ADONIS high contrast infrared imaging of Sirius-B

Context. Sirius is the brightest star in the sky and a strong source of diffuse light for modern telescopes so that the immediate surroundings of the star are still poorly known. Aims. We study the close surroundings of the star (2 to 25) by means of adaptive optics and coronographic device in the near-infrared, using the ESO/ADONIS system. Methods. The resulting high contrast images in the JHKs bands have a resolution of similar to 0.2 and limiting apparent magnitude ranging from m(K) = 9.5 at 3 from Sirius-A to m(K) = 13.1 at 10. These are the first and deepest images of the Sirius system in this infrared range. Results. From these observations, accurate infrared photometry of the Sirius-B white dwarf companion is obtained. The JH magnitudes of Sirius-B are found to agree with expectations for a DA white dwarf of temperature (T = 25 000 K) and gravity (log g = 8.5), consistent with the characteristics determined from optical observations. However, a small, significant excess is measurable for the K band, similar to that detected for dusty isolated white dwarfs harbouring suspected planetary debris. The possible existence of such circumstellar material around Sirius-B has still to be confirmed by further observations. Conclusions. These deep images allow us to search for small but yet undetected companions to Sirius. Apart from Sirius-B, no other source is detected within the total 25 field. A comparison of the flux expected from the faintest known brown dwarfs at the distance of Sirius demonstrates that the above limiting magnitudes correspond to a star of spectral type later than T5 at 5 and T7 at 10. Using theoretical spectra of brown dwarfs and planet-size objects, we also show that the end of the brown dwarf sequence is reached in the outer part of the image. The minimum detectable mass is around 10 M(Jup) inside the planetary limit, indicating that an extrasolar planet at a projected distance of similar to 25 AU from Sirius would have been detected.