Astron. Astrophys. 355, 581-594 (2000)

Exploring the brown dwarf desert with Hipparcos ^*

J.L. Halbwachs ¹, F. Arenou ², M. Mayor ³, S. Udry ³ and D. Queloz <sup>3,4

1</sup> Equipe "Populations stellaires et evolution galactique", Observatoire Astronomique de Strasbourg (UMR 7550), 11 rue de l'Université, 67 000 Strasbourg (halbwachs@astro.u-strasbg.fr)
² DASGAL, Observatoire de Paris-Meudon, bat 11, 5 place J. Janssen, 92195 Meudon Cedex, France (Frederic.Arenou@obspm.fr)
³ Geneva Observatory, 51 chemin des Maillettes, 1290 Sauverny, Switzerland (Michel.Mayor@obs.unige.ch; Stephane.Udry@obs.unige.ch; Didier.Queloz@obs.unige.ch)
⁴ Jet Propulsion Lab., 4800 Oak Grove Drive, Pasadena, CA 91109, USA

Received 5 July 1999 / Accepted 16 November 1999

Abstract

The orbital elements of 11 spectroscopic binaries with brown dwarf candidates ( between 0.01 and 0.08 ) are combined with the Hipparcos observations in order to derive astrometric orbits. Estimations of the masses of the secondary components are thus calculated. It appears that 5 secondary masses are more than above the limit of 0.08 , and are therefore not brown dwarfs. 2 other stars are still discarded at the level, 1 brown dwarf is accepted with a low confidence, and we are finally left with 3 viable candidates which must be studied by other means.

A statistical approach is developed, based on the relation between the semi-major axes of the photocentric orbit, , their errors, , and the frequency distribution of the mass ratios, q. It is investigated whether the set of values of and obtained for the sample is compatible with different frequency distributions of q. It is concluded that a minimum actually exists for between about 0.01 and 0.1 for companions of solar-type stars. This feature could correspond to the transition between giant planets and stellar companions. Due to the relatively large frequency of single brown dwarfs found recently in open clusters, it is concluded that the distribution of the masses of the secondary components in binary systems does not correspond to the IMF, at least for masses below the hydrogen-ignition limit.

Key words: astrometry stars: binaries: general stars: binaries: spectroscopic stars: formation stars: fundamental parameters stars: low-mass, brown dwarfs

* Based on photoelectric radial-velocity measurements collected at Haute-Provence observatory and on observations made with the ESA Hipparcos astrometry satellite.

Send offprint requests to: J.L. Halbwachs

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Contents

• 1. Introduction
• 2. The brown dwarf candidates observed with Hipparcos
  • 2.1. The sample
  • 2.2. Calculation of the semi-major axes of the astrometric orbits
  • 2.3. Validation of the method
  • 2.4. The masses of the brown dwarf candidates
  • 2.5. Calculation of the errors of the secondary masses
  • 2.6. Notes to individual objects
• 3. Does a brown dwarf desert exist in the distribution of the mass ratios?
  • 3.1. for different distributions of mass ratios
  • 3.2. Tests of the mass ratio distributions
• 4. Conclusions
• Acknowledgements
• Appendices
  • A: Calculation of the distribution functions of
• References

© European Southern Observatory (ESO) 2000

Online publication: March 9, 2000
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