4 Cluster and field stellar populations

 

 

Table 4: Selected quantities for the LMC and SMC clusters arranged in order of increasing age.

	Cluster	SWB class	R (arcsec)	$N_{{\rm star}}$	$V_{{\rm TO}}$ (mag)	age (Myr)
LMC	KMHK265	. . .	30	303	16.5	$50 \div 100$
	NGC 1902	II	40	440	17	$100 \div 150$
	KMHK264	. . .	30	241	17.5	$150 \div 200$
	NGC 1777	IV B	$25 \div 70$	804	19.5	$700 \div 800$
	IC 2146	V	60	2023	20.25	$1200 \div 1500$
	NGC 2155	VI	$16 \div 50$	1085	20.5	$1500 \div 2000$
SMC	NGC 299	. . .	25	271	14.5	$15 \div 20$
	NGC 220	III	30	511	16.5	$70 \div 100$
	NGC 222	II-III	25	361	16.5	$70 \div 100$
	NGC 231	. . .	30	449	16.5	$70 \div 100$
	NGC 458	III	65	1288	17.0	$100 \div 150$
	L45	. . .	30	334	17.0	$100 \div 150$
	L13	. . .	35	300	19.25	$450 \div 550$
	NGC 643	. . .	70	1127	19.5	$600 \div 700$
	L9	. . .	35	374	20.25 $\div$ 20.5	$1000 \div 1300$
	NGC 152	IV B	60	1862	20.25 $\div$ 20.5	$1000 \div 1300$

Figure 24: Isochrone fitting for IC 2146: solid and dashed line show the 1500 and 1000 Myr isochrones respectively.

Figure 25: Division in MS and evolved red stars regions of KMHK264 field CMD containing 1950 stars brighter than V = 21 mag.

Even though this paper is mainly devoted to the presentation of observational results, the CM diagrams presented in the previous section deserve some further comments. As a first point, let us summarise in Table 4 selected quantities for the studied stellar clusters, giving for each cluster the SWB class (Searle et al. 1980), the sky region used for producing the CM diagram, the number of stars ( $N_{{\rm star}}$) in the diagram, the estimated V magnitude of the upper MS termination ( $V_{{\rm TO}}$). The last column gives an estimate of the cluster ages as obtained from the $M_{V}^{{\rm TO}}$-age relation (Castellani et al., in preparation), assuming Z = 0.008, (m-M)_V = 18.6 for LMC and Z = 0.004, (m-M)_V = 19.0 for SMC. Because of the observational uncertainties in $V_{{\rm TO}}$ and lack of firm evaluations on the cluster metallicities in Table 4 we report just an indication of the range of ages which appears suitable for the various clusters. As expected from the SWB classification, one finds that clusters in both Clouds span a wide range of ages. To further discuss the comparison with theoretical predictions, as an example we show in Fig. 24 the stellar isochrones for the two extreme ages in Table 4 as overlapped on the CM diagram of stars in the LMC cluster IC 2146. One finds that, for the given chemical composition, the two isochrones indeed reasonably encompass the observed distribution of MS stars. Note that theoretical colours for red giants are freely governed by the assumptions on the efficiency of surface convection and the agreement between theory and observation is only marginally significant. This is not the case for luminosities, which keep being a firm theoretical prediction; the reasonable agreement between predicted and observed luminosities of the clumping He-burning red giants can be thus regarded as comforting evidence.

