First step to detect an extrasolar planet using simultaneous observations with the VLTI instruments AMBER and MIDI

Performed in November 2007 as a part of the MIDI Guaranteed Time Observation exoplanet program, the observation of the hot Jupiter-like exoplanet Gliese 86b constituted the first attempt of exoplanet detection with the VLTI instrument MIDI. It is also a technical achievement as the first VLTI observation using AMBER and MIDI simultaneously. Fringes were obtained for both instruments with the aim to correct the phase in N-band from the dispersion using the fringes in K-band. In N-band, the parent star has an estimated magnitude of 3.8, and a flux ratio planet/star of about 10 −3 is expected. After simulating the effect of the data reduction process of MIDI (EWS), it appears that the theoretical interferometric phase spectrum is a curved-like function with an amplitude (that we call arrow) of about 0.05 ◦ . According to the phase spectra of the calibrator HD9362, taken during the first night of observation, we estimate that a precision on the curvature measurement of about 0.33 ◦ is currently reached. Consequently, we are at least at a factor 6 from a possible detection. The AMBER data, obtained in parallel, were too noisy to be used to extrapolate and remove the corresponding dispersion in N band at the required level of precision.