Long-term monitoring in IC4665: Fast rotation and weak variability in very low mass objects

Abstract

We present the combined results of three photometric monitoring campaigns targeting very low mass (VLM) stars and brown dwarfs in the young open cluster IC4665 (age ~40 Myr). In all three runs, we observe ~100 cluster members, allowing us for the first time to put limits on the evolution of spots and magnetic activity in fully convective objects on timescales of a few years. For 20 objects covering masses from 0.05 to 0.5 Msol we detect a periodic flux modulation, indicating the presence of magnetic spots co-rotating with the objects. The detection rate of photometric periods (~20%) is significantly lower than in solar-mass stars at the same age, which points to a mass dependence in the spot properties. With two exceptions, none of the objects exhibit variability and thus spot activity in more than one season. This is contrary to what is seen in solar-mass stars and indicates that spot configurations capable of producing photometric modulations occur relatively rarely and are transient in VLM objects. The rotation periods derived in this paper range from 3 to 30h, arguing for a lack of slow rotators among VLM objects. The periods fit into a rotational evolution scenario with pre-main sequence contraction and moderate (40-50%) angular momentum losses due to wind braking. By combining our findings with literature results, we identify two regimes of rotational and magnetic properties, called C- and I-sequence. Main properties on the C-sequence are fast rotation, weak wind braking, Halpha emission, and saturated activity levels, while the I-sequence is characterised by slow rotation, strong wind braking, no Halpha emission, and linear activity-rotation relationship. Rotation rate and stellar mass are the primary parameters that determine in which regime an object is found. (abridged)

16 pages, 10 figures, accepted for publication in MNRAS

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• Angular Momentum Loss from Cool Stars: An Empirical Expression and Connection to Stellar Activity
• RACE-OC Project: Rotation and variability in young stellar associations within 100 pc
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• Rotation of young stars in Cepheus OB3b
• Employing a New BVIc Photometric Survey of IC 4665 to Investigate the Age of this Young Open Cluster
• Infrared Spectroscopic Observations of the Secondary Stars of Short Period Sub-Gap Cataclysmic Variables
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• Measuring the variability of directly imaged exoplanets using vector Apodizing Phase Plates combined with ground-based differential spectrophotometry
• Pre-main-sequence population in NGC 1893 region: X-ray properties
• Investigating the rotational evolution of very low-mass stars and brown dwarfs in young clusters using Monte Carlo simulations
• Photometric observations of NGC 281: Detection of 228 variable stars
• A deep staring campaign in the Sigma Orionis cluster. Variability in substellar members