Astron. Astrophys. 333, 524-530 (1998)

3. New Colour-Magnitude diagrams for four target clusters

CM diagram of clusters often do not permit a reliable estimate of the distance and age of a cluster. If the age is wrong, the chances of discovering  Scuti stars could be much smaller, than what one estimates based on the assumed age. If the distance (or reddening) is wrong, the magnitude of the  Scuti stars will not be as assumed and the exposure time not optimally chosen.

To illustrate what can be achieved by CCD colour photometry, we present the results for four clusters (also included in The book); see Viskum et al. (1997a) for more examples. The previous CM diagrams were not adequate (Fig. 1), mostly because the photoelectric photometry did not go faint enough. Photographic data generally is of low precision and not very useful, when it exists. In many other cases it is necessary to extend the dynamic range and the accuracy by new, modern observations.

Fig. 1. Colour-Magnitude diagram for NGC 1513, diamonds are data from the Open Cluster Database and crosses are CCD data from this investigation. Some stars are present in both sets and occur twice in the diagram

3.1. Two northern clusters

The observations were made in 1991 by Hans Kjeldsen at the Nordic Optical Telescope (ORM, La Palma). The Tek512 chip on the Aarhus-Tromso Low Dispersion Spectrograph (LDS) was used to take CCD frames in Strömgren b and y. Using Johnsson B and V data (Mermilliod, 1994) we have transformed the index to the Johnsson index, using a linear fit for stars that are present in both datasets.

3.1.1. NGC 1513

New data on this cluster (Fig. 1) extend the main sequence by two magnitudes. An impression of a fairly narrow main sequence is reinforced by the new data with some outliers (binaries, non-members). One of two observed giants are common to the two datasets. The age is given by Lyngå (1987) as , but the CM diagram indicates that we are seeing a younger cluster, just beyond . The main sequence covers a range of seven magnitudes and there are only two giants. This is confirmed by the age determined by del Rio & Huestamendia (1988).  Scuti stars might be present, but amplitudes are expected to be small as these stars will be unevolved.

3.1.2. NGC 2324

This cluster is situated in a crowded region and consequently the CM diagram (Fig. 2) is more perturbed by field stars than in the case of NGC 1513. The cluster contains a number of giants, and it is probably a cluster with an age close to in agreement with the published value (Lynga, 1987). With the reddening of , we get a turnoff colour of consistent with this age. The CCD observations again extend the CM diagram to fainter stars, although the position of the main sequence is hard to trace at faint magnitudes due to the presence of many field stars. There is a well developed population of giant stars and a group of stars between the giants and the turnoff point. Stars in this latter group are in the instability region and have magnitudes around . The cluster should be a good target to search for  Scuti stars, even though they are somewhat faint for performing time series photometry.

Fig. 2. Colour-Magnitude diagram for NGC 2324, diamonds and crosses as in the previous figure

We have short time series data for both clusters. However, due to problems with the field rotator on the telescope, which at the time was not fully operational, the guiding was poor. This introduced flat field problems that increase the low frequency noise. In NGC 2324 the exposures were not deep enough to give a precision at adequate to detect small amplitude Scuti stars, since the exposure time was limited by the bright stars. Nevertheless there are indications of variability for some stars.

3.2. Two southern clusters

The observations of the two southern clusters were made at La Silla by M. Gelbmann and U. Heiter, University of Vienna, using the Danish 1.54m telescope and the DFOSC instrument with a Loral chip. The field of view of this combination is quite large ().

3.2.1. NGC 2360

The data cover a range of 7 magnitudes, which means that the main sequence is well defined in the CM diagram (Fig. 3). The main sequence is rather broad and somewhat fuzzy due to the presence of field stars. The field is mildly crowded and in the outskirts of the cluster, field stars represent a fair fraction of the stars. The number of subgiants is small. Still, we expect that variables should be present in a cluster of this type with an age given by (Lynga, 1987). This value is based on a small reddening giving a fairly red . The reddening should be checked, as the morphology (only a small group of red giants) suggests it could be slightly younger.

Fig. 3. Colour-Magnitude diagram for NGC 2360, CCD data.

3.2.2. NGC 2489

The CM diagram (Fig. 4) is not very different from the previous one except for the lower content of field stars, So one might expect the parameters for this cluster to be similar to those of NGC 2360. In fact, the age is given as , a factor 3 smaller than for NGC 2360. This mainly reflects the difference in reddening: for NGC 2360, for NGC 2489 as given by Lyngå (1987). The reddening for NGC 2489 is even larger in Ramsay & Pollaco (1992), which give . The distance is almost the same to NGC 2360 and NGC 2489. If we assume, that the red giants have the same absolute magnitudes, then the apparent magnitude difference between red giants of 1 magnitude is consistent with a difference in reddening of 0.3-0.4.

Fig. 4. Colour-Magnitude diagram for NGC 2489, CCD data.

Both clusters look like good targets: they have the right age and distance and some evolved stars above the hot end of the main sequence. The magnitude of  Scuti stars would be bright enough that spectroscopy can be carried out to check membership, rotation etc.

© European Southern Observatory (ESO) 1998

Online publication: April 20, 1998
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