Transit observations at the observatory in Grossschwabhausen: XO-1b and TrES-1
M. Vanko, St. Raetz, M. Mugrauer, T.O.B. Schmidt, T. Roell, T. Eisenbeiss, M. Hohle, A. Seifahrt, A. Koeltzsch, Ch. Broeg, J. Koppenhoefer, R. Neuhaeuser
aa r X i v : . [ a s t r o - ph . E P ] J a n Title of your IAU SymposiumProceedings IAU Symposium No. 253, 2008A.C. Editor, B.D. Editor & C.E. Editor, eds. c (cid:13) Transit observation at the observatory inGroßschwabhausen: XO-1b and TrES-1
M. Vaˇnko , St. Raetz , M. Mugrauer , T.O.B. Schmidt , T. Roell ,T. Eisenbeiss , M. Hohle , A. Seifahrt , , A. Koeltzsch , Ch. Broeg , ,J. Koppenhoefer and R. Neuh¨auser Astrophysikalisches Institute und Universit¨ats-Sternwarte,Schillerg¨aßchen 2-3, 07745 Jena, Germanyemail: [email protected] Institut f¨ur Astrophysik, Georg-August-Universit¨at,Friedrich-Hund-Platz 1, 37077, G¨ottingen, Germany Space Research and Planetary Sciences, Physikalishes Institute, University of Bern,Sidlerstraße 5, 3012 Bern, Switzerland Max-Planck Institute of Extraterrestrial Physics,Giessenbachstraße, 85748 Garching, Germany
Abstract.
We report on observations of transit events of the transiting planets XO-1b and TrES-1 with the AIU Jena telescope in Großschwabhausen. Based on our ( IR ) photometry (in March2007) and available transit timings (SuperWASP, XO and TLC-project-data) we improved theorbital period of XO-1b ( P = 3.941497 ± P = 3.0300737 ± Keywords. binaries: eclipsing, planetary systems, stars: individual (GSC 02652-01324,GSC 02041 - 01657), techniques: photometric
1. Introduction
The transiting planets are giving us an opportunity to obtain important informationabout both the planet and the star. With precise measurements of transit events it ispossible to infer the relative size of the star and planet, the orbital inclination and thestellar limb-darkening function. Having spectroscopic measurements of the time-variableDoppler shift of the star and an estimate of the stellar mass we can determine planetarymass and the stellar radius.In this paper we present observations of two known transiting planets, XO-1b and TrES-1. The observations were carried out with the AIU Jena telescope at the observatoryin Großschwabhausen on March 2007. We have used the 25cm Cassegrain telescope in-stalled at the tube of the 0.9m telescope. The R and I -band images were obtained withthe Cassegrain-Telescope-Kamera (CTK) a CCD-camera with 37.7’x37.7’ field of viewand 1024x1024 pixels. (Mugrauer et al. (2008), in preperation).For both systems, XO-1b and TrES-1, we have observed one transit event in I -bandand R -band, respectively. Obtained data were reducted by standard IRAF procedures.For data analysis we have used IRAF task chphot , written by Ch. Broeg and based onthe standard IRAF routine phot (Broeg et al. 2005). The IRAF task chphot includesan algorithm which uses as many stars as possible and calculates one artificial compar-ison star (hereafter CS). This process is based on weighted averages of CSs, where analgorithm selects the most stable stars and computes the one artificial CS with the bestS/N ration. Finally, we have corrected data on systematic effects using the ”Sys-Rem”11920 M. Vaˇnkodetrending algorithm which is proposed by Tamuz et al. (2005) and implemented byJohannes Koppenhoefer. The main aim of our investigation is to improve orbital periodsof these transiting planets as well as to discuss possible changes of orbital periods basedon their O-C diagrams. For this purpose we collected all available transit times for eachtransiting system using SuperWASP, XO and TLC-project data.
