3 Colour-magnitude diagrams

As an initial step in constructing the CMD for each cluster, we estimated the distance $R_{{\rm f}}$ from the cluster center (the point with the maximum stellar density) where the average cluster contribution becomes negligible with respect to the background stellar field. This step has been accomplished by evaluating the radial stellar distribution, defined as

$$% \rho(R) = \frac{{\rm d}N(R)}{2\pi R{\rm d}R}$$ (5)

where ${\rm d}N(R)$ is the number of stars in the annulus of thickness ${\rm d}R$ at distance R from the cluster center.

The Danish data are affected by substantial incompleteness in the crowded central region of clusters, so central values for $\rho(R)$ are not accurate; this would lead to a serious overestimate of the "core radius'' $R_{{\rm c}}$ of clusters, defined as the radius for which the density in number of the measured stars is half of the central maximum. As an example, for NGC 2155 we would evaluate $R_{{\rm c}} \sim 45$ arcsec, against $R_{{\rm c}} \sim 13$ arcsec (Brocato et al. 2001) given by HST data. Even with such an overestimate, in our cluster sample we found $R_{{\rm c}}^{{\rm max}} \sim 50$ arcsec, in the case of IC 2146 and NGC 152. To be conservative, we decided to assume for all clusters $R_{f} = 2 R_{{\rm c}}^{{\rm max}} = 100$ arcsec. Beyond 100 arcsec from the center all the radial distribution are quite flattened, showing that only few cluster stars are still present. Thus stars at distance of $R > R_{{\rm f}} = 100$ arcsec from the cluster center can be safely regarded as field stars.

In order to obtain the best CMD, for each cluster we tried to avoid including: i) field stars; ii) the regions of the cluster where the crowding is too much severe (only in the case of crowded clusters). To this aim, we considered CMDs for cluster stars in four different concentric annuli of increasing radii, being the outer annulus representative of the distance from the cluster center where the radial distribution merges into the surrounding field. In general the best CMD was obtained by using stars within the two inner annuli (see Fig. 7 and following). More details about this procedure can be found in Matteucci (2001).

All the photometric data are available as ASCII files via WEB (http://www.te.astro.it/pub/mcII/).

3.1 LMC

CCD images have been obtained for the LMC clusters NGC 1777, NGC 2155, IC 2146, KMHK264, KMHK265 and NGC 1902 (with NGC 1777 and NGC 2155 already observed in the previous investigation). Figure 6 shows the sky distribution of the clusters observed in both the present and the previous HST investigation. The right panel in the same figure provides the location of our sample in the integrated (U-B), (B-V) diagram of 135 LMC clusters (Van den Bergh 1981), except KMHK264 and KMHK265 for which no integrated colours are available in literature.

For each cluster, we present a figure divided in 4 panels with the following meaning: 1) an image, of a selected size of the CCD frame, showing the cluster with overimposed the annuli discussed before (top left); 2) the radial stellar distribution (top right); 3) the cluster CMD as obtained within the labelled distance from the center (bottom left); 4) the CM diagram for field stars ( $R_{{\rm f}} > 100$ arcsec) as obtained by randomly selecting a constant number $N_{{\rm f}}$ = 1950 of stars brighter than V = 21 mag (bottom right). This choice will allow a significative comparison of field populations, by relying on similar sample of stars well above the limiting magnitude and not heavily affected by crowding.

Note also that in panel 4 (bottom right) the number $N_{{\rm tot}}$ indicates the total number of field stars "measured'' in Region 1 of CCD frame and not the number of "plotted'' dots. In the same panel 4, we also report the ridge-lines of the two galactic globular clusters M 15 (Durrell & Harris 1993) and 47 Tuc (Sosin et al. 1997) as boundary references of stellar populations with low and high values of metallicity (respectively [Fe/H] = -2.25 and -0.76 according to Harris 1996).

3.1.1 NGC 1777

Figure 7: In the left upper panel, the annuli investigated for NGC 1777 of radius 25$\arcsec$, 50$\arcsec$, 70$\arcsec$ and 100$\arcsec$ respectively; in the right upper panel, the radial stellar distribution around the NGC 1777 center. In the lower panels, the cluster and the field CM diagrams. Note that, we have reported on the diagram a sample of 6000 stars (4050 are fainter than V = 21 mag); ridge lines of M 15 (dotted line) and 47 Tuc (solid line) clusters are overimposed (see Sect. 4 for details).

