Investigation of transit-selected exoplanet candidates from the MACHO survey

Abstract

Context. Planets outside our solar system transiting their host star, i.e. those with an orbital inclination near 90°, are of special interest to derive physical properties of extrasolar planets. With the knowledge of the host star's physical parameters, the planetary radius can be determined. Combined with spectroscopic observations the mass and therefore the density can be derived from Doppler-measurements. Depending on the brightness of the host star, additional information, e.g. about the spin-orbit alignment between the host star and planetary orbit, can be obtained. Aims. The last few years have witnessed a growing success of transit surveys. Among other surveys, the MACHO project provided nine potential transiting planets, several of them with relatively bright parent stars. The photometric signature of a transit event is, however, insufficient to confirm the planetary nature of the faint companion. The aim of this paper therefore is a determination of the spectroscopic parameters of the host stars as well as a dynamical mass determination through Doppler-measurements. Methods. We obtained follow-up high-resolution spectra for five stars selected from the MACHO sample, which are consistent with transits of low-luminosity objects. Radial velocities were determined by means of cross-correlation with model spectra. The MACHO light-curves were compared to simulations based on the physical parameters of the system derived from the radial velocities and spectral analyses. Results. We show that all transit light-curves of the exoplanet candidates analysed in this work can be explained by eclipses of stellar objects, hence none of the five transiting objects is a planet.