It is a standard technique to fit theoretical isochrones to the observed
colour-magnitude diagrams and to select candidate cluster members from
the relative positions of the stars in the former. An evolutionary age
of 36 Myr has previously
been determined for IC 2391 by applying this technique to the photometry
of the early-type members. Assuming that coeval star formation has occurred,
then many of the low-mass members will still be contracting towards the
zero age main-sequence (ZAMS) and so it is important to use
pre-main-sequence isochrones in any analysis. The most comprehensive and
up-to-date computation of evolutionary tracks for low-mass stars
(
) are those of D'Antona & Mazzitelli (1994). We have used
the set of pre-main-sequence evolutions that have been modelled using the
Alexander opacities (Alexander et al. 1989) and the mixing length treatment
of Canuto & Mazzitelli (1990), as
D'Antona & Mazzitelli have shown
these to be in good agreement with observations of nearby M-dwarfs.
However, relating observed quantities such as V,B-V and R,R-I to the
stellar parameters 
and 
is a non-trivial
problem. Temperature calibrations have, in many instances, been determined
for a particular range of spectral types and these different calibrations are
not necessarily consistent. For example, Johnson (1966) derived a temperature
calibration for early-type stars; Mould & Hyland (1976) for K-stars;
Bessell
(1991) for late-K to mid-M spectral types; and
Reid & Gilmore (1984)
for M-stars.
In order to ensure some consistency, we have transformed the theoretical
isochrones of D'Antona & Mazzitelli (1994) to observed colours and
magnitudes as follows. For the late-K and M-stars (0.53<(R-I)<2.38), we
have converted 
to (R-I) and 
to MR
using the revised relations of Bessell (1995). Additionally, the empirical
relations of Caldwell et al. (1993) were used to transform (R-I) colour
to (B-V). For the earlier spectral types (late-F to late-K), we used the
theoretical colours and bolometric corrections of Kurucz computed by
Wood & Bessell (private communication) and which are available via
anonymous ftp from mso.anu.edu.au. This temperature calibration is quite
similar to the IR flux method temperature scale of
Blackwell & Lynas-Gray (1994).
In the photometric analysis we have assumed an evolutionary age of 36 Myr
for IC 2391 (Lynga 1987) and a distance modulus, (m-M)=6.05, which has
been derived using both photometric and spectroscopic techniques
(Becker
& Fenkart 1974). Adopting these values, we transformed a 36 Myr theoretical
isochrone to fit the different combinations of CMDs and reddened these by
the appropriate amount corresponding to a reddening value of E(B-V)=0.04
(Becker & Fenkart 1974).
In order to implement selection criteria for cluster membership, it is
necessary however to consider the various sources of error in matching the
observations to the theoretical isochrones. These include an uncertainty
of
10 Myr in the cluster age, based on the dispersion of
measurements found in the literature
0.2 magnitudes in the distance modulus (Becker & Fenkart 1974)
magnitudes, we adopted an
upper limit of E(B-V)=0.06 for the bright error limits. However, for the
faint error limits we have considered the possibility of negligible reddening
and have adopted a value of E(B-V)=0.00.
Candidate membership of the open cluster IC 2391 was based on the positions
of stars in the (V,B-V) and (R,R-I) CMDs. We also consider the effect
of binarity on the location of the theoretical isochrones (see, for example,
Dabrowski & Beardsley 1977). Such an effect will obviously depend on the
frequency of binaries and on the distribution of their mass ratios. However,
assuming that an undetected companion has a lower mass and hence redder
colour, its presence will cause its position to be shifted upwards in
brightness and redwards in colour. Hence, the maximum increase in
brightness allowing for a companion of equal mass would correspond to
0.75 magnitudes. This effect has been included in our bright error limit,
and possible binary members were identified if they were situated between
the single star, bright limit and the bright error limit. Objects were
selected as possible members of IC 2391 if they were situated between
the error limits as defined above. An object was then deemed a candidate
member of IC 2391 if it fulfilled the selection criteria for both
the (V,B-V) and (R,R-I) diagrams.
