FU Tau A is a young very low mass object in the Taurus star forming region which was previously found to have strong X-ray emission and to be anomalously bright for its spectral type. In this study we discuss these characteristics using new informati
on from quasi-simultaneous photometric and spectroscopic monitoring. From photometric time series obtained with the 2.2m telescope on Calar Alto we measure a period of ~4d for FU Tau A, most likely the rotation period. The short-term variations over a few days are consistent with the rotational modulation of the flux by cool, magnetically induced spots. In contrast, the photometric variability on timescales of weeks and years can only be explained by the presence of hot spots, presumably caused by accretion. The hot spot properties are thus variable on timescales exceeding the rotation period, maybe due to long-term changes in the accretion rate or geometry. The new constraints from the analysis of the variability confirm that FU Tau A is affected by magnetically induced spots and excess luminosity from accretion. However, accretion is not sufficient to explain its anomalous position in the HR diagram. In addition, suppressed convection due to magnetic activity and/or an early evolutionary stage need to be invoked to fully account for the observed properties. These factors cause considerable problems in estimating the mass of FU Tau A and other objects in this mass/age regime, to the extent that it appears questionable if it is feasible to derive the Initial Mass Function for young low-mass stars and brown dwarfs.
We have obtained a series of high-resolution optical spectra for the brown dwarf 2MASSW J1207334-393254 (2M1207) using the ESO Very Large Telescope with the UVES spectrograph during two consecutive observing nights (time resolution of ~12 min) and th
e Magellan Clay telescope with the MIKE spectrograph. Combined with previously published results, these data allow us to investigate changes in the emission line spectrum of 2M1207 on timescales of hours to years. Most of the emission line profiles of 2M1207 are broad, in particular that of Halpha, indicating that the dominant fraction of the emission must be attributed to disk accretion rather than to magnetic activity. From the Halpha 10% width we deduce a relatively stable accretion rate between 10^(-10.1...-9.8) Msun/yr for two nights of consecutive observations. Therefore, either the accretion stream is nearly homogeneous over (sub-)stellar longitude or the system is seen face-on. Small but significant variations are evident throughout our near-continuous observation, and they reach a maximum after ~8 h, roughly the timescale on which maximum variability is expected across the rotation cycle. Together with past measurements, we confirm that the accretion rate of 2M1207 varies by more than one order of magnitude on timescales of months to years. Such variable mass accretion yields a plausible explanation for the observed spread in the accretion rate vs. mass diagram. The magnetic field required to drive the funnel flow is on the order of a few hundred G. Despite the obvious presence of a magnetic field, no radio nor X-ray emission has been reported for 2M1207. Possibly strong accretion suppresses magnetic activity in brown dwarfs, similar to the findings for higher mass T Tauri stars.