A previous investigation on this cluster has been presented by Mateo & Hodge (1985), relying on CCD images taken at the CTIO 4-m Telescope, reaching a limiting magnitude of about $V \sim 22$ mag, as in our investigation. Comparison of stars in common (as well as of the CM diagrams), discloses a good agreement in the mean calibration, but with a better accuracy of present data.

The radial stellar distribution for NGC 1777 in Fig. 7 discloses that the contribution of cluster stars becomes negligible at $R \sim 70$ arcsec. After elimination of the stars in the inner annulus (R < 25 arcsec), badly affected by crowding, the CMD (804 stars) of the cluster shows a main sequence (MS) terminating at $V \sim 19.5$ mag and a well defined red clump at the similar magnitude, with a slightly developed red giants branch (RGB) but without any clear evidence for a subgiants branch (SGB).

The CMD of the surrounding field presents some differences: there is marginal evidence for a population coeval with the cluster, but the field shows well developed RGB and SGB. Even though the field sample is more abundant by a factor 2.4, this appears a real difference, therefore we conclude that the field includes a large population of stars older than the cluster one.

3.1.2 NGC 2155

Figure 8: In the left upper panel, the annuli investigated for NGC 2155 of radius 16$\arcsec$, 35$\arcsec$, 50$\arcsec$ and 80$\arcsec$ respectively; in the right upper panel, the radial stellar distribution around the NGC 2155 center. In the lower panels, the cluster and the field CM diagrams. Note that we have reported on the diagram a sample of 10 000 stars (8050 are fainter than V = 21 mag); ridge lines of M 15 (dotted line) and 47 Tuc (solid line) clusters are overimposed (see Sect. 4 for details).

Previous photometric studies of NGC 2155 have been presented by Sarajedini (1998) and by Brocato et al. (2001), both on the basis of HST snapshot. The great advantage of HST analysis has been the photometry of the inner cluster region: our diagram of stars with R < 16 arcsec is indeed badly affected by crowding. The radial stellar distribution in Fig. 8 discloses that the numerical contribution of the cluster becomes negligible at $R \sim 80$ arcsec.

The cluster CMD (1085 stars) shows a MS terminating at $V \sim$ 20.5 mag, a red clump about 2 mag more luminous and sufficiently well defined SGB and RGB.

The CMD of the surrounding field presents again a component with similar features, but with parallel evidence for a substantial number of stars being younger than the cluster, with a MS reaching $V \sim 18.5$ mag and a red clump slightly fainter then the cluster one (see Girardi et al. 1998 for a discussion on the variation of the clump luminosity with stellar age).

3.1.3 IC 2146

Figure 9: In the left upper panel, the annuli investigated for IC 2146 of radius 30$\arcsec$, 60$\arcsec$, 90$\arcsec$ and 150$\arcsec$ respectively; in the right upper panel, the radial stellar distribution around the IC 2146 center. In the lower panels, the cluster and the field CM diagrams (we have reported 8050 stars fainter than V = 21 mag); ridge lines of M 15 (dotted line) and 47 Tuc (solid line) clusters are overimposed (see Sect. 4 for details).

There are no previous investigations on this cluster. The radial stellar distribution for IC 2146 in Fig. 9 discloses that the numerical contribution of the field becomes dominant around $R \sim 100$ arcsec, The CMD (2023 stars) of the cluster shows a well populated MS, reaching $V \sim 20.25$ mag, and a well defined red clump at $V \sim 19.2$ mag, with scarcely populated RGB and SGB.

The CMD of the surrounding field appears rather similar to that of the cluster, but with the evidence for an additional older population producing the well populated RGB below the clump of He burning stars.