CCD photometry was determined for 1303 objects in the
field of IC 2391. Using the afore-mentioned selection criteria, 100 objects
were identified as being possible cluster members, of which 83 satisfied the
constraint for the (V,B-V) CMD and 34 satisfied the constraint for
the (R,R-I) CMD respectively. A subset of these objects (16 of the 83
and 10 of the 34) were located within the binary envelopes of the respective
CMDs. Seventeen objects satisfied both selection criteria and have been
classified as candidate members; these exhibit a range of colours
between approximately 0.4
(R-I)1.7 which corresponds to
spectral types between G8V and M4V. Identification of main-sequence
members of earlier spectral types was not possible, as these stars
were saturated on the CCD frames. The photometry and sky charts for
all 17 candidate members are presented in Table 4 (click here) and Fig. 4 (click here) respectively.
Figure 2: a) The (V, B-V) and b) (R, R-I)
colour-magnitude diagrams for the new photometric dataset. The solid line
corresponds to the locus of a reddened 36 Myr isochrone and the dotted lines
represent our estimates of upper and lower error limits for single stars
having membership of IC 2391. The dashed line indicates the bright limit
which accounts for the effect of binarity. Filled circles correspond to the
candidate members identified using the selection criteria described in
Sect. 5.3
Figure 3: The (R-I, B-V) colour-colour diagram for the subset of stars
that have been selected from the individual (V, B-V) and (R, R-I)
CMDs using the criteria discussed in Sect. 5.3. Objects that satisfied
both membership criteria are shown as filled circles. The solid line
corresponds to the reddened 36 Myr isochrone
Figure 4: Identification charts for the candidate members of IC 2391
For a further 125 objects, it was only possible to determine two-colour photometry in either BV or RI. In many cases, this was the consequence of their faintness and of the different limiting colour sensitivities. Additionally, some of the stellar profiles were severely contaminated by under-lying bad pixels or cosmic ray events. However, two of these objects (1316 & 1428) were identified as having possible cluster membership (see Table 4 (click here)).
| Id. |  |  | V | B-V | V-R | V-I |
| (J2000.0) | ||||||
| 162 | 08 40 05.68 | -53 05 59.971 | 13.065 | 1.252 | 0.677 | 1.329 |
| 302 | 08 40 31.18 | -53 11 44.415 | 14.605 | 1.384 | 0.724 | 1.544 |
| 311 | 08 40 32.50 | -53 10 32.571 | 12.295 | 0.902 | 0.466 | 0.980 |
| 314 | 08 40 32.99 | -53 10 29.804 | 13.217 | 1.099 | 0.646 | 1.276 |
| 348 | 08 40 38.40 | -53 06 29.594 | 14.405 | 1.310 | 0.792 | 1.570 |
| 362 | 08 40 39.75 | -52 45 58.903 | 12.700 | 1.265 | 0.711 | 1.377 |
| 448 | 08 40 52.26 | -52 46 46.917 | 14.426 | 1.316 | 0.798 | 1.566 |
| 480 | 08 40 53.89 | -52 42 14.523 | 14.142 | 1.267 | 0.719 | 1.510 |
| 581 | 08 40 59.89 | -52 44 27.479 | 13.752 | 1.386 | 0.722 | 1.524 |
| 586 | 08 40 59.99 | -52 44 21.420 | 13.572 | 1.267 | 0.691 | 1.470 |
| 711 | 08 41 06.96 | -52 51 04.148 | 13.880 | 1.237 | 0.705 | 1.385 |
| 729 | 08 41 08.02 | -53 00 04.475 | 17.955 | 1.591 | 1.357 | 3.097 |
| 872 | 08 41 15.40 | -53 00 14.300 | 13.040 | 1.050 | 0.591 | 1.182 |
| 950 | 08 41 18.30 | -52 47 52.839 | 14.045 | 1.463 | 0.856 | 1.617 |
| 955 | 08 41 18.65 | -52 58 54.409 | 18.718 | 1.460 | 1.312 | 3.005 |
| 1056 | 08 41 23.10 | -52 59 57.538 | 13.657 | 1.391 | 0.747 | 1.503 |
| 1144 | 08 41 26.57 | -53 04 26.286 | 14.401 | 1.463 | 0.760 | 1.602 |
| Id. | | | V | B-V | ||
| 1316 | 08 40 25.78 | -53 11 15.093 | 18.654 | 1.641 | ||
| Id. | | | R | R-I | ||
| 1428 | 08 41 39.62 | -52 59 34.208 | 12.613 | 0.780 | ||
The procedure discussed above for identifying cluster members is essentially the same as examining the (R-I, B-V) colour-colour diagram (see Stauffer et al. 1989). In Fig. 3 (click here), we have plotted this diagram for the subset of stars that have been selected from the independent CMDs. It is clear that many of the stars with B-V values redder than 1.5 and with R-I values less than 1.2 can be excluded from further consideration. These objects probably correspond to the reddened background giant population. However, stars that have B-V values bluer than 1.2 fall near the theoretical locus and even the two-colour diagram does not serve as a strong membership criterion. Hence, there may be a high contamination factor due to background objects in our candidate membership list. Unfortunately, due to the small image area of the RCA CCD, it would not have been observationally feasible to photometer a large number of offset fields in order to estimate the background contamination. Therefore, in the following section, we shall attempt to compare our results with the number density of low-mass objects found in the Pleiades.
