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ABSTRACT  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 information from quasi‐simultaneous photometric and spectroscopic monitoring. From photometric time series obtained with the 2.2‐m telescope on Calar Alto, we measure a period of ∼4 d 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 time‐scales of weeks and years can only be explained by the presence of hotspots, presumably caused by accretion. The hotspot properties are thus variable on time‐scales 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.

Magnetic activity and accretion on FU Tau A: clues from variability