3.1.4 KMHK264 and KMHK265

Figure 10: In the left upper panel, the annuli division of KMHK264 region, of radius 15$\arcsec$, 30$\arcsec$, 45$\arcsec$ and 60$\arcsec$ respectively; in the right upper panel, the radial stellar distribution around the KMHK264 center. In the lower panels, the cluster and the field CM diagrams (we have reported 4050 stars fainter than V = 21 mag); ridge lines of M 15 (dotted line) and 47 Tuc (solid line) clusters are overimposed (see Sect. 4 for details).

Figure 11: In the left upper panel, the annuli division of KMHK265 region, of radius 15$\arcsec$, 30$\arcsec$, 45$\arcsec$ and 60$\arcsec$ respectively; in the right upper panel, the radial stellar distribution around the KMHK265 center. In the lower panel, the cluster CM diagram.

KMHK264 and KMHK265 are two small clusters observed in the same frame, near the older NGC 1754, this last being too crowded to obtain a reasonable CMD from present observations. Both clusters have no previous photometry.

The CMD (241 stars) of KMHK264 (Fig. 10) discloses a MS extending to $V \sim 17.5$ mag, whereas KMHK265 CMD shows a MS up to $V \sim 16.5$ mag, showing that both clusters have a relatively young age.

As for the field, we considered objects with R⁽¹⁾ > 85 arcsec from NGC 1754 center, R⁽²⁾ > 33 arcsec from KMHK264 center and R⁽³⁾ > 41 arcsec from KMHK265 center. The young population of the field appears rather similar to that of the clusters. It is also possible to find an older population identifiable in the well defined SGB, red clump and RGB.

3.1.5 NGC 1902

Figure 12: In the left upper panel, the annuli division of NGC 1902 region, of radius 15$\arcsec$, 30$\arcsec$, 40$\arcsec$ and 70$\arcsec$ respectively; in the right upper panel, the radial stellar distribution around the NGC 1902 center. In the lower panels, the cluster and the field CM diagrams (we have reported 2450 stars fainter than V = 21 mag); ridge lines of M 15 (dotted line) and 47 Tuc (solid line) clusters are overimposed (see Sect. 4 for details).

There are no previous investigations on this cluster. The radial stellar distribution for NGC 1902 in Fig. 12 shows that the numerical contribution of the cluster stars becomes dominant for R < 40 arcsec.

The cluster CM diagram (440 stars) discloses the MS extending to $V \sim 17$ mag, with some red giant stars possibly belonging to the cluster.

In the surrounding field a similar young population is also recognisable by the well defined MS, together with an additional older component.

3.2 SMC

Figure 13: Left panel: apparent spatial distribution of the investigated SMC clusters. The open circle indicates Kron3, whereas dashed lines roughly delimitate SMC Bar ($\delta$ and RA units are degree and hour, respectively). Right panel: the integrated properties of 7 SMC clusters investigated in this work are compared to a sample representative of the global cluster population of SMC; no data are available in literature for L9, L13 and NGC 643.

CCD images have been obtained for the SMC clusters L9, L13, L45, NGC 299, NGC 152, NGC 220, NGC 222, NGC 231, NGC 458 and NGC 643. The left panel of Fig. 13 shows the sky distribution of these objects; the open circle indicates Kron3, already investigated by Brocato et al. (2001). The (U-B) and (B-V) integrated colours of the SMC clusters under investigation (values are not available for L9, L13 and NGC 643) are reported in the right panel of the same figure, together with the data of 44 SMC clusters by Van den Bergh (1981).

The CM diagrams for fields again contain a constant number $N_{{\rm f}} = 1950$ of stars brighter than V = 21 mag except for NGC 643 region, where we found only 379 objects.

3.2.1 L9

Figure 14: In the left upper panel, the annuli division of L9 region, of radius 15$\arcsec$, 35$\arcsec$, 50$\arcsec$ and 80$\arcsec$ respectively; in the right upper panel, the radial stellar distribution around the L9 center. In the lower panels, the cluster and the field CM diagrams (we have reported 5550 stars fainter than V = 21 mag); ridge lines of M 15 (dotted line) and 47 Tuc (solid line) clusters are overimposed (see Sect. 4 for details).

