Constraining the orbit of the possible companion to β Pictoris New deep imaging observations

Context. We recently reported on the detection of a possible planetary mass companion to beta Pictoris at a projected separation of 8 AU from the star, using data taken in November 2003 with NaCo, the adaptive-optics system installed on the Very Large Telescope UT4. Even though no second epoch detection was available, there are strong arguments to favor a gravitationally bound companion rather than a background object. If confirmed and located at a physical separation of 8 AU, this companion would be the closest planet ever imaged, and above all, could have formed via core-accretion. Its apparent magnitude would indicate a typical temperature of similar to 1500 K and a mass of similar to 8 M-Jup. Interestingly, a planet with such characteristics would explain the main morphological and dynamical peculiarities of the beta Pic system. Aims. Our goal was to re-observe beta Pic five years later to again detect the companion or, in the case of a non-detection, constrain its orbit. Methods. Deep adaptive-optics L'-band direct images of beta Pic as well as K-s-band Four Quadrant Phase Mask coronagraph images with were recorded with NaCo. Results. No point-like signal with the brightness of the companion candidate (apparent magnitudes L' = 11.2 or K-s similar or equal to 12.5) was detected at projected distances down to similar or equal to 6.5 AU in the present data from the star (by comparison, the same limit was reached at similar or equal to 5.5 AU in the better quality November 2003 data). As expected, the non detection does not allow us to rule out a background companion from an observational point of view. We show that the non detection is consistent with orbital motion. Using these data and previous K-s-band data obtained in 2004, we place strong constraints on the possible orbits of the companion.