ASTRONOMY & ASTROPHYSICS SEPTEMBER 2000, PAGE 451 SUPPLEMENT SERIES Astron. Astrophys. Suppl. Ser. 145, 451–465 (2000) Photometric catalog of nearby globular clusters? II. A large homogeneous (V,I) color-magnitude diagram data-base A. Rosenberg1, A. Aparicio2,I.Saviane3, and G. Piotto3 1 Telescopio Nazionale Galileo, vicolo dell’Osservatorio 5, I-35122 Padova, Italy 2 Instituto de Astrofisica de Canarias, Via Lactea, E-38200 La Laguna, Tenerife, Spain 3 Dipartimento di Astronomia, Univ. di Padova, vicolo dell’Osservatorio 5, I-35122 Padova, Italy Received February 24; accepted June 6, 2000 Abstract. In this paper we present the second and final 1. Introduction part of a large and photometrically homogeneous CCD color-magnitude diagram (CMD) data base, comprising As discussed in Rosenberg et al. (2000, hereafter Paper I), 52 nearby Galactic globular clusters (GGC) imaged in the the heterogeneity of the data often used in the litera- V and I bands. ture for large scale studies of the Galactic globular cluster The catalog has been collected using only two tele- (GGC) properties has induced us to start a large survey of scopes (one for each hemisphere). The observed clus- both southern and northern GGCs by means of 1-m class ters represent 75% of the known Galactic globulars with telescopes, i.e. the 91 cm European Southern Observatory (m − M)V ≤ 16.15 mag, cover most of the globular clus- (ESO)/Dutch telescope and the 1 m Isaac Newton Group ter metallicity range (−2.2 ≤ [Fe/H] ≤−0.4), and span (ING)/Jacobus Kapteyn telescope (JKT). We were able Galactocentric distances from ∼ 1.2to∼ 18.5kpc. to collect the data for 52 of the 69 known GGCs with In particular, here we present the CMDs for the (m − M)V ≤ 16.15. Thirty-nine objects have been ob- 13 GGCs observed in the Northern hemisphere. The served with the Dutch Telescope and the data have been remaining 39 Southern hemisphere clusters of the catalog already presented in Paper I. The images and the photom- have been presented in a companion paper (Rosenberg etry of the remaining 13 GGCs, observed with the JKT et al. 2000). We present the first CCD color magnitude are presented in this paper. A graphical representation of diagram for NGC 6779 (M 56). the spatial distribution of our cluster sample is given in All the CMDs extend from at least ∼ 2 magnitudes Fig. 1. below the turn-off (i.e. Vlim ≥ 22) to the tip of the red As a first exploitation of this new data base, we have giant branch. The calibration has been done using a large conducted a GGC relative age investigation based on the number of standard stars, and the absolute calibration is best 34 CMDs of our catalog (Rosenberg et al. 1999, here- reliable to a ∼ 0.02 mag level in both filters. after Paper III), showing that most of the GGCs have This catalog, because of its homogeneity, is expected the same age. We have also used the data base to obtain a to represent a useful data base for the measurement of the photometric metallicity ranking scale (Saviane et al. 2000, main absolute and relative parameters characterizing the hereafter Paper IV), based on the red giant branch (RGB) CMD of GGCs. morphology. There are many other parameters that can be mea- Key words: astronomical data base: miscellaneous — sured from an homogeneous, well calibrated CMD data catalogs — stars: Hertzsprung-Russell (HR) and C−M base: the horizontal branch (HB) level, homogeneous red- diagrams — stars: population II — globular clusters: dening and distance scales, etc. We are presently working general on these problems. However, we believe it is now the time to present to the community the complete data base to give to anyone interested the opportunity to take advan- Send offprint requests to: A. Rosenberg, e-mail: [email protected] tage of it. ? Based on observations made with the 1 m Jacobus Kapteyn In the next section, we will describe the observations Telescope operated on the island of La Palma by the ING in collected at the JKT in 1997. The data reduction and cal- the Spanish Observatorio del Roque de Los Muchachos of the ibration is presented in Sect. 3, where a comparison of the Instituto de Astrof`ısica de Canarias. calibration of the northern and southern clusters is also 452 A. Rosenberg et al.: VI photometric catalog of nearby GGC’s. II. 2. Observations The data were collected on May 30-June 2 1997. For almost 2 nights we had quite good seeing conditions (FWHM between 0.65 and 0.85 arcsec). Unfortunately, too few standards were observed during this run. In order to ensure an homogeneous calibration, we