Issue |
A&A
Volume 419, Number 1, May III 2004
|
|
---|---|---|
Page(s) | 249 - 267 | |
Section | Stellar structure and evolution | |
DOI | https://doi.org/10.1051/0004-6361:20034022 | |
Published online | 23 April 2004 |
Rotation and accretion of very low mass objects in the σ Ori cluster*
Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
Corresponding author: A. Scholz, scholz@tls-tautenburg.de
Received:
27
June
2003
Accepted:
27
December
2003
We report on two photometric monitoring campaigns of Very Low Mass (VLM) objects in the
young open cluster around σ Orionis. Our targets were pre-selected with multi-filter photometry
in a field of 0.36 sqdeg. For 23 of these objects, spanning a mass range from 0.03 to 0.7 ,
we detect periodic variability. Of these, 16 exhibit low-level variability, with amplitudes of less
than 0.2 mag in the I-band, which is mostly well-approximated by a sine wave. These periodicities are probably
caused by photospheric spots co-rotating with the objects. In contrast, the remaining variable targets
show high-level variability with amplitudes ranging from 0.25 to 1.1 mag, consisting of a periodic
light variation onto which short-term fluctuations are superimposed. This variability pattern is very similar
to the photometric behaviour of solar-mass, classical T Tauri stars. Low-resolution spectra of a few of
these objects reveal strong Hα and Ca-triplet emission, indicative of ongoing accretion processes.
This suggests that 5-7% of our targets still possess a circumstellar disk. In combination with previous results
for younger objects, this translates into a disk lifetime of 3-4 Myr, significantly shorter than for
solar mass stars. The highly variable objects rotate on average slower than the low-amplitude variables,
which is expected in terms of a disk-locking scenario. There is a trend towards faster rotation with
decreasing mass, which might be caused by shortening of the disk lifetimes or attenuation of magnetic
fields.
Key words: techniques: photometric / stars: low-mass, brown dwarfs / stars: rotation / stars: formation / stars: activity / stars: magnetic fields
© ESO, 2004
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