HD 179949b: a close orbiting extrasolar giant planet with a stratosphere?

We have carried out a search for the 2.14-mu m spectroscopic signature of the close orbiting extrasolar giant planet, HD 179949b. High-cadence time-series spectra were obtained with the Cryogenic high-resolution InfraRed Echelle Spectrograph at Very Large Telescope, Unit 1 on two closely separated nights. Deconvolution yielded spectroscopic profiles with mean signal-to-noise ratios of several thousand, enabling the near-infrared contrast ratios predicted for the HD 179949 system to be achieved.

Recent models have predicted that the hottest planets may exhibit spectral signatures in emission due to the presence of TiO and VO which may be responsible for a temperature inversion high in the atmosphere. We have used our phase-dependent orbital model and tomographic techniques to search for the planetary signature under the assumption of an absorption line dominated atmospheric spectrum, where T and V are depleted from the atmospheric model, and an emission line dominated spectrum, where TiO and VO are present.

We do not detect a planet in either case, but the 2.120-2.174-mu m wavelength region covered by our observations enables the deepest near-infrared limits yet to be placed on the planet/star contrast ratio of any close orbiting extrasolar giant planet system. We are able to rule out the presence of an atmosphere dominated by absorption opacities in the case of HD 179949b at a contrast ratio of F-p/F-* similar to 1/3350, with 99 per cent confidence.