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Publications of the Astronomical Society of the Pacific 106: 404-412, 1994 April

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Publications of the Astronomical Society of the Pacific 106: 404-412, 1994 April Publications of the Astronomical Society of the Pacific 106: 404-412, 1994 April CCD Photometry of the Galactic Globular Cluster NGC 6535 in the Β and Passbands Ata Sarajedini1 Kitt Peak National Observatory, National Optical Astronomy Observatories,2 P.O. Box 26732, Tucson, Arizona 85726-6732 Electronic mail: [email protected] Received 1993 October 15; accepted 1994 February 1 ABSTRACT. The first CCD color-magnitude diagram (CMD) in Β and V is presented for the Galactic globular cluster NGC 6535. From this CMD, which extends below the main-sequence turnoff, we draw the following conclusions: (1) The horizontal branch (HB) is predominantly blue in nature with no RR Lyrae variables known to be cluster members. Nonetheless, based on a comparison with clusters which have blue HBs and RR Lyraes (Ml5 and M79), we infer a mean HB magnitude of <^RR> = 15.73 ±0.11 for NGC 6535. (2) Again, via a direct comparison with the blue HBs of M15 and M79, we derive a cluster reddening oíE{B—V) =0.44±0.02. (3) When combined with the apparent color of the red-giant branch at the level of the HB, (B—V)g= 1.18 ±0.02, the derived reddening yields a metal abundance of [Fe/H] = —1.85 ±0.10, similar to that of NGC 6397. (4) Application of the AFT0_hb and Δ(5— F)SGB_To cluster dating techniques reveals no perceptible age difference between NGC 6535 and NGC 6397. (5) A significant population of nine blue-straggler candidates is detected in NGC 6535. However, this is too few to facilitate a meaningful analysis of their radial distribution. 1. INTRODUCTION mag below the main-sequence turnoff. The remaining por- tions of Sec. 3 discuss, among other things, the cluster The first color-magnitude diagram (CMD) of this rel- horizontal branch, reddening, metallicity, age, and blue atively small Galactic globular cluster was constructed by straggler content. The results are then summarized in Sec. Liller (1980, hereafter referred to as L80). She setup a 4. photoelectric sequence composed of 14 stars in the vicinity of NGC 6535 and proceeded to derive photographic pho- 2. PROCEDURES tometry for a total of 103 stars. The cluster CMD dis- 2.1 Observations played a predominantly blue horizontal branch (HB) with an estimated magnitude at the instability strip of F(HB) The observations of NGC 6535 were obtained with the = 15.8 ±0.1. Liller also discovered two RR Lyrae vari- direct-imaging CCD cameras on the 0.9-m and 1.5-m tele- ables: star VI with F= 16.59, i?—F=0.67, and V2 with scopes at Cerro Tololo Inter-American Observatory. Table 1 shows the observing log. The Texas Instruments (TI) F= 17.2:, i?—F=0.4:. However, she concluded that they W must be field stars because they are significantly fainter CCD was binned 2x2 and has an image scale of 0 49/ pixel with a field size of 3.3 arcmin square. The Tektronix than the HB. / The next photometric study of NGC 6535 was that by (TEK) CCD has an image scale of 0' 24/pixel with a field size of 8.1 arcmin square. Figure 1 shows an image of NGC Anthony-Twarog and Twarog (1985, hereafter referred to 6535 in the V filter taken with the TEK CCD. The square as ATT). They present a CMD based on Video Camera region indicated in Fig. 1 represents the TI cluster field. images of a field located southeast of the cluster center. Furthermore, a region located 10 arcmin south of the clus- Unfortunately, photometric error and the presence of ex- ter was observed to monitor field-star contamination. Note tensive field-star contamination (the cluster is at I—IT, h m s o that Trager et al. ( 1993) list 8.4 arcmin as the cluster tidal b= +10° and «1950= 18 01 17 , ^950= —00 18!0) reduced radius. All of the images were taken under photometric the usefulness of their CMD. However, they were able to conditions except for those on 1993 July 17. On the pho- conclude that the number of blue stragglers in NGC 6535 tometric nights, standard stars with B—V colors between seems to be unusually small. —0.1 and 1.5 drawn from the lists of Graham (1982) and In this paper, we present the tirst Β, V CCD photometry Landolt ( 1983) were observed at airmasses ranging from 1 for NGC 6535. This cluster was observed as part of a to 1.3. The instrumental setup and the processing of the TI survey of Galactic globular clusters in search of blue- images has already been discussed in the previous papers of straggler stars. The observations and reductions are de- this series (see below). As for the TEK images, they were scribed in Sec. 2. The resulting CMD, presented in Sec. 3, ñat-fielded with dome flats. Twilight sky flats were ob- extends from the upper giant branch to approximately 2 tained to serve as illumination correction frames, but no correction of this sort was required. Visiting Astronomer, Cerro Tololo Inter-American Observatory, Na- tional Optical Astronomy Observatories. 