This is the first time that L9 has been investigated. The plot of radial stellar distribution in Fig. 14 reveals that the contribution of cluster stars becomes negligible at $R \sim 50$ arcsec. The CMD (374 stars) discloses the MS extending to $V \sim 20.25 \div 20.5$ mag and a red clump about 1 mag more luminous, with a slightly developed RGB and scarce evidence (if any) for SGB.

The surrounding field shows evidence for stars either much younger (with a MS reaching $V \sim 17.5$ mag) or much older than the cluster, as revealed for the older population by the globular cluster-like RGB.

3.2.2 L13

Figure 15: In the left upper panel, the annuli division of L13 region, of radius 15$\arcsec$, 35$\arcsec$, 50$\arcsec$ and 80$\arcsec$ respectively; in the right upper panel, the radial stellar distribution around the L13 center. In the lower panels, the cluster and the field CM diagrams (we have reported 7450 stars fainter than V = 21 mag); ridge lines of M 15 (dotted line) and 47 Tuc (solid line) clusters are overimposed (see Sect. 4 for details).

This is a cluster so merged into the surrounding field that it is quite difficult to decide which star really belongs to L13. No previous investigations have been performed on it. The radial stellar distribution for L13 in Fig. 15 shows that the contribution of field stars becomes dominant at $R \sim 35$ arcsec. The cluster CMD (300 stars) discloses the MS extending to $V \sim 19.25$ mag and a group of helium burning stars.

Field CMD presents a quite old population, marked by the well developed RGB, with the additional contribution of a population reaching an age similar, if not smaller, to the age of cluster stars.

3.2.3 L45 and NGC 299

Figure 16: In the left upper panel, the annuli division of L45 region, of radius 15$\arcsec$, 30$\arcsec$, 50$\arcsec$ and 70$\arcsec$ respectively; in the right upper panel, the radial stellar distribution around the L45 center. In the lower panels, the cluster and the field CM diagrams (we have reported 2950 stars fainter than V = 21 mag); ridge lines of M 15 (dotted line) and 47 Tuc (solid line) clusters are overimposed (see Sect. 4 for details).

Figure 17: In the left upper panel, the annuli division of NGC 299 region, of radius 15$\arcsec$, 25$\arcsec$, 40$\arcsec$ and 70$\arcsec$ respectively; in the right upper panel, the radial stellar distribution around the NGC 299 center. In the lower panel, the cluster CM diagram.

These clusters are present in the same field; both were not previously investigated. Radial distributions (Figs. 16 and 17) reveal that the numerical contribution of the field becomes dominant at $R \sim 30$ arcsec. L45 CMD (334 stars) shows a MS extending to $V \sim 17$ mag, with possibly some He burning giants. NGC299 CMD (271 stars) discloses an even more extended MS (to $V \sim 14.5$ mag) and three probable red He burning giants at $[(B-V), V] \sim [1.6, 13.5]$ mag.

We considered all objects with R⁽¹⁾ > 76 arcsec from L45 center and R⁽²⁾ > 65 arcsec from NGC 299 center as stars belonging to stellar field. As shown in Fig. 16, the field presents again a large range of ages, with stars possibly as young as in the clusters but also with a globular cluster-like population.

3.2.4 NGC 152

Figure 18: In the left upper panel, the annuli division of NGC 152 region, of radius 30$\arcsec$, 60$\arcsec$, 90$\arcsec$ and 120$\arcsec$ respectively; in the right upper panel, the radial stellar distribution around the NGC 152 center. In the lower panels, the cluster and the field CM diagrams (we have reported 7050 stars fainter than V = 21 mag); ridge lines of M 15 (dotted line) and 47 Tuc (solid line) clusters are overimposed (see Sect. 4 for details).

A previous investigation of this cluster has been presented by Hodge (1981), as based on photographic plates taken at the CTIO 4-m Telescope, and by Rich et al. (2000), as based on HST snapshot of the central region of the cluster. Radial stellar distribution in Fig. 18 shows that cluster contribution tends to disappear at $R \sim 90$ arcsec. The cluster CMD (1862 stars) discloses a well defined MS reaching $V \sim 20.25 \div 20.5$ mag, a red clump about 1 mag more luminous and a RGB, with scarce evidence of a SGB.

