El. Martin et al., Membership and multiplicity among very low mass stars and brown dwarfs in the Pleiades cluster, ASTROPHYS J, 543(1), 2000, pp. 299-312
We present near-infrared photometry and optical spectroscopy of very low ma
ss stars and brown dwarf candidates in the Pleiades open cluster. The membe
rship status of these objects is assessed using color-magnitude diagrams, l
ithium and spectral types. Eight objects out of 45 appear to be nonmembers.
A search for companions among 34 very low mass Pleiades members (M less th
an or equal to 0.09 M-.) in high spatial resolution images obtained with th
e Hubble Space Telescope (HST) and the adaptive optics system of the Canada
-France-Hawaii telescope produced no resolved binaries with separations lar
ger than 0."2 (a similar to 27 AU; P similar to 444 yr). Nevertheless, we f
ind evidence for a binary sequence in the color-magnitude diagrams, in agre
ement with the results of Steele & Jameson for higher mass stars. We apply
the lithium test to two objects: CFHT-P1-16, which lies in the cluster bina
ry sequence but is unresolved in images obtained with the Hubble Space Tele
scope; and CFHT-P1-18, which is binary with 0."33 separation. The first obj
ect passes the test, but the second object does not. We conclude that CFHT-
Pl-16 is an Pleiades brown dwarf binary with separation less than 11 AU and
that CFHT-Pl-18 is a foreground system. We compare the multiplicity statis
tics of the Pleiades very low mass stars and brown dwarfs with that of G-an
d K-type main-sequence stars in the solar neighborhood. We find that there
is some evidence for a deficiency of wide binary systems (separation >27 AU
) among the Pleiades very low mass members. We briefly discuss how this res
ult can fit with current scenarios of brown dwarf formation. We correct the
Pleiades substellar mass function for the contamination of cluster nonmemb
ers found in this work. We find a contamination level of 33% among the brow
n dwarf candidates identified by Bouvier et al. Assuming a power-law IMF ac
ross the substellar boundary, we find a slope dN/dM similar to M-0.53, impl
ying that the number of objects per mass bin is still rising but the contri
bution to the total mass of the cluster is declining in the brown dwarf reg
ime.