6. Discussion

6.1. Background contamination

Given the low galactic latitude of IC 2602 (b=-4.9) it is to be expected that the stars selected using theoretical isochrones will be contaminated to a large extent by background stars. In order to gauge the scale of this contamination a similar selection process was applied to the photometry of the "offset'' field. The selection process yielded 43 "primary candidate members'' with V<18.5.

It should be noted that in using the offset field to gauge the background contamination we are assuming that the distribution of stars in it is representative of the background stars to be found in the cluster fields. This comparison suggests that nearly all of the stars selected are background stars. However, see the discussion in Sect. 6.2 (click here) for further comments on the location of the offset field.

6.2. Comparison with the results of Randich et al.

  In their study of IC 2602 using the ROSAT X-ray satellite, Randich et al (1995) detected 110 X-ray sources in an area, 68 of which they identify with at least one optical counterpart. 4 of these X-ray sources lie in our field IC 2602a: R64, R69, R76, R80. No detected X-ray sources lie in our second cluster field. One X-ray source, R25, lies in our offset field. Randich et al. identify 7 stars as possible optical counterparts to these X-ray sources (6 in field IC 2602a, 1 in our offset field) based on photometrically selected cluster members and additional bright stars located close to the X-ray position.

Table 4 (click here) shows the results of our photometry for the stars which Randich et al. indicate as optical counterparts. Photometry for the X-ray selected stars is taken from Prosser et al. (1996) where available. The X-ray source R80 is clearly identified with the bright star HD 308016 whose magnitude of V=10.66 places it beyond the bright limit of our photometry.

   

ID V V-R V-I Memb.

R25 16.38 -- 2.75 Y

of-6078 16.15 1.00 2.67 Y

R64 16.53 -- 2.51 Y?

F41 16.66 0.93 2.62 Y

R69A 18.0 -- 5.40 ?

f1-514 16.71 2.08 5.12 N

R69B 14.12 -- 2.36 N

f1-10 14.20 1.11 2.42 N

R69C 17.07 -- 2.87 Y

f1-738 17.16 1.04 2.94 Y?

R76 16.81 -- 2.97 Y?

F71 16.81 1.07 2.88 Y

Comparing the remaining 6 stars, there are differences in both the V magnitude and the V-I colours between the two sets of photometry somewhat larger than the errors in Table 2 (click here). Given that Randich et al.'s objects are active stars, however, they are quite likely to be variable. van Leeuwen et al. (1987) have found variable stars in the Pleiades with amplitudes of up to . Walter et al. (1992) have recorded variability on a naked T-Tauri star of amplitude , . Thus a part of the discrepancy at least might be reasonably attributed to such variability. Since the majority of Randich et al.'s photometry is unpublished we are unable to make a more detailed comparison between the photometric datasets at this point.

The positions of the detected X-ray sources in the region of IC 2602 are shown in Fig. 7 (click here). The figure also shows the positions of our observed fields. It should be noted that there are no X-ray sources in the the field IC 2602b and that there are relatively few in the region surrounding it. This lack of X-ray sources is consistent with the lack of primary candidate members found from the photometry of that field. It should also be noted that the offset field contains one X-ray source, R25. Both our photometry and that of Randich et al. indicated that this star is a probable cluster member. If this star is a true cluster member then the offset field may be positioned too close to the cluster to give an accurate indication of the background contamination. If this is the case then we are over-estimating the level of background contamination as the "primary candidate members'' will contain real cluster members as well as background stars which lie in the appropriate areas of the colour-magnitude diagrams.

  
Figure 7: The region of IC 2602 showing X-ray sources corresponding to photometric cluster members (filled circles) and non-members (open circles) from Randich et al. (1995)

6.3. Comparison with the Pleiades

The Pleiades, by virtue of its proximity, is one of the most extensively studied young (age Myr) open clusters. Hambly & Jameson (1991) have studied its mass-distribution and luminosity function. We may use their results to independently examine the effects of background contamination on our data.

By taking the star numbers for the inner radius of the Pleiades as an approximate model for our region of IC 2602, and scaling to allow for the different distance moduli and angular extents of the two clusters, we should see roughly 22 cluster members in our cluster fields. Insofar as this comparison is valid, this suggests that the level of contamination is less than that suggested by the number of "primary candidate members'' located in the offset field, assuming that the two clusters have similar star densities and mass functions. We have made no allowances for the differences in "richness'' between the two clusters.

