4. High proper motion stars in STARNET
4.1. Verification of high proper motions
Prior to the physical interpretation of the large proper motions of a number of stars, these proper motions have to be carefully investigated. As mentioned, STARNET proper motions are derived from AC and GSC. For a certain number of stars in STARNET misidentifications of their positions on the AC and/or GSC plates can occur because of the large ( years) epoch difference. We decided to cross-check large proper motions either by comparison with the proper motion given in another catalogue, if available, or by testing the star's position on the plates of a third epoch. This is by no means an easy task, because accurate astrometric data in this magnitude range are rare.
Table 4 summarizes the stars with large proper motions from Tables 1, 2 and 3. The sample can be split into stars with proper motions close to the mean proper motion of the Pleiades cluster ( mas/y, mas/y) and those with proper motions randomly far off the mean motion of Tau-Aur. The proper motions of 15 stars in Table 4 could be confirmed by comparison with their positions on the digitized POSS I plates or by independent proper motion measurements. This is indicated by a -sign. The digitized POSS I is well suited to check the erroneously large proper motions because in this case there should be a large, easily detectable offset from the expected position. This turned out to be the case for the 6 remaining stars; their proper motions seem to be erroneous in STARNET
Table 4. Stars with conspicuous proper motions in our investigation. The last column indicates the result of the proper motion check via POSS I: a -sign indicates that the proper motion could be confirmed, a ?-sign indicates a doubtful proper motion.
4.2. Pleiades membership
In the proper motion plot (Fig. 2) we find a secondary crowding of stars with proper motions similar to that of the Pleiades. Indeed the star NTTS 034903+2431 is found to match both photometric and proper motion membership criteria of the Pleiades cluster by Schilbach et al. (1995, their star No. 36000) and classified as highly probable Pleiades member. SAO 76411 A, on the other hand, corresponds to the star Pels 178, which was included in the photometric investigation of the Pleiades cluster by van Leeuwen et al. (1986) and classified as non-member.
Fig. 6 shows the distribution on the sky of the stars from the upper part of Table 4 with the exception of SAO 76411 A. The direction to the centre of the Pleiades cluster is indicated by a filled circle. The arrows show the proper motions of the stars from Table 4 relative to the mean motion of the Pleiades in PPM and STARNET. Two stars close to the centre of the Pleiades have small motions relative to the Pleiades cluster; one of them is the star classified as highly probable proper motion member by Schilbach et al. (1995). Three stars about away from the Pleiades centre show no relative motion and should be checked further photometrically for membership. Finally, the stars RXJ 0409.2+2901, RXJ 0439.4+3332A, RXJ 0448.0+0738 and HD 287017 could have been ejected from the Pleiades a few million years ago. This is consistent with the findings of Kroupa (1995), who expects from the result of numerical N-body simulations of star clusters that during the lifetime of a cluster a certain fraction of stars can be ejected with velocites up to 100 km/s due to close encounters between binary systems. The ejection rate is estimated to be higher in the earlier phases of cluster evolution.
© European Southern Observatory (ESO) 1997
Online publication: April 28, 1998