ADONIS high contrast infrared imaging of Sirius-B

Abstract

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 ~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 ~25 AU from Sirius would have been detected.