reobserved all the same cluster fields (plus numerous standard fields) in June 24, 1998, a night that was photometric and with a stable seeing. Observations were done at the 1 m Jacobus Kapteyn telescope (JKT), located at the Roque de Los Muchachos Observatory. The same instrumentation was used in both runs: A thinned Tektronix CCD with 1024 × 1024 pixels projecting 0.331 arcsec on the sky and providing a field of view of 5.6 arcmin square. The detector system was linear to 0.1% over the full dynamic range of the 16 bit analog-to-digital converter. Three short (10 s), two medium (45 s ÷ 120 s) and one large (1800 s) exposures were taken for one field of each of the proposed objects in each filter. Also a large number of standard stars were obtained during the second observing run. This was done in order to have all the clusters in ex- actly the same system, making internal errors negligible. 3. Data reduction and calibration The data were reduced following the same procedure de- scribed in Paper I. The absolute calibration of the observations is based on a set of standard stars of the catalog of Landolt (1992). Eighteen standard stars were observed; specifically, the ob- served fields were PG1525, PG1633, Mark-A and PG2213. At least 4 exposures at different airmasses were taken dur- ing the night for each standard field, making a total of ∼ 80 individual measures per filter. Fig. 1. Galactocentric distribution of all the GGCs with (m − Thanks to the relatively stable seeing conditions, for the aperture photometry we used for all the standards M)V ≤ 16.15 mag. In the upper panel, the projection of the GGC location over the Galactic plane is presented. The open (and the cluster fields) a 12 pixel aperture (4 arcsec). The circles represent the GGCs presented in Paper I, the open aperture magnitudes were normalized by correcting for the squares the GGCs whose photometry and CMDs are discussed exposure time and airmass. Using the standards observed in this paper, and the asterisk indicates the position of the at different airmasses, we estimated for the extinction co- GGC Pal 1 (Rosenberg et al. 1998). The clusters which are not efficients in the two filters: AV =0.11 and AI =0.06. For presented in our catalog are marked by open triangles. The the calibration curves we adopted a linear relation. The lower panel shows the location of the same GGCs in a plane best fitting straight lines are: perpendicular to the Galactic plane. The Milky Way is also V − v(ap) = (0.017 0.003) × (V − I) − (2.232 0.002); schematically represented I − i(ap) = (0.053 0.003) × (V − I) − (2.679 0.002). The magnitude differences vs. the standard color are plot- ted in Fig. 2, where the solid lines represent the above equations. 3.1. Comparison between the two catalogs discussed for three objects observed with both telescopes. In order to assist the reader, in Sect. 4 we present the Apart from the 13 GGCs presented in the following main parameters characterizing our clusters. Finally, the sections, we re-observed three additional clusters selected observed fields for each cluster, and the obtained CMDs from the southern hemisphere catalog presented in are presented and briefly discussed in Sect. 5. Paper I. We covered the same spatial region, in order A. Rosenberg et al.: VI photometric catalog of nearby GGC’s. II. 453 Fig. 2. Calibration curves for the V (upper panel)andI (lower panel) filters for the June 24th 1998 run to test the homogeneity of the two catalogs created using the two different telescopes. In Fig. 3 the magnitudes of 170 stars (with photo- metric error smaller than 0.025 mag and V magnitude brighter than 18) in NGC 5897 from the JKT and the ESO/Dutch telescope data sets are compared. The dif- ferences in magnitude and color between the two observ- ing runs are: 0.00 0.02, −0.01 0.02 and 0.01 0.02 in V , I and V − I, respectively. For the other two clus- Fig. 3. Comparison between the JKT and the ESO/Dutch tele- ters, we have: ∆V JKT =0.02 0.02, ∆IJKT =0.01 0.02 scope magnitudes for the 170 stars in NGC 5897 with errors DUT DUT ≤ − JKT 0.025. The differences in magnitude and color between the and ∆(V I)DUT =0.01 0.02 for 163 stars selected in − JKT JKT two runs are: 0.00 0.02, 0.01 0.02 and 0.01 0.02 in V , I NGC 6093, and ∆VDUT =0.01 0.02, ∆IDUT =0.00 0.02 and V −I, respectively. Notice also that the slopes of the differ- − JKT and ∆(V I)DUT =0.01 0.02 for 249 stars selected in ences in color vs. magnitudes or color of one of the runs (upper NGC 6171 (selection criteria as in the case of NGC 5897).