High angular resolution detection of β Pictoris b at 2.18 μm

Context. A giant planet was recently discovered around the young star β Pictoris. This planet is the closest to its parent star ever imaged. With an estimated mass of about 9 M Jup and separation of 8–15 AU, it explains most of the peculiarities of β Pictoris  and its disk.Aims. Previous detections were made in the L ′ band (3.8 μ m) and at 4.05 μ m. We recorded new K s -band data (2.18 μ m) in order to measure its color and get an additional estimate of its mass and effective temperatureMethods. Angular differential K s -band images of β  Pictoris were recorded with NaCo in March and April 2010.Results. The companion is detected at K s . This independently confirms the physical nature of β  Pictoris b inferred from the L ′ and NB_4.05 bands. The increase of the projected separation between October–December 2009 and April 2010 observations is consistent within error bars with the expected orbital motion. Using the absolute K s photometry, “hot start” evolutionary models predict a mass of 7–11 M Jup in agreement with previous estimates. Moreover, this mass is compatible with T eff  = 1700 ± 300 K derived from the comparison of the K s  − L ′ color with those generated using synthetic spectra.

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