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Published December 2010 | Published
Journal Article Open

Precision Photometric Monitoring of Very Low Mass σ Orionis Cluster Members: Variability and Rotation at a Few Myr


We present high-precision photometry on 107 variable low-mass stars and brown dwarfs in the ~3 Myr σ Orionis open cluster. We have carried out I-band photometric monitoring within two fields, encompassing 153 confirmed or candidate members of the low-mass cluster population, from 0.02 to 0.5M_⊙. We are sensitive to brightness changes on timescales from 10 minutes to two weeks with amplitudes as low as 0.004 mag, and find variability on these timescales in nearly 70% of cluster members. We identify both periodic and aperiodic modes of variability, as well as semi-periodic rapid fading events that are not accounted for by the standard explanations of rotational modulation of surface features or accretion. We have incorporated both optical and infrared color data to uncover trends in variability with mass and circumstellar disks. While the data confirm that the lowest-mass objects (M <0.2 M_⊙) rotate more rapidly than the 0.2–0.5 M_⊙ members, they do not support a direct connection between rotation rate and the presence of a disk. Finally, we speculate on the origin of irregular variability in cluster members with no evidence for disks or accretion.

Additional Information

© 2010 American Astronomical Society. Received 2010 July 12; accepted 2010 October 25; published 2010 December 2. This research has been supported by grants to L.A.H. from the NASA Origins and ADP programs. A.M.C. thanks the CTIO Telescope Operations staff for help in carrying out observations. Observation time on SMARTS consortium facilities was awarded through the National Optical Astronomy Observatory, operated by the Association of Universities for Research in Astronomy, under contract with the National Science Foundation. We are grateful for helpful comments from the referee, Adam Kraus, Krzysztof Findeisen, and Nairn Baliber.

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