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The formation mechanisms and chemical compositions of brown dwarfs likelyspan a diverse range. If they are formed predominantly in isolation bygravitational collapse like dwarf stars then their compositions might followthose of dwarf stars, spanning a relatively narrow range in metallicities (~-0.4 to +0.4 dex) and predominantly oxygen-rich (C/O <~ 0.7). On the otherhand, giant planets in the solar system are all super-solar in metallicity([C/H] > 0.5~dex), which is thought to be a consequence of formation bycore-accretion in a circumstellar disk. In this study we determine theatmospheric chemical compositions of three brown dwarfs and find them to beneither characteristic of dwarf stars nor giant planets. We derivehigh-precision atmospheric chemical abundances using high-S/N HST near-infraredspectra of three mid T Dwarfs which, together with two previously studiedobjects, display distinctly sub-solar metallicities and span C/O ratios of0.4-1.0. These abundance patterns indicate either an old sub-stellar populationand a multitude of formation environments or objects formed in circumstellardisks followed by ejection later. We show that for a simple synthetic modelpopulation of brown dwarfs with the same spectral-type as our sample thepredicted age distribution is predominantly older than the Sun and, hence, moremetal poor, consistent with our derived abundances. The diverse C/O ratios wefind are indicative of either different chemical reservoirs in their formationenvironments or different formation pathways. Our results open the possibilitythat cool brown dwarfs may provide important probes of early galactic chemicalevolution and inhomogeneity.

Atmospheric Compositions of Three Brown Dwarfs and Implications for their Formation Conditions