Search for Sirius companions

Observatoire de Paris Paris, France

Jean-Marc Bonnet-Bidaud

Service d’Astrophysique, DSM/DAPNIA/Sap, CEA Saclay

bobi@discovery.saclay.cea.fr

Many 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/actual/nouvelle/bobi_en.html),
illustrates the problem, along with a more detailed
information.

Peer reviewed publication and references:

J. M. Bonnet-Bidaud, F. Colas, J. Lecacheux, 2000,
Astronomy & Astrophysics
360, 991