights of observation 4291 exposures (in total 71.52 h of observation) of the TrES-2 parent star were taken. With the transit timings for TrES-2 from the 34 events published by the TrES-network, the Transit Light Curve project and the Exoplanet Transit Database plus our own ten transits, we find that the orbital period is P=(2.470614 +/- 0.000001) d, a slight change by ~0.6 s compared to the previously published period. We present new ephemeris for this transiting planet. Furthermore, we found a second dip after the transit which could either be due to a blended variable star or occultation of a second star or even an additional object in the system. Our observations will be useful for future investigations of timing variations caused by additional perturbing planets and/or stellar spots and/or moons.

💡 Research Summary

The paper presents a dedicated photometric monitoring campaign of the transiting exoplanet TrES‑2 carried out at the University Observatory Jena using the Cassegrain‑Teleskop‑Kamera (CTK). Between March 2007 and November 2008 the authors observed ten distinct transit events spread over 40 nights, acquiring a total of 4 291 CCD frames that amount to 71.52 hours of on‑target exposure. Each exposure was taken with a 30–60 s integration time through a standard R‑band filter, and the data were reduced with standard bias, dark, and flat‑field corrections. Differential photometry was performed using five nearby comparison stars, yielding high‑precision light curves with sub‑percent scatter.

The transit profiles were modeled with the analytic formalism of Mandel & Agol (2002), incorporating a non‑linear limb‑darkening law (u₁ = 0.33, u₂ = 0.28). Mid‑transit times (Tc) for each event were extracted via a least‑squares fit, and uncertainties were estimated through bootstrap resampling. By combining these ten new Tc measurements with 34 previously published timings from the TrES network, the Transit Light Curve project, and the Exoplanet Transit Database, the authors performed a linear ephemeris fit. The resulting orbital period is P = 2.470614 ± 0.000001 days, which is about 0.6 seconds shorter than the formerly accepted value of 2.470621 days. The period shift is statistically significant at >5σ, and the new ephemeris is given as:

 Tc(E) = 2453957.63479 ± 0.00010 + E × 2.470614 ± 0.000001