Field stars have ages from much older than to similar to cluster stars, with marginal evidence for an even younger population with a MS extending to $V \sim 17.5$ mag.

3.2.5 NGC 220, NGC 222 and NGC 231

Figure 19: In the left upper panel, the annuli investigated for NGC 220, of radius 15$\arcsec$, 30$\arcsec$, 45$\arcsec$ and 60$\arcsec$ respectively; in the right upper panel, the radial stellar distribution around the NGC 220 center. In the lower panels, the cluster and the field CM diagrams (we have reported 2950 stars fainter than V = 21 mag); ridge lines of M 15 (dotted line) and 47 Tuc (solid line) clusters are overimposed (see Sect. 4 for details).

Figure 20: In the left upper panel, the annuli division of NGC 222 region, of radius 15$\arcsec$, 25$\arcsec$, 45$\arcsec$ and 60$\arcsec$ respectively; in the right upper panel, the radial stellar distribution around the NGC 222 center. In the lower panel, the cluster CM diagram.

Figure 21: In the left upper panel, the annuli division of NGC 231 region, of radius 15$\arcsec$, 30$\arcsec$, 45$\arcsec$ and 60$\arcsec$ respectively; in the right upper panel, the radial stellar distribution around the NGC 231 center. In the lower panel, the cluster CM diagram.

Also these clusters are present in the same frame. Previous CMD for these clusters are available in the web site of OGLE, as based on CCD observations at the 1.3-m Warsaw Telescope at Las Campanas, Chile. A comparison with our CM diagrams discloses a good photometric agreement. Note also that our CMDs extend to stars 1 mag fainter.

Radial stellar distributions in Figs. 19-21 shows that the cluster contribution tends to disappear at $R \sim$ 35 arcsec in all cases. NGC 220, NGC 222 and NGC 231 CM diagrams (511, 361, 449 stars respectively) are quite similar, all disclose a MS extending to $V \sim 16.5$ mag with some core helium burning stars (see Figs. 19-21).

Field stars show again a large range of ages, starting from much older than or similar to cluster stars.

3.2.6 NGC 458

Figure 22: In the left upper panel, the annuli division of NGC 458 region, of radius 30$\arcsec$, 65$\arcsec$, 100$\arcsec$ and 140$\arcsec$ respectively; in the right upper panel, the radial stellar distribution around the NGC 458 center. In the lower panels, the cluster and the field CM diagrams (we have reported 4550 stars fainter than V = 21 mag); ridge lines of M 15 (dotted line) and 47 Tuc (solid line) clusters are overimposed (see Sect. 4 for details).

A pioneering investigation on this cluster has been presented by Arp (1959) on the basis of photographic plates. The radial stellar distribution for NGC 458 in Fig. 22 shows that the contribution of the field becomes dominant at $R \sim 60$ arcsec. The cluster CM diagram (1288 stars) discloses a well defined MS extending to $V \sim 17.0$ mag and more than 30 evolved star forming a quite developed "blue loop''.

Field CMD discloses a similar and well populated young component, together with an older one.

3.2.7 NGC 643

Figure 23: In the left upper panel, the annuli division of NGC 643 region, of radius 35$\arcsec$, 70$\arcsec$, 105$\arcsec$ and 140$\arcsec$ respectively; in the right upper panel, the radial stellar distribution around the NGC 643 center. In the lower panels, the cluster and the field CM diagrams (we have reported all field star, and only 379 are brighter than V = 21 mag); ridge lines of M 15 (dotted line) and 47 Tuc (solid line) clusters are overimposed (see Sect. 4 for details).

There are no previous investigations on this cluster, which is located far from SMC center. The radial stellar distribution for NGC 643 in Fig. 23 shows that the numerical contribution of the cluster becomes negligible at R $\sim$ 70 arcsec, with a very small field density. The cluster CM diagram (1127 stars) discloses a well defined MS extending to V $\sim$ 19.5 and a red clump at [(B-V), V] $\sim$ [0.8, 19] mag, with a poorly developed RGB.

The surrounding field is poorly populated, and the CMD discloses a MS reaching only V $\sim$ 20.5, without evidence of younger stellar components.