CFHT Information Bulletin Number 37, Semester 97II

Brown Dwarfs in the Pleiades

J. Bouvier, B. Wallace, J. Stauffer, E. Martin

We have used the CFHT UH8k mosaic CCD camera to survey 2.5 square degrees of the Pleiades in R and I to detection limits of about R ~ 25.0 and I ~ 23.3 (completeness limits of R = 23.5 and I = 22.6). This is the widest area, deep survey for brown dwarfs in the Pleiades conducted to date, covering of order 15% of the effective surface area of the cluster to a sensitivity limit which should allow detection of Pleiades age brown dwarfs to about 0.03 solar mass if the theoretical models are approximately correct. The bright limit for our survey, set by the magnitude where stars saturate in our I band images, corresponds to about 0.1 solar mass at Pleiades age.

Of the ~100,000 objected detected in our survey area, we identify sixteen stars as forming a Pleiades brown dwarf sequence in our I vs. R-I color-magnitude diagram (cf. Figure 1, front cover). These brown dwarf candidates have I magnitudes ranging from ~ 17.8 to I ~ 20, and inferred masses from the hydrogen burning mass limit to about 0.04 or 0.05 solar mass. Below I = 20, we no longer see an identifiable locus of Pleiades members in the CM diagram either because few or no objects in this mass range formed or because the R-I colors of Pleiades age brown dwarfs change abruptly (grain formation?) at this mass. In any event, by surveying a statistically significant fraction of the Pleiades to very deep limits, we estimate that brown dwarfs in the mass range to which we are sensitive are unlikely to compose more than 5% of the total mass of the Pleiades. If most stars in the galactic disk form in rich clusters, this suggests that brown dwarfs also comprise only a very small fraction of the mass of the disk.



CFHT Information Bulletin Number 37, Semester 97II

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