Abstract
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 find that Pleiades stars with K=10.5--13 are rather redder than the NextGen isochrones. We also identify this effect amongst alphaPer sources from the literature, but find no evidence of it for field stars from the literature. It is possible that the I-K colour of youthful stars is affected by the presence of magnetic activity. In the 2-colour diagram we find no colour change across the M dwarf gap (which would be indicative of dust formation). For Teff=2800--3150K, the colours of Praesepe and Pleiades sources differ, which may result from gravity sensitive molecular opacities. Praesepe sources cooler than 2500K agree with the dusty models, but Pleiades sources agree with the NextGen models down to 2000K. It is possible that dust formation does not begin until lower Teffs in sources with lower surface gravities. 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 0.5-0.35Mo, in contrast to solar type stars.
Highlights
Low-mass star and brown dwarf (BD) populations in open clusters make excellent test-beds for our theoretical understanding of these objects
In the previous section we found that the NextGen isochrones fit the IK colour magnitude diagrams (CMDs) very well for I-K∼1–3.5, except for Pleiades sources from K=10.5–13, which are slightly red
This is all consistent with the gradual changeover from NextGen to Dusty isochrones that was seen in the I-K CMDs (IK CMDs), and suggests that these models should provide accurate NIR isochrones in the blue and red photometric ranges
Summary
Low-mass star and brown dwarf (BD) populations in open clusters make excellent test-beds for our theoretical understanding of these objects. With a well constrained cluster age, and assuming uniform composition, we can effectively compare theoretical isochrones to observed colour magnitude diagrams (CMDs) and two-colour diagrams without the complications introduced by a spread in metallicity and a range in surface gravities for a given mass. We compare NextGen and Dusty theoretical isochrones to our observed CMDs and two-colour diagrams, and identify photometric ranges where the isochrones agree well with observations. We join these isochrone sections together using additional empirical points, and discuss the overall form of the cluster sequences. The fainter surveys cover a combined area of 7.6 square degrees, and range in photometric depth from I=19.521.8 (unless otherwise stated, I refers to Cousins I). The Magazzu survey covered 800 square arcminutes down to I=21.2, and identified 1 BD candidate which they called Roque Pr 1
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
