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.
Original language | English |
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Pages (from-to) | 1271-1279 |
Journal | Monthly Notices of the Royal Astronomical Society |
Volume | 419 |
Issue number | 2 |
DOIs | |
Publication status | Published - 11 Jan 2012 |
Keywords
- accretion
- accretion discs
- stars: activity
- brown dwarfs
- stars: low-mass
- stars: rotation