Accretion in brown dwarfs down to nearly planetary masses

We show that in accreting ultra low‐mass stars and brown dwarfs, the CaII λ 8662 emission line flux correlates remarkably well with the mass accretion rate ( $ \dot M $ ), just as it does in higher mass classical T Tauri stars (CTTs). A straightforward measurement of the CaII flux thus provides an easier $ \dot M $ determination technique than detailed modeling of the H α emission line profile (except at the very lowest accretion rates, where CaII does not appear to be in emission for ultra low‐mass objects, and H α modeling is required). Using optical high‐resolution spectra, we infer $ \dot M $ from CaII emission for young ultra low‐mass objects down to nearly the deuterium‐burning (planetary‐mass) limit. Our results, in combination with previous determinations of $ \dot M $ in CTTs, illustrate that the accretion rate declines steeply with mass, roughly as $ \dot M $ ∝ M * 2 (albeit with considerable scatter). A similar relationship has been suggested by previous studies; we extend it down to nearly the planetary regime. The physical reason for this phenomenon is not yet clear; we discuss various possible mechanisms. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)