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Astron. Astrophys. 322, 455-459 (1997)

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4. Discussion

Photometry alone can not extract Pleiades members to a very high confidence level, but is the most time-efficient tool for finding potential members, whose proper motion later can be measured. In this survey it is shown that the I vs [FORMULA] diagram clearly sorts out the Pleiades sequence from the background. As an example of efficiency, consider the completeness limit of this survey ([FORMULA]), reached in 20 minutes integration at seeing 0.6". The same BD in V ([FORMULA]) would demand [FORMULA] hours and in R ([FORMULA]) [FORMULA] hour. In J it takes only [FORMULA] 1 minute. Thus even though CCDs have larger surface area, they cannot compete with IR-arrays for objects as red as presumed BDs. In case of K, the needed integration time is [FORMULA] minutes. Adding more complicated reductions to that and the fact that I and J is enough to extract the Pleiades sequence there is no need for K data in the first step of a survey like this.

Possible contamination of the Pleiades candidates could come from reddened background giants, M-dwarfs and unresolved galaxies. The number of contaminating background giants in the surveyed area was estimated from the model by Bahcall & Soneira (1984) to be [FORMULA] 0.03, thus being negligible. The interstellar extinction adopted was [FORMULA] mag/kpc ([FORMULA] mag/kpc) (Lucke 1978; Cardelli et al. 1989) out to 2 kpc, then decreasing exponentially with the same scale-factor as for the giants, which is expected to be an upper limit of the possible extinction, and thus gives an upper limit of the number of contaminating giants. Due to the excellent seeing during the observing run it is believed that the contamination by galaxies on the stellar sample is negligible for [FORMULA].

The LF from Gould et al. (1996) was used to estimate the number of foreground M-dwarfs for [FORMULA] to [FORMULA]. Thus it is not likely, although possible, that NOT1 is a foreground M-dwarf. Since late M-dwarfs are presumed to be rare, it is interesting that a few candidates turn up in this survey, see Table 3. Both NOT2 and NOT3 appear quite close to the Pleiades zone in Fig. 2. Could they be low mass Pleiades BDs? NOT3 was in a part of the field that was also covered by Jameson & Skillen (1989). It was found in their original data, however far too faint for astrometry of sufficiently high accuracy for a proper motion measurement. NOT2 and NOT3 will though be observed in future runs.

Towards this part of the Pleiades the total reddening is [FORMULA] mag (Stauffer & Hartmann 1987), which corresponds to [FORMULA] mag (Cardelli et al. 1989). Thus if these stars really are M-dwarfs, [FORMULA] is less than 0.1 mag even for the most distant candidate and does not affect classification significantly.

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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