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Magellanic Clouds Stellar Clusters. II. New $B,V$ CM Diagrams for 6 LMC

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Magellanic Clouds Stellar Clusters. II. New $B,V$ CM Diagrams for 6 LMC A&A 387, 861–889 (2002) Astronomy DOI: 10.1051/0004-6361:20020376 & c ESO 2002 Astrophysics Magellanic Clouds stellar clusters. II. New B; V CM diagrams for 6 LMC and 10 SMC clusters? A. Matteucci1,V.Ripepi2, E. Brocato3, and V. Castellani1 1 Dipartimento di Fisica, Universit`a di Pisa, Piazza Torricelli 2, 56100 Pisa, Italy 2 Osservatorio Astronomico di Capodimonte, Via Moiariello 16, 80131 Napoli, Italy 3 Osservatorio Astronomico di Collurania, Via M. Maggini, 64100 Teramo, Italy Received 10 August 2001 / Accepted 29 January 2002 Abstract. We present new CCD photometry for 6 LMC and 10 SMC stellar clusters taken at the ESO 1.54-m Danish Telescope in La Silla, to extend a previous investigation on Magellanic Clouds clusters based on HST snap- shots. Thanks to the much larger area covered by the Danish detector, we investigate the spatial distribution of cluster stars, giving V ,(B − V ) CM diagrams for both clusters and surrounding fields. Evidence of a complex history of star formation in the Clouds is outlined, showing that old field populations in both Clouds have metal- licities much lower than normally adopted for them (Z =0.008 and Z =0.004 for LMC and SMC respectively), with SMC field stars more metal poor than in the LMC. Observational data concerning the red clump of field stars in both Clouds are briefly discussed. Key words. galaxies: clusters: general – galaxies: stellar content – galaxies: Magellanic Clouds 1. Introduction order to allow the investigation of stellar populations in both Clouds. The Magellanic Clouds (MC) are well-known targets for Section 2 gives information on the observations and on the investigation of simple stellar populations, since they the adopted data reduction procedures. The CM diagrams present a great number of populous young and intermedi- of clusters and surrounding fields are presented in Sect. 3 ate age clusters, which allow to test evolutionary theories where the major features of the diagrams are briefly dis- for a combination of masses and metallicities not present cussed. In Sect. 4 we discuss the stellar populations of the in our Galaxy. In a previous paper (Brocato et al. 2001) fields, whereas a study of the red clumps in both LMC B,V we have recently presented CMDs for 21 globular and SMC fields is presented in Sect. 5. In Sect. 6 we fi- clusters in the LMC and 1 in the SMC, as obtained on the nally present some brief concluding remarks. basis of HST snapshots. In this paper we extend this inves- tigation by using observational data from the ESO 1.54-m Danish Telescope. 2. Observations, data reduction and calibration With respect to the previous survey, the present study obviously relies on a worse seeing, but presents also the B (ESO # 450 filter) and V (ESO # 451 filter) images relevant advantage of providing data in a larger field of were collected by one of us (E. B.) during five nights, view (∼12.05 × 120), covering the whole cluster and a large from Nov. 28 to Dec. 2 1997 (we shall refer to the nights portion of the surrounding stellar field. As we shall discuss as N1 to N5, where N1 = Nov. 28 night), with the ESO later on, the quality of the CM diagrams presented in this 1.54-m Danish Telescope (La Silla, Chile). The instrument paper appears largely comparable with the ones in the pre- installed at the telescope was the DFOSC, equipped with vious investigation. In this way we shall add 4 clusters to a LORAL/LESSER C1W7 CCD (2052 × 2052 pixels). the previous sample of the LMC clusters (2 of our 6 LMC The scale was 0.39 arcsec/pixel. The log of observations clusters were already investigated in the previous paper), is reported in Table 1. extending the investigation to 10 clusters in the SMC in The original images were pre-reduced following the usual procedures, i.e. bias subtraction and flat-fielding. Send offprint requests to: E. Brocato, However, the edges of the frames have been trimmed to e-mail: [email protected] remove the overscan columns and the regions of the CCD ? Based on observations carried out at the European where flat-fielding showed dramatic differences with