2.2 Standard Stars 2NOAO is -operated by the Association of Universities for Research in Astronomy, Inc. under cooperative agreement with the National Science The reader is referred to Sarajedini and Da Costa ( 1991, Foundation. hereafter referred to as Paper 1 ) for a full discussion of the 404 © 1994. Astronomical Society of the Pacific © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System NGC 6535 PHOTOMETRY 405 Table 1 NGC 6535 Observing Log Frame Center Date Telescope Exposures CCD Airmass FWHM (arcsec) 1.5'East 18 August 1987 0.9-m 3 χ 1800s Β TI#1 1.26 2.4 3 χ 1200s V 1.24 2.3 10'South 29 August 1989 0.9-m 3 χ 1200s Β Ή #3 1.16 2.6 3 χ 900s V 1.24 2.1 Center 17 July 1993 1.5-m 1 χ 1200s Β TEK#1 1.20 1.5 Ix 900s V 1.24 1.5 Center 17 July 1993 1.5-m Ix 900s Β TEK#1 1.29 1.4 Ix 600s V 1.34 1.4 standard-star reduction procedure. For the night of 1987 were reduced first. The positions of these stars were then August 18, the equations which form the photometric cal- applied to the TI frames, daophot varied the positions ibration are and magnitudes of these stars in order to fit the PSF. Note that allstar ii has an option whereby, at designated b=B+a - (0.128±0.006) (£- V) + (0.187±0.006 B points in the iterative PSF-fitting scheme, the local sky + (0.00128 ±0.00105 ) C/TB rms=0.008 mag (1) background is redetermined in a user-specified annulus around each star after all of the known stars have been y= V+av+ (0.0232 ±0.0036) (5— V) + (0.133 subtracted from the frame. In this way, a more robust estimate of the local sky is derived. ±0.002)Xr rms=0.005 mag, (2) Because the TI #1 CCD suffered from poor photon and for the night of 1989 August 29, the equations are sensitivity over a region ^30 pixels wide along one edge, any star located in this area was automatically deleted. b=B+aB- (0.146± 0.004) (^- V) -f (0.190± 0.007)Z5 After further editing using the image diagnostics output by daophot (see Paper 1), the resultant photometry from -f (0.00696 ±0.00189)ί/Γ5 rms=0.020 mag, (3) each frame pair was matched to form colors. The photo- v=: V+av+ (0.0177±0.0028) (5— V) + (0.147± 0.004) metric data were then transformed to the standard system using Eqs. (1) and (2) in the manner described in Papers (0.000659±0.000118) rms=0.012 mag. 1 and 2. Magnitude offsets between each B,V frame pair (4) and the mean photometric zeropoint were never greater than ±0.01 mag; therefore each pair was adjusted to the In this formulation, b and υ refer to the instrumental mag- mean zeropoint of all of the frames and the magnitudes of nitudes (corrected for exposure time), Β, V, and {Β— V) stars in common were averaged, weighted by the inverse are the standard-system values, and X is the airmass of square of their errors. each observation. Note that, in Eq. (1), the coefficient of The nonphotometric data (1993 July 17) were cali- the UTB term is only slightly larger than its error. How- ever, this term is needed in order to eliminate the trend brated in a two-step procedure. First, we used the photo- seen in the residuals as a function of UT . electric sequence setup by L80 supplemented by photo- B graphic photometry for one of her brightest and reddest stars in order to solve for the color coefficients in the fol- 2.3 Ouster Frames lowing equations: The reduction procedure for the cluster frames is similar to that of Sarajedini ( 1992, hereafter referred to as Paper B— V=aB_ y-\-b B_ v(b—v), (5) 2) and Sarajedini ( 1993, hereafter referred to as Paper 3). v=V+a +b {B-V). (6) Briefly, each of the 16 CCD frames was reduced separately v v using the second-generation daophot ii photometry pack- The B—V colors of the stars used to perform these fits age (Stetson 1987). Typically 10 to 13 uncrowded stars on ranged from 0.36 to 1.55.
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