Transit of the Planetary Companion to HD209458b

Following the discovery of the transit of the planetary companion to HD209458 (Charbonneau et al. 2000, Henry et al. 2000), an attempt has been made to observe the transit from ULO with the 24-inch (0.6-m) Radcliffe refracting telescope. This work was carried out by Andrew Swan (4th-year MSci student, 2000/01) and Dr Steve Fossey.

Data were obtained in the autumn of 2000, on the nights of September 23 and 30. The field of HD209458 (V=7.65) was imaged with the Wright Instruments CCD camera, and included the reasonably bright comparison star HD209346 (V=8.33).

Images were obtained mainly in the R band, with some B-band data also obtained on each night. Exposure times in the R-band were limited to 20 seconds to avoid saturation by the brighter stars, and the data are sampled about every minute in the R-band sequences.

Photometry was carried out with the DAOPHOT routines in IRAF. Differential magnitudes between HD209458 and HD209346 were corrected for colour-dependent extinction effects (due to the changing airmass); colour terms were determined using the star BD +18 4914 (V=11.1) [its (B-V) colour is similar to HD209458, while HD209346 is somewhat bluer].

Transit of HD209458b in R-band

The results for the R-band are shown in the figure above. The two nights' data have been phased to the time of mid-transit. The transit is clearly detected, showing a dip of the expected magnitude (1.64%, Brown et al. 2001). Unfortunately, few data are available before first contact, and those obtained in the later phases and after last contact were obtained at high airmass (X>2) and become quite noisy. The main source of noise in these data is believed to be scintillation in the atmosphere, which becomes worse at high airmass. The most precise data are those around mid-transit from the night of Sep 23; these were obtained close to the meridian at ULO, and show an rms scatter of about 0.2-0.3 % (0.002 - 0.003 mag). This suggests that under good conditions, by increasing the sampling rate and averaging over several exposures, it should be possible to obtain differential photometry to millimagnitude precision in a few minutes for targets of this brightness.

   Steve Fossey