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1. Introduction THE ASTRONOMICAL JOURNAL, 116:1295È1300, 1998 September ( 1998. The American Astronomical Society. All rights reserved. Printed in U.S.A. VI PHOTOMETRY OF THE POSTÈCORE-COLLAPSE GLOBULAR CLUSTER NGC 6558 AND THE ADJACENT BULGE FIELD POPULATION1 R. M.RICH,2 S.ORTOLANI,3 E. BICA,4 AND B. BARBUY5 Received 1998 March 10; revised 1998 April 21 ABSTRACT We present VIcolor-magnitude diagrams (CMDs) of the globular cluster NGC 6558 and its surround- ing Ðeld, obtaining cluster parameters. The cluster has a blue extended horizontal branch and a depleted red giant branch, characteristics already observed in other postÈcore-collapse clusters in the bulge, such as NGC 6522, NGC 6540, and HP 1. These clusters do not belong to the metal-rich stellar population, and they may deÐne a distinct class in the bulge. We derive a reddening of E(B[V ) \ 0.50 and a dis- tanced_ B 6.3 kpc. The CMD morphology is consistent with a metallicity range of [1.6 \ [Fe/H] \ [1.2. We also report photometry of the bulge Ðeld population (b \[6¡ on the minor axis) in which NGC 6558 is imbedded. As is the case for other bulge Ðelds, there is a prominent red clump and a strong descending red giant branch, similar to that seen in lower latitude bulge Ðelds; we estimate the metallicity of the bulge population to lie between that of 47 Tuc and the nearly solar-metallicity cluster NGC 6553 (i.e., approximately [0.3 dex). Key words: globular clusters: individual (NGC 6558) 1. INTRODUCTION that the frequent shocking experienced by bulge clusters may contribute to or accelerate dynamical evolution in In the course of a study of ages and metallicities of Ðeld these globular clusters. stars in the Galactic bulge(Ortolani, Rich, & Renzini 1998), NGC 6558 is also designated as GCl B1807[3146, GCl we imaged the globular cluster NGC 6558 and its surround- 89(Alter, Ruprecht, & Vanysek 1970), and ESO 456-SC62. ing Ðeld at 6¡ on the minor axis. Very deep photometry of [ The coordinates are (B1950.0) a \ 18h07m03s.3, d \ the bulge Ðeld population is also being obtained from a [31¡46@27A, or (J2000.0) a \ 18h10m18s.4, d \[31¡45@49A. Hubble Space Telescope (HST ) Wide Field Planetary The Galactic coordinates arel \ 0¡.201, b \[6¡.025 a rela- Camera 2 (WFPC2) observation made near the cluster tively low absorption bulge region (Blanco 1988). (Rich et al. 1998) and will be reported in a separate paper. Early investigators were uncertain as to whether NGC Several factors have contributed to a renewed interest in 6558 was an open or a true globular cluster (Rosino 1962 the globular clusters of the bulge. First, high-resolution and references therein). Rosino (1962) discovered several imaging from the ground and space has made age determi- RR Lyrae stars in the cluster, which, along with its central nations and secure distance measurements possible. This concentration, clearly conÐrmed its classiÐcation as a body of work has identiÐed a separate population of globu- globular cluster. CCD photometry reveals NGC 6558 to be lar clusters associated with the Galactic bulge (Barbuy, very concentrated, with a postÈcore-collapse structure with Bica, & Ortolani 1998). HST imaging of the metal-rich a core radius ofrc \ 2A and a half-light radius rh \ 129A globular cluster NGC 6553 with WFPC2 shows that it is as (Trager,King, & Djorgovski 1995). old as 47 Tuc(Ortolani et al. 1995), and the analysis of the Because of the difficult crowded Ðeld and sparse popu- same data shows that the bulge Ðeld population is the same lation of this cluster, no CMD has been reported in the age as the metal-rich clusters. The second factor contrib- literature up to now. Globular clusters in the direction of uting to a renewal of interest in the bulge clusters is the the bulge have often revealed surprising CMD morphol- large number of clusters with collapsed cores and other ogies when they are studied with modern CCD detectors in indications of being dynamically evolved. These include the good seeing. For example, a recent case is the cluster HP 1, metal-rich clusters NGC 6388 and NGC 6441(Rich et al. previously thought to be metal-rich but now shown 1997), which have anomalous blue horizontal branches, and (Ortolani,Bica, & Barbuy 1997) to have an extreme blue NGC 6522(Shara et al. 1998), which is depleted of red horizontal branch (HB). In the latter study and inBarbuy et giants. In this paper, we report on another bulge globular al.