As for the surrounding fields, one finds in all cases evidence of an old globular cluster-like population, with an age of several billion years, as indicated by the occurrence of well-developed RGBs. A simple inspection of the CM diagrams does not reveal clear differences in the slope of these RGBs, either within the single Clouds, or between the two Clouds, excluding the occurrence of large variations in the metallicity of these earlier populations. However, comparison with ridge-lines for the two selected galactic globular clusters (M 15 and 47 Tuc), as presented in panel 4 of all figures for the given distance moduli and adopting mean foreground reddening values from Schlegel et al. (1998) E(B-V) = 0.075 and 0.037 for LMC and SMC respectively (except in the cases of NGC 2155 and NGC 643 fields, for which Schlegel et al. values are 0.051 and 0.068 respectively), reveals several relevant features. One finds that the metallicity of LMC old field stars appears a bit lower than stars in 47 Tuc, i.e., lower than $[{\rm Fe}/{\rm H}] \sim -0.76$. Moreover, one finds that the old population in SMC appears even more metal-poor, roughly between the metallicities of M15 and 47 Tuc, i.e. $-2.25 < [{\rm Fe}/{\rm H}] < -0.76$ (Harris 1996). One finds that reasonable variations in the adopted distance moduli and/or reddening cannot affect such a conclusion which, in turn, is not surprising, since it was already found (e.g. Olszewsky et al. 1996 and references therein) that field stars in both Clouds can be much less metallic than the nominal adopted metallicities (see e.g. Westerlund 1997, who reports $[{\rm Fe}/{\rm H}]_{{\rm LMC}} = -0.30 \pm 0.2$ and $[{\rm Fe}/{\rm H}]_{{\rm SMC}} = -0.65 \pm 0.2$for cool stars in the fields of the two galaxies). As a further support to this occurrence, Dolphin et al. (2001) have recently presented observational evidence for a metal poor ( $[{\rm Fe}/{\rm H}] \sim -1.3 \pm 0.3$) population in the SMC halo region in the field of NGC 121.

In all fields one finds evidence of additional younger populations, but with the last episode of star formation placed at various ages. Not surprisingly, in all cases the age of the cluster appear within the range of ages covered by the surrounding field, sometimes in correspondence of the last episode of star formation (IC 2146), sometimes much earlier (e.g., L9). Only NGC 643 seems to be younger than the surrounding population, but we tend to attribute such peculiarity to the very low statistical significance of the field sample.

To go deeper in this scenario, let us discuss separately the fields of the two Clouds.

4.1 LMC fields

Stellar population studies, as based on CMD investigations, have already been extensively carried out with the aim of constraining the LMC star-formation history. In particular, one of the most interesting issues under discussion is whether common star-formation events involved the entire LMC or different regions of the Cloud experienced different star-formation histories. Bertelli et al. (1992), Vallenari et al. (1996a) and Vallenari et al. (1996b), on the basis of CCD ground observations of fields out of the Bar, found evidence that star-formation activity is correlated with position in the LMC. In the vicinity of the LMC main body, a significant young population component is present with an age of 0.06-0.1 Gyr; this age seems to increase to 0.4-0.5 Gyr at 4$^\circ$ from the LMC center and to more than 1 Gyr beyond 5$^\circ$; in a western outer field young stars seem to be completely absent. Westerlund et al. (1995) investigated a region in the NW ($\sim$4$^\circ$ from the center) and one in the SW ($\sim$5$^\circ$), finding a young component (0.2-0.6 Gyr) present only in the first zone; they have also found similar characteristics between fields and included clusters. Westerlund et al. (1998) studied three northeastern fields whose youngest population appears to follow an analogous trend: the inner field shows a very young component that is completely absent in the outer one. Santos et al. (1999) investigated 14 fields covering the central distance range 4-8 kpc and confirmed the previous general behaviour. Geha et al. (1998), on the basis of HST deep observations, studied two fields in the NW region and one in the NE and found that they contain statistically indistinguishable stellar populations, in apparent contrast to the other quoted works.

 

 

Table 5: Results of the star counts for the selected regions, containing 1950 stars, of observed LMC fields, named as the included cluster. We assume, as statistical fluctuation on these numbers, the Poisson error.

Field	NB	NR	NB/NR
KMHK264	1212 $\pm$ 35	738 $\pm$ 27	1.64 $\pm$ 0.11
NGC 1902	1296 $\pm$ 36	654 $\pm$ 26	1.98 $\pm$ 0.13
NGC 2155	1202 $\pm$ 35	748 $\pm$ 27	1.61 $\pm$ 0.11
NGC 1777	910 $\pm$ 30	1040 $\pm$ 32	0.88 $\pm$ 0.06
IC 2146	930 $\pm$ 30	1020 $\pm$ 32	0.91 $\pm$ 0.06

 

 

Table 6: Results of the star counts for the selected regions, containing 1950 stars, of observed SMC fields, named as the included cluster. We assume, as statistical fluctuation on these numbers, the Poisson error.

