next previous
Up: CCD photometry of

5. Results

The selection of candidate cluster members was made by placing theoretical isochrones on colour-magnitude diagrams and selecting stars from their positions with respect to the isochrones. The theoretical isochrones of D'Antona & Mazzitelli (1994), and in particular those using Alexander, Rodgers & Iglesias opacities with the Canuto & Mazzetelli convection model, remain the most comprehensive for the low-mass stars with which we are concerned here. However, the calibrations used to transform the isochrones from the theoretical quantities of tex2html_wrap_inline1355 and tex2html_wrap_inline1357 to the observed colours and magnitudes are not well defined.

For 4000 K tex2html_wrap_inline1359 3500 K we used temperature scales and bolometric corrections from Bessell (1995) and for tex2html_wrap_inline1361, temperature scales and bolometric corrections of Kurucz computed by Wood & Bessell (private communication) which are available via anonymous ftp from mso.anu.edu.au. For cooler stars Stauffer et al. (1995) have made comparisons of a 70 Myr isochrone, using several different temperature scales and bolometric corrections, with known Pleiades members. The best agreement was achieved using temperature scales from Kirkpatrick et al. (1993) with bolometric corrections from Bessell (1991). Thus, for tex2html_wrap_inline1363, we have used temperature scales from Kirkpatrick et al. (1993) with bolometric corrections from the more recent paper by Bessell (1995).

Given the uncertainty in the literature regarding the age of the cluster, ranging from 8 Myr to 36 Myr, it was decided to use 10 and 40 Myr isochrones as the limits for selection of cluster members. The isochrones were transformed to allow for a distance modulus of 6.0 and reddening of EV-I=0.044 (Randich et al. 1995) and broadened to allow for the 0.2 mag uncertainty in the distance modulus. These limits were further broadened to allow for the photometric errors. These errors for field IC 2602a are shown in Table 2 (click here).

We have also taken into account the effect of binarity on the location of stars with respect to the theoretical isochrones. The size of the effect depends of the composition of the binary. We assume that if the unresolved companion has a lower mass and hence redder colour, it will have the effect of shifting the position to a brighter and redder position in the colour-magnitude diagram. For a companion of equal mass, the increase in brightness corresponds to 0.75 mag. However, Dabrowski & Beardsley (1977) have shown that the increase in magnitude in the case of some binaries is as large as 0.8 mag, so we have decreased the bright selection limit by 0.8 mag to allow for the presence of binaries. We note that the sequence of existing members from Prosser et al. (1996) show a width of tex2html_wrap_inline1369 mag. Given the increase in photometric error introduced though undersampling we feel that our broader selection criteria are justified.

   

Magnitude range Err(V) Err(V-R) Err(V-I)
12<V<13 0.02 0.04 0.03
13<V<14 0.03 0.06 0.04
14<V<15 0.04 0.08 0.05
15<V<16 0.05 0.10 0.06
16<V<17 0.07 0.15 0.10
17<V<18 0.08 0.17 0.12
18<V<19 0.13 0.14 0.13
19<V<20 0.13 0.14 0.13
Table 2: Photometric errors for field IC 2602a

Primary candidate members of the cluster were those stars which were located between the selection limits in both (V, V-I) and (R, R-I) colour-magnitude diagrams. Stars were selected as secondary candidate members if they fell between the limits in one or other of the diagrams but not both. Closer inspection of the secondary candidate members reveals that many of them are very unlikely to be true cluster members, having colours that place them far from the selection limits in the non-selecting diagram.

A V vs. V-I colour-magnitude diagram for field IC 2602a can be seen in Fig. 2 (click here), for the second cluster field (IC 2602b) in Fig. 3 (click here) and for the "offset'' field in Fig. 4 (click here).

  figure324
Figure 2: A V vs. V-I colour magnitude diagram for the first cluster field showing the stars selected as primary (filled circles) and secondary (open circles) candidate cluster members. The solid lines are 10 Myr and 40 Myr isochrones, the dashed lines show the selection limits including all sources of uncertainty discussed in the text, and the dotted line shows the bright selection limit before any allowance was made for binarity

  figure333
Figure 3: A V vs. V-I colour magnitude diagram for the second cluster field with symbols as per Fig. 2 (click here)

  figure338
Figure 4: A V vs. V-I colour magnitude diagram for the offset field with symbols as per Fig. 2 (click here)

  figure343
Figure 5: Instrumental r vs. instrumental tex2html_wrap_inline1419 for cluster field IC 2602a. The solid dots are the photometrically selected primary candidate members for that field

  figure348
Figure 6: Instrumental r vs. instrumental tex2html_wrap_inline1419 for cluster field IC 2602b. The solid dots are the photometrically selected primary candidate members for that field

It is clear from the colour-magnitude diagrams that the number of stars increases dramatically for V>18.5 in field IC 2602b, due to an increased scatter from the main field population. In view of this increased level of contamination we will consider only the primary candidate members with tex2html_wrap_inline1427. In field IC 2602a we detect 45 primary candidate members. In field IC 2602b we detect 33 primary candidate members. The primary candidate cluster members are listed in Table 3gif.



1 The results of the tex2html_wrap_inline1201 data are shown in Fig. 5 (click here), as a plot of R minus an instrumental tex2html_wrap_inline1201 magnitude (with an arbitrary zero-point) versus R for field IC 2602a. The figure also shows the position of our photometrically selected primary candidate members. Figure 6 (click here) shows a similar diagram for field IC 2602b.

Photometry of all the stars observed can be obtained electronically from the Armagh Observatory WWW server (http://star.arm.ac.uk/tex2html_wrap_inline1437dcf/ic2602.html) or by anonymous ftp upon request.


next previous
Up: CCD photometry of

Copyright by the European Southern Observatory (ESO)
web@ed-phys.fr