Although the total number of identified candidate members is small, it
should be noted that approximately 25% of these objects are located
within the limits for binarity, which is compatible with the binary
frequency found in the Pleiades (Bettis 1975).
In recent years, many studies have been directed at the Pleiades in an
attempt to determine the luminosity and mass functions (see, for example,
Stauffer et al. 1991b; Hambly et al. 1991b;
Schilbach et al. 1995).
Unfortunately, this photometric study does not permit the construction of
a luminosity function for IC 2391 (due to the small number of members that
have been found). However, it is worthwhile to consider the luminosity
function that is currently accepted for the Pleiades and to estimate what
the total number of members to be expected in the area of sky (0.06 sq.
degrees) that was photometered for IC 2391.
Stauffer et al. (1991b) have identified 369 Pleiads in the magnitude range,
, distributed across 16 square degrees of sky.
This magnitude range is similiar to that obtained in this study of IC 2391.
As the Pleiades and IC 2391 open clusters have different distance moduli,
we have applied a scaling factor in our calculation to account for the
observed differences in their spatial extent. We have adopted angular
diameters that have been based on the distribution of the early-type
members in each cluster (Lynga 1987), and based on these observations made
for the Pleiades field, approximately seven members are expected to be
found in our sample for IC 2391.
However, Pleiads identified by Stauffer et al. (1991b) are
distributed over a large area of sky, whereas our IC 2391 photometry has
been obtained close to the cluster core where the star density may be
expected to be greater. In fact, Hambly et al. (1991a) presented membership
numbers as a function of radius which show that this scenario is true for
the Pleiades. We have taken the star numbers for the inner 0.6
radius of the Pleiades, and estimate that approximately 24 members are to be
expected in our sky sample, which is the same order of magnitude as the
number of selected candidate members. These statistics would suggest that
the background contamination of candidates is not severe. However, it
should be noted that these estimates implicitly assume that both IC 2391
and the Pleiades have similar star densities and mass functions, an
assumption which we can not comment further upon here.
From an inspection of the
CMDs (see Fig. 2 (click here)), it would appear that very few members are to be found
for spectral types later than M0V in IC 2391. This observation may
be the result of having identified a small sample of candidate members and
furthermore, a greater contamination of background objects may exist
at GK-spectral types for the reasons discussed in Sect. 5.3. However,
Foster et al. (1996) observed a sharp decline of cluster members in
IC 2602 (
Myr) at a spectral type of about M4V
(
). This dataset contained photometry of an offset
field and this clearly showed that the selection criteria identified
a significant excess number of stars in the cluster field compared with
the former. Foster et al. were confident that the contamination of the
selected possible members due to background objects was small, implying
that this observed decline in the late-type members is real.
Intermediate-resolution spectra of our candidates would enable us to
obtain spectral classifications which, combined with the derived radial
velocities, would permit confirmation of cluster membership.
Reid & Hawley (1996) have found a similar result in the old
open cluster, M 67. In this case they have attributed the effect to
dynamical "boiling-off'' of the lower mass cluster members due to
gravitational interaction of the cluster with passing massive
objects. IC 2391, however, is too young for this mechanism to have
operated.
Previously, only one investigation has been directed at the lower
main-sequence of IC 2391.
Stauffer et al. (1989) identified 10 GKM
pre-main-sequence stars using both photometric and spectroscopic
techniques. However, we did not obtain photometric measurements for any
of these objects. This is a result of several factors. First, our brighter
magnitude limit is 
2 magnitudes fainter than that of Stauffer et
al. Secondly, we avoided fields within several arcminutes of the bright
stars (
) as their reflected starlight contaminated the CCD frame, and
thirdly, our sky coverage was severely curtailed by bad weather experienced
throughout the run. Therefore, a direct comparison is not possible.