6 Comparison with X-ray observations

  Preibisch et al. (1996) detected 116 X-ray sources in deep pointings with the ROSAT Position Sensitive Proportional Counter (PSPC) and the ROSAT High Resolution Imager (HRI). From these 116 sources we identified 26 with optical counterparts from our sample of 1431 objects using a search radius of 5arcsec. By increasing the search radius to 10arcsec and 15arcsec, respectively we found additional 8+8 identifications, with one identification of the same star in the proper motion sample with two different X-ray sources. The relatively small number of optical counterparts compared to the identification of Preibisch et al. (1996) is due to the selection criteria for our sample of starlike objects (Sect. 2.3).

The 26 X-ray sources identified within 5arcsec with optical sources show a strong concentration in the proper motion diagram (see Fig. 10) at $\mu_x=2.9 (2.9)$mas/yr and $\mu_y=-8.6 (2.7)$mas/yr (mean values with their dispersions given in parentheses). These mean values and dispersions are in good agreement with those obtained for the cluster stars in our membership determination (see group A in Table 4) and with the mean proper motion and dispersion of the Per OB 2 association (Sect. 5.4).

Most of the X-ray sources identified with optical sources are within the $20\hbox{$^\prime$}\times 20\hbox{$^\prime$}$ field (see Fig. 11). All X-ray sources within this small field were classified as cluster members by Preibisch et al. (1996).

Note that the scale in Fig. 10 is the same as in Fig. 9 but the start and end values of the two plots are different. Figure 9 does not show the proper motion dispersion of cluster and distant field stars but the highly probable members of both distributions ($P_{\rm cl} \gt 90$% and $P_{\rm df} \gt 90$%).

The distribution of the membership probabilities $P_{\rm cl}$ (dotted line) and $P_{\rm df}$ (dashed line) for the 26 X-ray sources identified within 5arcsec is shown in Fig. 12. There are no foreground stars ($P_{\rm cl} < 5$% and $P_{\rm df} < 5$%) among the 26 X-ray sources. Their cluster membership is more probable than a membership in the group of distant field stars.

  

Figure 10: Proper motions of starlike objects (from our sample of 1431 objects) identified with X-ray sources from Preibisch et al. (1996). Small circles show the 26 objects found within 5arcsec search radius, additional identifications by increasing the search radius to 10arcsec (8 objects) and 15arcsec (8 objects), respectively are shown by + and $\times$. Two close X-ray sources were identified with the same object in the proper motion sample

  

Figure 11: The one square degree field around IC348 with all X-ray sources from Preibisch et al. (1996) identified with optical sources from our sample of 1431 stars. The dashed line shows the main field of the ROSAT PSPC observations, open circles show the 26 objects found within 5arcsec search radius, additional identifications by increasing the search radius to 10arcsec (8 objects) and 15arcsec (8 objects), respectively are shown by + and $\times$. All X-ray sources within the small $20\hbox{$^\prime$}\times 20\hbox{$^\prime$}$ field (dotted square) were classified as members of IC348 by Preibisch et al. (1996)

  

Figure 12: Histogram of membership probabilities $P_{\rm cl}$ (dotted line) and $P_{\rm df}$ (dashed line) for the 26 X-ray sources identified within 5arcsec with optical sources from our proper motion survey