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 theoretical NextGen isochrones of the Lyon Group fit the K , I-K CMDs well for stars with I-K \sim 1.5-3.5 .
However Pleiades stars with K \simeq 10.5–13 ( M _ { K } \simeq 5–7.5 ) are rather redder than the isochrones .
We also identify this effect amongst \alpha Per 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 \simeq 4.3-6/J-K \simeq 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 \Delta I \sim 0.5 , \Delta J \sim \Delta K \sim 0.3 ) , probably resulting from a sharp local drop in the magnitude-mass relation .
Then the sequence is quite flat from I-K \sim 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 \sim 0.1 too blue in H-K , and \sim 0.1 too red in J-H for T _ { eff } > 4000K .
However , they are in reasonable agreement with the data at T _ { eff } \sim 3200K .
For T _ { eff } \sim 2800–3150K , the colour of Pleiades and Praesepe sources are significantly different , where Praesepe sources are \sim 0.1 bluer in J-H and up to \sim 0.1 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 2500K .
However , Pleiades sources remain consistent with the NextGen models ( and inconsistent with the dusty models ) down to T _ { eff } s of \sim 2000K .
It is possible that dust formation does not begin until lower T _ { eff } s 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 ^ { +11 } _ { -10 } % .
We also find some evidence suggesting that the BF-q distribution is flat for 0.5-0.35M _ { \odot } , in contrast to solar type stars .