Millimagnitude Photometry for Transiting Extrasolar Planetary Candidates. IV. Solution to the Puzzle of the Extremely Red OGLE-TR-82 Primary

We present precise new V-, I-, and K_s-band photometry for the planetary-transit candidate star OGLE-TR-82. V-band images acquired in good seeing with the VIMOS instrument at the Very Large Telescope allowed us to measure V=20.61+/-0.03 mag for this star despite the presence of a brighter neighbor about 1" away. This faint magnitude answers the question why it has not been possible to measure radial velocities for this object. One transit of this star has been observed with the GMOS-S instrument on Gemini South in the i and g bands, which allowed us to verify that this is not a false positive, to confirm the transit amplitude measured by OGLE, and to improve the ephemeris. The transit is better defined in the i-band light curve (with a depth of A_i=0.034 mag), than in the g band (A_g=0.1 mag), in which the star is significantly fainter. Near-IR photometry obtained with the SOFI array at the ESO New Technology Telescope yields K=12.20+/-0.10 and V-K=8.40+/-0.10, so red that it is unlike any transit candidate studied before. With the new data, we consider two possible configurations for the system: (1) a nearby M7 V star or (2) a blend with a very reddened, distant red giant. The first hypothesis would give a radius for the companion of R_p=0.3+/-0.1 R_J, i.e., the size of Neptune. Quantitative analysis of near-IR spectroscopy finally shows that OGLE-TR-82 is a distant, reddened, metal-poor early K giant, confirmed by direct comparison with stellar templates, which gives as a best match a K3 III star. Therefore, we rule out a planetary nature for the companion, and conclude that this system is a main-sequence binary blended with a background red giant. As a case study, a system that can so mimic a planetary transit presents a lesson for future transit surveys.

Based on observations collected with the Gemini South telescope (queue observing, program GS-2005B-Q-9) and with the Very Large Telescope at Paranal Observatory (J. M. F. and D. M., Visiting Astronomers) and the ESO New Technology Telescope at La Silla Observatory (S. R., F. P., and D. M., Visiting Astronomers) for ESO programs 075.C-0427, 075.B-0414, and 076.C-0122.