2. XO-1b
We have observed the transit of the exoplanet XO-1b on March 11th 2007. Accordingto the ephemeris provided by McCullough et al. (2006): T c ( E ) = (2453808 . E · . d, (2.1)this transit corresponds to epoch 92. During observation we have performed 161 I -bandexposures. Our mean photometric accuracy is 0.008 mag. Due to the variable observa-tional conditions, the photometric accuracy at the beginning of the night was a bit worsethen at the end. For data analysis we have used aperture photometry on all availableimages. To get, as good as possible transiting light curve, we have used only the moststable CSs in the field. After the first run of the artificial-comparison-star-algorithm wehave rejected 168 CSs. After repetition of the algorithm we have rejected other faint stars Figure 1. ( left plot ) Relative I -band photometry of XO-1b. ( right plot ) Transit timing residualsfor XO-1b. The dashed line shows the ephemeris given by McCullough et al. (2006). The bestfit (dotted line) is representing by the updated ephemeris. with low S/N and stars suspected of variability. With the remaining 28 most stable starswe calculated the artificial CS. In the resulting light curves we have used Sys-Rem. Thealgorithm works without any prior knowledge of the effects. The number of effects thatshould be removed from the light curves is selectable and can be set as a parameter.Using Sys-Rem with two effects the transit itself is disappearing from the light curve.Thus we have used only one effect.To determine the time of the center transit we have used the fit based on the systemparameters of Holman et al. (2006). With the help of the χ -test, we have determinedthe time of the midtransit as follows: T c ( HJD ) = (2454171 . ± . d. (2.2)The final time series is plotted in Fig. 1 (left plot). Except of our transit, observed atthe observatory in Großschwabhausen, we found 16 other transit times for XO-1b in theliterature. Together with our transit, all 17 points allow us to study the (O-C) diagram ransit observation of XO-1b and TrES-1 T . In this case we have used the transit with epoch 20, according tothe ephemeris of McCullough et al. (2006), observed in the TLC-Project. Hence, thetransit time for the epoch 0 is T c (HJD) = (2453887.74679 ± T . We gotthe best χ with an orbital period of P = (3.941497 ± σ from the new ”zero” line (solid line in Fig. 1 - right plot).The resulting ephemeris which is in a good agreement with our observation is: T c ( HJD ) = (2453887 . E · . d. (2.3)
3. TrES-1
The other observation at the observatory in Großschwabhausen was performed onMarch 15th 2007. We observed one transit of TrES-1 with the same 24.5cm Cassegraintelescope. The light curve consists of 88 R -band images with 60s exposures. Unfortu-nately, our observation was aborted too early and the last part of the light curve is miss-ing. This transit corresponds to epoch 326 of the ephemeris given by Winn et al.(2007): T c ( E ) = (2453186 . E · . d. (3.1)In this case our mean photometric accuracy is 0.009 mag. The data reduction andanalysis was carried out in the same way as in the case of XO-1b. For calculation of theartificial CS we have used 34 most stable stars with a good S/N. As we did not have lightfrom out of transit which allow us to find the systematic effect, the Sys-Rem was notused. The determination of the transit center was done in the same way as in the case ofXO-1b. After normalization we did a fit of the light curve using the system parametersby Winn et al. (2007). Using the theoretical light curve and the χ -test, it was possibleto estimate the center of the transit even without the egress. The minimal value of χ corresponds with the following midtransit time: T c ( HJD ) = (2454174 . ± . d. (3.2)The resulting light curve is shown in the Fig. 3 (left plot). For TrES-1 we found 15midtransit times in the literature. The transit of epoch 40.5 (forced e = 0) according tothe ephemeris given by Winn et al. (2007) is even a secondary transit observed by Char-bonneau et al. (2005). With all 16 available transit times we determined the transit andsecondary-eclipse timing residuals for TrES-1. The calculated times (using the ephemerisof Winn et al. (2007)) have been subtracted from the observed times. The resulting (O-C)diagram is shown in the Fig. 2 (right plot). The point lie on the horizontal line. It meansthat data are consistent with a constant period which confirmes the ephemeris given byWinn et al. (2007).22 M. Vaˇnko Figure 2. ( left plot ) Relative R -band photometry of TrES-1. ( right plot ) Transit timing residualsfor TrES-1. The dashed line shows the ephemeris given by Winn et al. (2007). The data pointsare consistent with the constant period.
4. Conclusions and Discussion
During March 2007 we have observed transits of known transiting planets XO-1b andTrES-1 at the university observatory in Großschwabhausen. Using the theoretical lightcurve and χ -test we determined the time of the midtransit for XO-1b ( T c (HJD) =(2454171.53188 ± T c (HJD) = (2454174.60958 ± P = 3.941497 ± P = 3.0300737 ± T c (HJD) = (2453887.74679 + E · References
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