Brown dwarfs and low-mass stars in the Pleiades and Praesepe: membership and binarity

We present near-infrared J-, H- and K-band photometry and optical spectroscopy of low-mass star and brown dwarf (BD) candidates in the Pleiades and Praesepe open clusters. We flag non-members from their position in K, I -K and J, J - K colour-magnitude diagrams (CMDs), and J - H, H - K two-colour diagrams. In general, the dust-free NextGen model isochrones of the Lyon Group fit the K, I - K CMDs well for stars with I - K similar to 1.5-3.5. However, Pleiades stars with K similar or equal to 10.5-13 (M-K similar or equal to 5-7.5) are rather redder than the isochrones. We also identify this effect amongst alphaPer sources from the literature, but find no evidence of it for field stars from the literature. The NextGen isochrones fit the J, J - K CMDs of both clusters very well in this photometric range. It is possible that the I - K colour of youthful stars is affected by the presence of magnetic activity. The Lyon Group's Dusty isochrones fit both K, I -K and K, J - K Pleiades CMDs well for I - K similar or equal to 4.3-6/J - K similar or equal to 1.1-1.4. In between these colour ranges the Pleiades cluster sequence comprises three portions. Starting at the bluer side, there is a gap where very few sources are found (the gap size is DeltaI similar to 0.5, DeltaJ similar toDeltaK similar to 0.3), probably resulting from a sharp local drop in the magnitude-mass relation. Then the sequence is quite flat from I - K similar to 3.5-4. Finally, the sequence turns over and drops down to join the Dusty isochrone. We also compare model atmosphere colours to the two-colour diagrams of the clusters. The NextGen models are seen to be similar to0.1 too blue in H - K and similar to0.1 too red in J - H for T-eff > 4000 K. However, they are in reasonable agreement with the data at T-eff similar to 3200 K. For T-eff similar to 2800-3150 K, the colours of Pleiades and Praesepe sources are significantly different, where Praesepe sources are similar to0.1 bluer in J - H and up to similar to0.1 redder in H - K. These differences could result from gravity-sensitive molecular opacities. Cooler Praesepe sources then agree well with the dusty models, suggesting that dust is beginning to form in Praesepe sources around 2500 K. However, Pleiades sources remain consistent with the NextGen models (and inconsistent with the dusty models) down to T-eff values of similar to2000 K. It is possible that dust formation does not begin until lower T-eff values in sources with lower surface gravities (and hence lower atmospheric pressures). We also identify unresolved binaries in both clusters, and estimate mass ratios (q) for Pleiades BDs. Most of these have q > 0.7, however, 3/18 appear to have lower q values. We determine the binary fraction (BF) for numerous mass ranges in each cluster, and find that it is generally rising towards lower masses. We find a BD BF of 50(-10)(+11) per cent. We also find some evidence suggesting that the BF-q distribution is flat for 0.5-0.35 M., in contrast to solar-type stars.