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We have obtained a series of high-resolution optical spectra for the browndwarf 2MASSW J1207334-393254 (2M1207) using the ESO Very Large Telescope withthe UVES spectrograph during two consecutive observing nights (time resolutionof ~12 min) and the Magellan Clay telescope with the MIKE spectrograph.Combined with previously published results, these data allow us to investigatechanges in the emission line spectrum of 2M1207 on timescales of hours toyears. Most of the emission line profiles of 2M1207 are broad, in particularthat of Halpha, indicating that the dominant fraction of the emission must beattributed to disk accretion rather than to magnetic activity. From the Halpha10% width we deduce a relatively stable accretion rate between10^(-10.1...-9.8) Msun/yr for two nights of consecutive observations.Therefore, either the accretion stream is nearly homogeneous over (sub-)stellarlongitude or the system is seen face-on. Small but significant variations areevident throughout our near-continuous observation, and they reach a maximumafter ~8 h, roughly the timescale on which maximum variability is expectedacross the rotation cycle. Together with past measurements, we confirm that theaccretion rate of 2M1207 varies by more than one order of magnitude ontimescales of months to years. Such variable mass accretion yields a plausibleexplanation for the observed spread in the accretion rate vs. mass diagram. Themagnetic field required to drive the funnel flow is on the order of a fewhundred G. Despite the obvious presence of a magnetic field, no radio nor X-rayemission has been reported for 2M1207. Possibly strong accretion suppressesmagnetic activity in brown dwarfs, similar to the findings for higher mass TTauri stars.

Emission Line Variability of the Accreting Young Brown Dwarf 2MASSW J1207334-393254: From Hours to Years