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 and
to the observed colours and magnitudes are not well
defined.
For 4000 K 3500 K we used temperature scales and
bolometric corrections from Bessell (1995) and for
, 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
, 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 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 |
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).
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
Figure 3: A V vs. V-I colour magnitude diagram for the second cluster
field with symbols as per Fig. 2 (click here)
Figure 4: A V vs. V-I colour magnitude diagram for the offset field
with symbols as per Fig. 2 (click here)
Figure 5: Instrumental r vs. instrumental for cluster
field IC 2602a. The solid dots are the photometrically selected
primary candidate members for that field
Figure 6: Instrumental r vs. instrumental 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 .
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 3
.
1
The results of the data are shown in Fig. 5 (click here),
as a plot of R minus an instrumental
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/dcf/ic2602.html) or by
anonymous ftp upon request.