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

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3. Analysis

3.1. Extraction technique

Several different kinds of errors define the zone of potential Pleiades members in the colour magnitude diagram. The maximum error in the absolute magnitude of a single Pleiades member is not likely to exceed 0.2 mag because of uncertainty of its distance. The photometric error for [FORMULA] is less than 0.1 mag. For binaries, the worst case is considered. Two identical companions get 0.75 mag brighter than a single star of the same colour. Therefore the bright edge is raised by 0.75 mag. The age of the Pleiades is also a source of error. Basri et al. (1996) argue that the age is [FORMULA] 115 Myr while the "canonical" age deduced from the upper main sequence turn-off is 75 Myr. To account for the worst possible case, the 70 Myr sequence -1.05 mag defines the bright limit, and the 120 Myr model +0.30 mag defines the faint limit. An extension of the model colours in Chabrier et al. (1996) based on the Brett (1995) models was kindly provided by Plez (1996). Next generation models from Allard et al. (1996) were also included. Both models show excellent agreement with the data (Fig. 1 and 2).

[FIGURE] Fig. 1. Colour-magnitude diagram for all stars detected in both I and J. NOT1 is a Pleiades candidate and other NOT stars are probable background M-dwarfs. Ticks ([FORMULA]): 0.035, 0.045, 0.055, 0.060, 0.070, 0.075, 0.078, 0.080, 0.090, 0.100, 0.125, 0.150, 0.200 (Allard) and 0.045, 0.060, 0.070, 0.075, 0.080, 0.090, 0.100, 0.125, 0.150, 0.200, 0.220 (Brett,Plez). The two field stars are BRI0021-0214 ([FORMULA] M9.5V) and RG0050-2722 (M8V), reduced in the same way as the program fields. The models and the two field stars have been shifted to the distance and general reddening of the Pleiades.
[FIGURE] Fig. 2. All stars detected in both I and K. Symbols are the same as in Fig. 1.

Note the clear gap between the background field stars and the Pleiades sequence and that all previously known members within the field were detected with this technique.

3.2. Extracted objects

As shown in Fig. 1 and 2, 6 objects in the sample ([FORMULA]) can be regarded as potential cluster members (Table 1). Of these, 5 were previously known as proper motion members (Hambley et al. 1993; Rebolo et al. 1995). The new candidate (NOT1) is very close to a much brighter star, discovered only as a consequence of the excellent seeing conditions during the observations. A finding chart and coordinates of NOT1 are given in Fig. 4. Its colours and magnitude indicate a binary of equally sized components, each of [FORMULA], slightly heavier than PPL15 (Basri et al. 1996). It is, however, also consistent with being a foreground M-dwarf (see Sect. 4).

[FIGURE] Fig. 3. Comparing LF predicted for four different IMF-indices (bars) and five recent surveys (curves). All LFs were normalized to 10 counts in [FORMULA]. Indicated are also approximate completeness limits for the surveys. Jameson & Skillen (1989) (J89) is only indicated for completeness limit, since all of their candidates fall 1.5 - 2 mag below the Pleiades sequence.
[FIGURE] Fig. 4. Finding chart for the Pleiades candidate NOT1 (RA [FORMULA], Dec [FORMULA] (J2000.0, Epoch 1995.9), errors [FORMULA] in both coordinates). The size of the field is [FORMULA] 2'x2'. North is down. East is right.

The last two stars in Table 1 were previously regarded as Pleiades candidates from RI photometry (Jameson & Skillen 1989), but can be judged as clear nonmembers from Fig. 1, which is the same result as Zapatero Osorio et al. (1996) got from their RI survey.

3.3. Comparison with other works

Several papers have recently been published on the Pleiades. The proper motion survey by Hambley et al. (1993), using POSS and UKSTO Schmidt plates, covered most of the cluster, complete to [FORMULA], corresponding to a star of 0.10 [FORMULA], slightly above the hydrogen burning limit. Their limiting magnitude was [FORMULA] mag fainter and they reached objects of [FORMULA] 0.08 [FORMULA], right at the BD limit.

Jameson & Skillen (1989) imaged 175 [FORMULA] in RI. Some of their fields overlap with ours and after checking their original data, we concluded that their completeness limit is [FORMULA] and [FORMULA]. Their R limit corresponds to [FORMULA] on the Pleiades sequence. They found 9 BD candidates, all of which later have been shown to fall significantly below the Pleiades sequence in colour magnitude diagrams (Zapatero Osorio et al. 1996). Rebolo et al. (1995) announced the discovery of a BD (Teide1) in the Pleiades as a result of a CCD survey, 175 [FORMULA]. This survey was later extended to 578 [FORMULA], complete to [FORMULA], and another candidate (Calar3) was found (Zapatero Osorio et al. 1996). Both candidates have passed all membership tests hitherto, including the lithium test (Rebolo et al . 1996) and can be regarded as genuine BDs. Stauffer et al. (1994) surveyed [FORMULA] 1500 [FORMULA]. Their survey is complete to [FORMULA] corresponding to [FORMULA] in the Pleiades domain of the V vs [FORMULA] diagram. They found 15 very red Pleiades candidates, among them PPL15, which for a cluster age of 75 Myr would be of mass [FORMULA] [FORMULA]. However, Basri et al. (1996) observed PPL15 at Keck and suggested, based on the lithium line strength, that its age is around 115 Myr, thus increasing the mass from 0.065 [FORMULA] to 0.077 [FORMULA]. Basri also concludes, based on the non-detection of lithium in the BD candidate HHJ3 (Hambley et al. (1993)), that it cannot be younger than 110 Myr. Thus the Pleiades age is confined approximatively between 110 Myr and 125 Myr. Simons & Becklin (1992) used I vs [FORMULA] diagrams and compared fields within the cluster to reference fields outside the cluster. They found an excess of [FORMULA] BD candidates in 200 [FORMULA] in the cluster field. Williams et al. (1996) found 8 stars in the magnitude interval [FORMULA], using V and K instead of IJK, a result consistent with ours. Steele et al. (1993) present [FORMULA] candidates with IJK photometry in the same magnitude range, a subsample from Hambley et al. (1993), complete to [FORMULA] and therefore not included in Table 2. All surveys except Simons & Becklin (1992) agree reasonably well within error bars.

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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