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Search for Sirius companions
November 24, 2000Many direct or indirect observations have suggested the existence of a second companion (in addition of the white dwarf Sirius-B) around the brilliant star Sirius-A. The presence of a second faint star could in particular explain a change of color of Sirius, as suggested by historical texts. A team of astronomers of the CEA (Service of Astrophysics) and of Observatoire de Paris (IMCCE and DESPA) have recently obtained a new image of the sky around Sirius, using a coronographic device. This image, compared with a preceding observation, allowed for the first time, by using the large proper motion of Sirius, to eliminate certain possible candidates and to constrain the possible characteristics of the suggested second companion.
Because of the enormous diffusion created by the bright Sirius-A, the star field around Sirius is very difficult to reach, and remained long unexplored. A first observation was carried out in 1985 with the 1.5m telescope of European Southern Observatory (ESO) by J.M. Bonnet-Bidaud of the CEA Service of Astrophysics and C. Gry of the Marseilles LAS. It revealed for the first time about fifteen faint stars close to Sirius-A.
A second image was obtained, in January 1999, by J.M. Bonnet-Bidaud (CEA), F. Colas (Obs.Paris, IMCCE) and J. Lecacheux (Obs. Paris, DESPA), thanks to a coronographic device adapted on the 1m telescope of Pic du Midi. In both cases an astrometrical study made it possible to measure the precise position of stars within a field of 3 arcmin around Sirius. In the 13 years interval between the two images, Sirius-A moved 14 seconds of arc and a companion in orbit would have to undergo the same displacement. The superposition of the two images shows that no star of the field moved by more than 0.5 arcsec. This method thus makes it possible to eliminate as probable companion all the stars detected in the field. These images show, in addition, that Sirius passed, in 1937, at an apparent angular distance of less than 7 arcsec of a star of magnitude 12. The alleged presence of a second companion, claimed on the basis of visual observations during the period 1920-1930, is thus most probably the result of this fortuitous conjunction with a background star.
The new observations lead the authors to the conclusion that only a brown dwarf, similar to the weakest currently observed, with a magnitude of 18-19, could still be an undetected companion in the observed field. Also, the most central area (< 30 arcsec) around Sirius remains for the moment unexplored.
A program is currently carried out with ESO to observe the area nearest to Sirius with high spatial resolution using adaptive optics. Let us note that Kuchner and Brown also recently obtained constraints on the possible companions of Sirius, with the Space Telescope (NICMOS) in a very limited region close to the star.
Among all the binary stars comprising a white dwarf, Sirius is a singular system, and moreover, a different color has been reported in old historical texts. These characteristics, still largely unexplained, could well be the result of the evolution of a more complex system,comprising a third star still undetected.
An animated presentation of the intertwining trajectories of Sirius-A and -B in the picture of the stellar field, on the WEB site of Observatoire de Paris (http://www.obspm.fr), illustrates the problem, along with a more detailed information.
Observatoire de Paris and CNRS
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