Field	NB	NR	NB/NR
L45	1124 $\pm$ 34	826 $\pm$ 29	1.36 $\pm$ 0.09
NGC 220	1102 $\pm$ 33	848 $\pm$ 29	1.30 $\pm$ 0.08
NGC 458	1224 $\pm$ 35	726 $\pm$ 27	1.69 $\pm$ 0.11
L9	876 $\pm$ 30	1074 $\pm$ 33	0.82 $\pm$ 0.05
L13	756 $\pm$ 27	1194 $\pm$ 35	0.63 $\pm$ 0.04
NGC 152	610 $\pm$ 25	1340 $\pm$ 37	0.46 $\pm$ 0.03

To approach such a scenario in a more quantitative way, we made use of our field samples (cf. Sect. 3.1), dividing the CMDs for V < 21 mag into two regions separating the MS from evolved red stars and counting the objects in each zone. Figure 25 gives an example of this procedure in the case of the KMHK264 field, while Table 5 reports star counts for LMC fields, where N_B, N_R indicate the number of MS and evolved red stars, respectively. We can now connect these results with the field positions in the LMC. More internal fields (KMHK264 and NGC 1902) present greater N_B and, correspondingly, a more extended MS (see also Figs. 10 and 12); southern fields (NGC 1777 and IC 2146), instead, present greater N_R and lack a young population component (see also Figs. 7 and 9): this evidence appears in good agreement with previous suggestions. The case of NGC 2155 field merits a particular remark: it presents indeed a large N_B as more internal fields, but the MS does not extend to high luminosities, so the field has an abundant component younger than cluster stars, but not as young as in the fields around KMHK264 or NGC 1902. This evidence is in agreement with Bertelli et al. (1992), which studied a smaller area near this field showing the same characteristic.

4.2 SMC fields

With respect to the Large Cloud, the field of the SMC and the Cloud formation history were less investigated. The most extensive work (Gardiner & Hatzidimitriou 1992) has been the search for main-sequence stars younger than 2 Gyr in an area covering six UK Schmidt Telescope survey fields, virtually corresponding to the entire SMC outer region. These authors concluded that such MS objects are absent in the NW, while the younger population is considerably distributed over a large part of the eastern (the SMC "Wing'') and southern areas, with a rapidly increasing concentration with decreasing distance from the optical center. A similar distribution has been noted for SMC clusters (Van den Bergh 1991), younger clusters being more concentrated towards the SMC Bar and the older ones more dispersed. Our sample allows us to extend the investigation towards more internal regions, not covered by the Gardiner & Hatzidimitriou (1992) study.

Following the same procedure used for the LMC fields, we have performed star counts on MS and evolved regions of the CMD of stars brighter than V = 21 mag: Table 6 gives N_B and N_R for the various fields. A numerically consistent young population is present in L45, NGC 220 and NGC 458 fields: from respective CMDs (Figs. 16, 19 and 22) it appears that MS termination becomes fainter with increasing distance from the SMC center. L9, L13 and NGC 152 fields are located in SW region at 1.5$^\circ$-2$^\circ$ from SMC center: they have detectably smaller N_B, but their CMDs (Figs. 14, 15 and 18) still show a young MS population, even if not with identical features. The L9 field CMD is morphologically similar to NGC 458 (MS termination $\sim$17.0 mag), while the NGC 152 field CMD presents a less developed MS, with a few stars brighter than V = 18.5 mag. We have not performed this analysis on the NGC 643 field, that is located 4$^\circ$ from SMC center in the outer SE region, since only 379 stars (see Fig. 23) brighter than V = 21 mag have been identified. However, the very young population appears completely absent, in agreement with Gardiner & Hatzidimitriou (1992) who found few bright MS in this region.

A comparison between the CMDs of studied clusters and the surrounding fields suggests that near the SMC Bar they are quite similar, with the youngest field component resembling cluster populations (see Figs. 16, 19 and 22), while in the SW region clusters appear older than the youngest field population. This is clearly evident in the L9 and NGC 152 cases (see Figs. 14 and 18).