In summary, the evidence suggests that the level of contamination due to background stars lies somewhere between 50%, as suggested by Pleiades mass functions, and 73% as suggested by comparing the "offset'' field with field IC 2602a, although it is likely to be well below the latter figure, given that the "offset'' field appears to be located within the cluster. Such levels of contamination might well be expected given the location of the cluster, and the broad selection limits used.

6.4. Luminosity function

In Fig. 8 (click here), which shows the distribution of primary candidate members with V magnitude, several features are apparent. Firstly, for 12<V<14 there is a clear excess of cluster members in field IC 2602a in comparison with cluster field IC 2602b and the offset field. This excess is not apparent in the range 14<V<16 where an effectively similar number of primary candidate members was selected in each cluster field. A larger number of stars were selected in the offset field in this magnitude range. In each colour-manitude diagram there is an obvious field-giant branch with 1.4<V-I<1.9. This may be a cause of contamination in the candidate list for V<14.5, but a reason why this should be worse in the offset field in comparison with the other two fields is unclear. There is a sharp falloff in stars in all fields in the range 16<V<19. Because of the likely high level of field contamination, further dicussion of the cluster luminosity function is not appropriate at this time.

  
Figure 8: The distribution of primary candidate members in IC 2602a ( solid line), IC 2602b (dashed line) and the offset field (dotted line) with magnitude

6.5. brightness

Given that the filter had a passband of 70 Å, that the equivalent width of in an active late-M dwarf is Å and that the equivalent width in absorption for an inactive M dwarf is Å, we would expect a magnitude difference between active and inactive stars of mag, assuming that cluster late-type stars are similar to solar neighbourhood M dwarfs. This magnitude difference is comparable to both the scatter in the plot and the errors in . Thus, we were unable to make any further selection on the basis of the magnitudes.

We were able to determine magnitudes for 20 of the 45 photometrically selected stars in cluster field IC 2602a (the rest being too bright). 13 of the 20 stars lie to the bright side of the mean level, 7 of these more than from the mean level ( Å). Of the stars that lie below the mean, none are more than from the mean. Similarly, for cluster field IC 2602b we have determined magnitudes for 18 of the 33 primary candidate members. The scatter in the data is much larger ( Å), and so only two stars lie more than from the mean, both in emission. The data for the primary candidate members are shown in Table 5 (click here).

 

ID R ID R

F1 14.55 -6.79 -0.36 F36 14.78 -6.13 2.92

F2 14.22 -6.74 2.92 F38 14.47 -6.24 -4.11

F4 14.34 -6.82 -2.26 F41 15.73 -5.83 25.25

F6 13.70 -6.87 -3.50 F42 17.01 -5.92 19.30

F7 14.25 -6.81 -1.63 F46 16.12 -5.88 20.96

F8 17.02 -6.85 -7.65 F49 14.46 -6.23 -3.50

F9 14.88 -6.72 3.60 F53 14.79 -6.16 0.93

F10 14.55 -6.80 -1.00 F54 16.92 -5.91 20.13

F13 14.49 -6.80 -1.00 F57 16.89 -5.98 14.50

F14 13.80 -6.87 -3.50 F59 14.17 -6.18 -0.36

F16 14.95 -6.74 2.92 F60 15.09 -6.10 4.28

F18 14.27 -6.65 9.22 F61 16.88 -5.79 28.82

F19 16.48 -6.40 24.37 F62 15.00 -6.18 -0.36

F20 14.41 -6.59 13.72 F64 14.53 -6.19 -1.00

F22 13.97 -6.68 9.22 F66 14.45 -6.22 -2.88

F24 14.14 -6.81 -1.63 F67 14.09 -6.15 2.25

F30 14.04 -6.78 2.25 F69 14.67 -6.16 0.93

F32 14.23 -6.66 8.50 F70 14.06 -6.12 4.28

F34 13.58 -6.24 -3.50 F71 15.74 -5.84 24.37

 

For field IC 2602a the data reinforces the likelyhood of a significant fraction of the photometrically selected star being true cluster members; 35% of those stars with magnitudes being well in emission. For field IC 2602b the results are also consistent with the reduced number of photometrically selected stars, and the lack of any X-ray detections in that field.