J. Bergeat and
Received 20 November 1997 / Accepted 23 February 1998
More than 300 galactic carbon-rich stars have been observed by the HIPPARCOS satellite. Parallaxes and proper motions are given with an unprecedented accuracy. Carbon stars are however distant giants and there are only 4 stars with a parallax accuracy better than (26 stars if ). Direct use of observed data is thus severely hampered by statistical biases. We propose here a model of the distribution of observed parallaxes as deduced from the cumulative distribution of proper motions.
A simple law for true parallaxes is found to give a good fit to observed parallaxes. The minimum true parallax of this magnitude-limited sample amounts to mas. True parallaxes are then generated for statistical purposes. They are found to agree with the relative photometric angular diameter of Knapik & Bergeat (1997) following the theoretical relation Diagrams such as Fig. 3b show no distortion even at small parallaxes which suggests they are free of systematic effects. Finally, the space distribution of the HIPPARCOS carbon stars is discussed. An evaluation of the Malmquist bias is obtained and the local (presumably unbiased) distribution of carbon stars is found to be A flat subsystem or slab is proposed It is shown that the HIPPARCOS sample is reasonably complete for with the possible exception of the faint R0-R2 stars. The Malmquist bias appears also as negligible for the bright N-variables down to No clear correlation with the local arms structure and interstellar extinction could be seen in this magnitude-limited sample of the Sun vicinity. The space density of carbon stars is estimated to range from 40 to 70 stars which is in good agreement with previous estimates of Fuenmayor (1981).
Key words: stars: AGB and post-AGB stars: carbon stars: fundamental parameters Galaxy: stellar content
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© European Southern Observatory (ESO) 1998
Online publication: May 15, 1998