Millimagnitude Photometry for Transiting Extrasolar Planetary Candidates IV: The Puzzle of the Extremely Red OGLE-TR-82 Primary Solved

摘要

We present precise new V, I, and K-band photometry for the planetary transitcandidate star OGLE-TR-82. Good seeing V-band images acquired with VIMOSinstrument at ESO VLT allowed us to measure V=20.6+-0.03 mag star in spite ofthe presence of a brighter neighbour about 1" away. This faint magnitudeanswers the question why it has not been possible to measure radial velocitiesfor this object. One transit of this star has been observed with GMOS-Sinstrument of GEMINI-South telescope in i and g-bands. The measurement of thetransit allows us to verify that this is not a false positive, to confirm thetransit amplitude measured by OGLE, and to improve the ephemeris. The transitis well defined in i-band light curve, with a depth of A_i=0.034 mag. It ishowever, less well defined, but deeper (A_g=0.1 mag) in the g-band, in whichthe star is significantly fainter. The near-infrared photometry obtained withSofI array at the ESO-NTT yields K=12.2+-0.1 and V-K=8.4+-0.1, so red that itis unlike any other transit candidate studied before. Due to the extreme natureof this object, we have not yet been able to measure velocities for this star,but based on the new data we consider two different possible configurations:(1)a nearby M7V star, or (2) a blend with a very reddened distant red giant. Thenearby M7V dwarf hypothesis would give a radius for the companion ofR_p=0.3+-0.1 R_J, i.e. the size of Neptune. Quantitative analysis of near-IRspectroscopy finally shows that OGLE-TR-82 is a distant, reddened metal poorearly K giant. This result is confirmed by direct comparison with stellartemplates that gives the best match for a K3III star. Therefore, we discard theplanetary nature of the companion. Based on all the new data, we conclude thatthis system is a main-sequence binary blended with a background red giant.