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Astron. Astrophys. 352, 567-573 (1999)

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The density of methane brown dwarfs: observational and theoretical constraints

F. D'Antona 1, E. Oliva 2 and A. Zeppieri 1

1 Osservatorio Astronomico di Roma, Monte Porzio Catone, 00040 Roma, Italy
2 Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy

Received 18 June 1999 / Accepted 1 October 1999


Methane brown dwarfs (CH4bds ), i.e. objects similar to the low mass star Gliese 229B, are characterized by very unusual colours: J-K[FORMULA]0 and I-J[FORMULA]5. An analysis of the ESO public images, which cover an area of 37 sq. arcmin in the three filters, yields one clear CH4BD candidate with J=20.2 and two fainter sources with J[FORMULA]23. The resulting observed density of methane dwarfs ranges from [FORMULA]100 to [FORMULA]500 objects per sq. degree with J[FORMULA]23.1, quite high but compatible, within the errors, with what we derive from simulations of the stellar population of brown dwarfs in the disk of the Galaxy adopting reasonable (although still speculative) hypotheses on the evolution of BD colours with cooling. The predictions presented here can be useful in constraining the results from future searches of these objects in the infrared. Deep imaging of several sq. degrees of sky down to J[FORMULA]22, K[FORMULA]22 and I[FORMULA]27 are necessary to significantly improve the observational database.

Key words: stars: low-mass, brown dwarfs – stars: luminosity function, mass function – infrared: stars

Send offprint requests to: F. D'Antona (dantona@coma.mporzio.astro.it)

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© European Southern Observatory (ESO) 1999

Online publication: December 2, 1999