(1998), the properties of these clusters are reviewed. cluster with a depleted red giant branch and other anom- While no CMD is available for NGC 6558, several alies in its color-magnitude diagram (CMD). We suggest studies report integrated properties.ZinnÏs (1980) photo- metry yields E(B[V ) \ 0.41 and [Fe/H] \[1.63, and ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ that ofBica & Pastoriza (1983) yields E(B[V ) \ 0.54 and 1 Based on observations collected at the European Southern Observa- [Fe/H] \[1.11. The spectroscopy ofZinn & West (1984) tory, La Silla, Chile. derives a revised metallicity of [Fe/H] \[1.44. They also 2 Department of Astronomy, Mail Code 5242, Columbia University, New York, NY 10027; rmr=astro.columbia.edu. derive a radial velocityvr \[135 km s~1. In the course of 3 Departimento di Astronomia, Universita` di Padova, vicolo constructing a grid of star cluster spectra, Bica & Alloin dellÏOsservatorio 5, I-35122 Padova, Italy; ortolani=astrpd.pd.astro.it. (1986a, 1986b) ranked the line strengths of NGC 6558 4 Departamento de Astronomia, Universidade Federal do Rio Grande relative to a large sample. They concluded that the do Sul, C.P. 15051, 91500-970 Porto Alegre, RS, Brazil; bica=if.ufrgs.br. spectral properties are consistent with E(B V ) 0.40 and 5 Departamento de Astronomia, Instituto Astronoü mico e GeoÐ sico, [ \ Universidade de Sa8 o Paulo, C.P. 9638, 01065-970 Sa8 o Paulo, SP, Brazil; [Fe/H] \[1.3.WebbinkÏs (1985) compilation reported barbuy=orion.iagusp.usp.br. E(B[V ) \ 0.43,V \ 16.7, a distanced \ 8.8 kpc, and a HB _ 1295 1296 RICH ET AL. Vol. 116 TABLE 1 tance and reddening. We also discuss other CMD LOG OF OBSERVATIONS properties of this cluster, which is in the direction of the bulge but not metal-rich. We describe our observations in Exposure Time Seeing ° 2 and discuss the cluster and Ðeld CMDs in° 3. We Filter (s) (arcsec) derive reddening and metallicity from our CMDs in ° 4 V ................ 10 1.1 and present a case for a new family of bulge globular clus- 3]180 1.1 ters in° 5. Our concluding remarks are given in ° 6. I ................. 7 1.35 2 ] 120 1.25 2. OBSERVATIONS AND REDUCTIONS NOTE.ÈAll observations taken 1993 June 15 NGC 6558 was observed with the NTT at the European with the NTT and EMMI instrument. Southern Observatory using the ESO Multi-Mode Intru- ment (EMMI) in the imaging/focal reducer mode. Images metallicity [Fe/H] \[1.51, whereas that of Harris (1996) were obtained through the red arm, equipped with the gives E(B[V ) \ 0.42,V \ 16.7, d_ \ 8.7 kpc, and [Fe/ 2024 ] 2024 pixel Loral antireÑection-coated CCD No. 34 H] \[1.44. HB detector. The pixel size is 15 km, corresponding to0A.35 on Recently,Hazen (1996) identiÐed some new variables the sky, and the array gives a full Ðeld [email protected] ] [email protected]. The within the cluster tidal radius. From the Ðve RR Lyrae log of observations is provided in Table 1. stars, which are probable members, she deduces B \ 16.8 The data were trimmed, bias-subtracted, and Ñat-Ðelded ^ 0.08 and a cluster distance of 6.6 kpc, this latterHB distance using MIDAS routines. Details of the reduction procedure being considerably lower than the catalog values. for these crowded, often di†erentially reddened Ðelds are In the present study, we report new VI photometry for described for the clusters NGC 6553, NGC 6528, and Liller NGC 6558 obtained at the ESO New Technology Tele- 1 byOrtolani, Barbuy, & Bica (1990) and Ortolani, Bica, & scope (NTT), and from the CMD we derive the cluster dis- Barbuy(1992, 1996), respectively. FIG. 1.ÈI image of NGC 6558 obtained with the NTT equipped with EMMI. Dimensions [email protected] ] 10.7.@ North is at the top and east is to the left. No. 3, 1998 VI PHOTOMETRY OF NGC 6558 1297 DAOPHOT II was used to extract the instrumental mag- nitudes. These magnitudes have been calibrated using Landolt(1983, 1992) stars and the same calibration equa- tions described inOrtolani, Barbuy, & Bica (1994): V \ v]0.02(V [I)]23.88^0.015 mag , I \ i]0.015(V [I)]23.61^0.015 mag , reduced to 1 s exposure time and 1.2 air masses. Because of crowding e†ects arising in the transfer of the aperture mag- nitudes from standards to the Ðeld stars, the zero-point calibration errors are dominant, estimated to be about ^0.03 mag. The CCD shutter time uncertainty (0.7 s), for a typical 20 s exposure time for the standard stars, produces an additional 3% uncertainty, which is propagated to the calibrations of the long-exposure cluster frames. The Ðnal magnitude zero-point uncertainty amounts to ^0.04. The atmospheric extinction was corrected with the standard La Silla coefficients(CV \ 0.20, CI \ 0.12 mag per air mass). We measured 12 Landolt stars during the two photometric nights used to deÐne the calibration transformations. Some FIG. 2.ÈV vs.
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