Results of a Deep Imaging Survey of One Square Degree of the Pleiades for Low‐Luminosity Cluster Members

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Abstract
We have obtained relatively deep imaging in two colors, with limiting magnitudes of V ~ 22.5 and Ic ~ 21, of about one square degree of the Pleiades open cluster. Our primary goal was to identify new candidate brown dwarf members of this ~100 Myr-old open cluster. In the process, we have also obtained V and Ic photometry for a large number of brighter proper-motion members of the cluster for which only photographic photometry had been available up until now. We identify six candidate objects whose locations in a V versus (V - I)c color-magnitude diagram are consistent with their being Pleiades members near or below the hydrogen-burning mass limit. We have obtained near-infrared photometry for several of these brown dwarf candidates, as well as for a selection of previously identified very low mass Pleiades members, and we use this new photometry to help determine whether the objects identified in the visual are indeed Pleiades members or not. Finally, we have obtained a moderate-resolution spectrum of one of the new brown dwarf candidates with the Keck LRIS spectrograph. The spectrum shows that the star has a spectral type of M7, which is compatible with its photometric colors, and that it has an Hα equivalent width of about 14 Å and a marginally detected lithium 6708 Å absorption equivalent width of about 0.4 Å. These spectroscopic characteristics suggest that this object (MHObd1) is indeed a Pleiades member with a mass at or slightly above the hydrogen-burning mass limit.