Astron. Astrophys. 336, 490-502 (1998)

1. Introduction

Imaging surveys for brown dwarfs have generally targeted young open clusters or star-forming regions because they provide context for any identified brown dwarf and because brown dwarfs are brighter and warmer when they are young. Based either on the number of papers published or on the number of confirmed brown dwarfs identified, by far the most productive of these regions has been the Pleiades open cluster. Astronomers have voted with their telescope time to make the Pleiades the prime hunting ground for brown dwarfs because its properties are closest to the ideal one would desire. Those properties include: (a) its age of about 100 Myr, which is old enough so that the lithium test (Rebolo, Martín and Magazzù 1992) is useful but young enough so that objects at the hydrogen burning mass limit (HBML) are still relatively warm and hence easily detected in the optical where large format detectors are available; (b) its distance of about 125 pc, which is near enough so that the apparent magnitude of the HBML is relatively bright and optical imaging surveys can detect objects below the HBML even with relatively small telescopes; (c) its "richness", with more than 800 catalogued members, and (d) its relatively high galactic latitude, which minimizes the background density of old disk dwarfs and entirely avoids having to distinguish brown dwarf candidates from heavily reddened, distant giants.

Imaging surveys to detect brown dwarfs in the Pleiades were begun almost a decade ago. However, for a number of years interpretation of those surveys was problematic because theoretical evolutionary tracks were not reliable enough to confidently demarcate the location of the HBML at Pleiades age (Stauffer et al. 1994) and there was no predicted signpost that denoted an object as a brown dwarf based on information obtainable from low resolution spectroscopy or photometric colors. This unsatisfactory state of affairs was resolved by the detection of lithium in a high-resolution spectrum of one of the Pleiades brown dwarf candidates, PPL 15, by Basri, Marcy & Graham (1996) using the Keck I telescope. Non-detection of a number of brighter Pleiades members (Marcy, Basri & Graham 1994; Martín, Rebolo & Magazzù 1994; Oppenheimer et al. 1997) and subsequent detection of lithium in two other Pleiades brown dwarf candidates (Rebolo et al. 1996) now require the HBML to be somewhere in the range 17.5 I 18.5, and suggest that the Pleiades is somewhat older than previously thought, with a probable age of 100-130 Myr (Basri, Marcy & Graham 1996).

Table 1 provides a summary of the Pleiades brown dwarf imaging surveys conducted to date. The final two columns, and in particular the final column, are somewhat subjective and reflect our own personal biases or guesses in many cases. None of these Pleiades brown dwarf candidates is cool enough to have detectable photospheric methane, the most nearly certain indicator that an extrasolar, high gravity object is a brown dwarf (Burrows et al. 1997). For Table 1, we therefore restrict the appelation "confirmed brown dwarf" to objects with detected lithium and with inferred effective temperatures compatible with model predictions for Pleiades age brown dwarfs. Recent models (Baraffe et al. 1998) predict that at the age of the Pleiades (120 Myr, see below), a 0.075 object will have depleted lithium by a factor of 50 relative to its initial abundance, while a 0.070 object will have depleted it only by a factor of 3. Therefore, the lithium borderline in the Pleiades closely match the HBML, located at 0.072 according to Baraffe's et al. models. Because of the detection of approximately undepleted lithium abundances in Teide 1 and Calar 3, confirmed members of the Pleiades fainter than I 18.5 could also be considered as confirmed brown dwarfs, but as yet the kinematic information for any of the candidate Pleiades brown dwarfs this faint is too uncertain to certify Pleiades membership. Therefore, for now, we only have 3.5 confirmed Pleiades brown dwarfs.

Table 1. Imaging surveys for Pleiades brown dwarfs

A general characteristic of Table 1 is that the surveys to date have either been large area but relatively shallow or deep but with relatively limited coverage. This was simply the result of the available technology - photographic plates, as used by Hambly, Hawkins and Jameson (1993=HHJ), come in quite large formats but have low quantum efficiency, while CCD's (in the optical) and InSb or HgCdTe arrays (in the near-IR) are very sensitive but have historically come in small formats. With the development of CCD-mosaic cameras on large telescopes, it has recently become possible to obtain deep and wide-field surveys in the optical. In this paper, we report the results of the first survey for Pleiades brown dwarfs conducted using a CCD-mosaic camera. We have used the Canada-France-Hawaii telescope and the UH 8Kx8K camera to survey 2.5 square degrees of the Pleiades to a completeness limit of 22nd magnitude in I and 23rd in R. We have identified more than 15 brown dwarf candidates in these images, and provide their coordinates, I magnitudes and colors, and finding charts for these objects. We also discuss our initial attempts to constrain the Pleiades mass function below the HBML using these data.

© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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