re- Southern Observatory, La Silla, Chile. spect to the central part of the CCD image. The resulting Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20020376 862 A. Matteucci et al.: MC stellar clusters. II. Table 1. Log of the observations. Cluster Filter Observation Number of Min, max exposure Seeing (Galaxy) night observations time (s) (arcsec) KMHK264a B N4 2 60, 300 1.3–1.4 (LMC) V N4 2 30, 200 1.2–1.3 NGC 1777 B N1 5 120, 300 1.6–1.8 (LMC) V N1 5 60, 200 1.6–2.0 NGC 1902 B N1, N4, N5 4 40, 300 1.2–1.8 (LMC) V N1, N4, N5 4 20, 200 1.1–1.6 NGC 2155 B N3 3 60, 600 1.2 (LMC) V N3 4 30, 300 1.1–1.2 IC 2146 B N4 3 60, 1200 1.2–1.3 (LMC) V N4 4 30, 600 1.2 L9 B N5 3 60, 600 1.3–1.4 (SMC) V N5 4 30, 300 1.1–1.4 L13 B N5 2 120, 600 1.3–1.4 (SMC) V N5 3 60, 300 1.2–1.4 L45b B N5 3 60, 600 1.3–1.4 (SMC) V N5 3 30, 300 1.3–1.4 NGC 152 B N4 4 60, 1200 1.2–1.4 (SMC) V N4 5 30, 600 1.1–1.2 NGC 220c B N1, N2, N4, N5 8 40, 300 1.3–1.9 (SMC) V N1, N2, N4, N5 10 20, 200 1.3–1.8 NGC 458 B N1, N3, N4, N5 5 60, 300 1.2–1.9 (SMC) V N1, N3, N4, N5 5 30, 300 1.1–1.8 NGC 643 B N2, N3 5 120, 1200 1.2–2.2 (SMC) V N2, N3 12 30, 600 1.1–2.3 a The same field of KMHK264 includes also the KMHK265 cluster. b The same field of L45 includes also the NGC 299 cluster. c The same field of NGC 220 includes also the NGC 222 and NGC 231 clusters. frame dimensions were 1911 × 1811 pixels, for a total field of view of ∼12.04 × 11.08. Photometry was performed using the point-spread function (PSF) technique with the DAOPHOT II package (Stetson 1994). Raw instrumental magnitudes and errors were obtained by fitting the PSF to all identified objects using the routine ALLSTAR (Stetson 1994). A detailed test of the resulting photometry revealed a region of the frames where stellar images appeared elongated because of optical aberrations. ALLSTAR deconvolved these objects into more stars than really present, providing a bad fit with high residuals. We eliminated from our star lists all the objects identified in this region, named as “Region 2” in Fig. 1. The field of view of the resulting “good” field (Region 1) is ∼113.4 arcmin2. For a given stellar field, each frame was reduced inde- pendently; then the frame with the best seeing was cho- Fig. 1. CCD frame region (Region 2) affected by optical aber- sen in each filter as a reference for coordinate and magni- rations. The center of the dividing circumference is (500, 905) tude transformations. Subsequently, all the frames where and its radius is 1120 pixels. A. Matteucci et al.: MC stellar clusters. II. 863 Fig. 2. Instrumental errors in magnitude (σV ) and colour (σB−V ) as a function of the calibrated magnitudes (V ) for the stars in the frames of NGC 458. Table 2. Standard stars used for calibration. Night Reference field Standard stars No. of B, V observations per field N1 Selected Area 95 (Landolt 1992) 42, 101, 105, 107 1, 2 E3 Region (Graham 1982) R 1, 1 N3 Selected Area 95 (Landolt 1992) 42, 101, 105 1, 1 E2 Region (Graham 1982) I, m, o, s, t 3, 3 E3 Region (Graham 1982) T, k, v 1, 1 N4a T Phe (Landolt 1992) A, C, D, E, F 6, 6 E2 Region (Graham 1982) I, m, o, s, t 3, 3 N5a T Phe (Landolt 1992) A, C, D, E, F 4, 4 E2 Region (Graham 1982) I, m, o, s, t 2, 2 a Photometric night. transformed into the reference system to perform suitable where V ,(B −V ) indicate Johnson magnitude and colour, average operations in order to obtain a set of mean instru- respectively, whereas va and (b − v)a are instrumental re- mental magnitudes (weighted according to the photomet- sults corrected for atmospheric extinction and normalised ric quality of the frames). Finally, B and V photometry for to an exposure time of 1 second, as given by the following each field were matched in order to obtain the instrumen- relations: tal magnitudes (v), colours (b − v) and errors (see Fig. 2). To transform instrumental magnitude into the ba = b +2.5logt − BX (3) Johnson system, several standard stars from the E-regions va = v +2.5logt − V X (4) (Graham 1982) and from equatorial fields (Landolt 1992) were observed every night, except N2 (see Table 2 for a where B and V are the adopted extinction coefficients detailed list of standards used); however, the best photo- and X the airmass. Coefficients α, β, γ, δ were found metric night turned out to be N4.
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