McMahan Observatory

Exoplanet Wasp-2b

The stars Wasp-1 (also listed in SIMBAD as TYC 2265- 107-1, GSC 02265-00107, and 2MASS J00204007+3159239 ) and Wasp-2 (also listed in SIMBAD as GSC 00522-01199, 2MASS J20305413+0625463, and UCAC2 34018636) were identified in the 2004 SuperWASP wide field transit survey as high potential hosts for transiting exoplanets. Between August 31, 2006 and September 19 2006, Wasp-1 and Wasp-2 were observed using he newly commissioned SOPHIE spectrograph on the 1.93-m telescope at the Observatoire de Haute-Provence. Radial velocity measurements were taken which ruled out blended eclipsing binary stars as the light curve variations recorded for these two stars in the 2004 SuperWASP survey. The complete transit light curve was observed using a CCD camera and R band filters on the 60-cm telescope at the Observatoire Francois-Xavier Bagnoud at St-Luc on September 12, 2006 [A. Collier Cameron, et al, arXiv:astro-ph/0609688v1 September 25, 2006].

Discovery of exoplanets Wasp-1b and Wasp-2b was announced on September 26th and the data presented here was taken at McMahan Observatory on September 30, 2006 (UT), during the next transit of Wasp-2b following the announcement . It is interesting to note that standard stochastic error of less than 0.01 mag could be achieved with a 10" aperture telescope. During the sequence, Wasp-2 reached a zenith distance equal to airmass 2 at approximately JD2453000+1008.7670, the 15 point from the end of the sequence. Starting at approximately this airmass, the scatter of the points started to increase due to lower S/N ratio resulting from haze and light pollution near the horizon. S/N ratios at low airmass were approximately 280 for Wasp-2 and 220 for the reference star, GSC 00522-00780. At 2.0 airmass the S/N ratios had degraded to less than 230 and 180, respectively. The reduction in differential magnitude during the transit was at least twice the standard error of measurement during the transit period and is, therefore, statistically significant. The variation in the differential magnitude curve of the reference star vs a check star (GSC 00522-01100) does not show a similar dip in magnitude during the transit period, confirming that the variation in the light curve of Wasp-2 is independent of variations in atmospheric conditions or instrumentation. The gap in the light curve near the egress occurred while taking a calibration star measurement.

Measurements were corrected for atmospheric extinction using the first order term only. Empirically determined second order terms were not statistically different from zero.