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

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. Liller claims that her photoelec- the TI frames and 50 to 60 such stars on the TEK frames tric V and B—V measures are good to <0.015 mag. In- were used to construct the point-spread function (PSF), deed, the RMS residuals for Eqs. (3) and (4) were 0.015 which was then fit to all of the detected profiles on each and 0.009 mag, respectively, for the long-exposure frames frame. In the case of the TEK CCD images, the shape of and 0.019 and 0.013 mag for the short-exposure frames. the PSF was made to vary quadratically across the frames. The zeropoints of Eqs. (3) and (4) were then set by com- Three iterations of find, phot, and allstar ii were ap- parison with the photometric data of 1987 August 18. plied to each image. Because of the poor seeing conditions Once the photometric calibration was complete, stars present on the night of 1987 August 18, the TEK images appearing on the various framç pairs were matched. The

Fig. 1— Reproduction of a 900-s V exposure of NGC 6535. The numbered stars are from Table 2. North is to the right and East is at the top. resulting error in each V and B—V measurement is based on the frame-to-frame scatter in these values. In particular, 14 we adopt the standard error of the mean, determined using the small-sample statistical formulae of Keeping ( 1962), as the 1σ error in each V and B—V measurement. 16 .· c : '·· > 3. COLOR-MAGNITUDE DIAGRAM 18 Figure 2 shows the color-magnitude diagram (CMD) of 932 stars in the direction of NGC 6535. These data are tabulated in Table 2, wherein Ν gives the number of de- 20 tections. Only stars with at least two detections, V errors less than 0.050 mag, and B—V errors less than 0.071 mag I , . . ι are shown in Fig. 2 and Table 2. In limiting ourselves to 0.5 1.0 1.5 these stars, we ensure that only the highest quality pho- Β-V tometry is included in the CMD. Figure 3 shows the CMD Fig. 2— Color-magnitude diagram for NGC 6535 based on 932 stars. for stars in the off-cluster region. These have been subject Only those with at least two detections, V errors less than 0.050 mag, and to the same editing criteria as the data in Fig. 2. B—V errors less than 0.071 mag are shown.

Table 2 NGC 6535 Photometry NGC 6535 Off-Cluster Field V B-V Ν

1 1023.97 6.89 18.83 1.39 2 184.56 14.43 18.25 1.10 3 1454.45 18.41 18.64 0.97 4 1708.28 28.17 19.51 0.97 5 1963.69 36.50 19.08 1.01 6 71.63 40.09 19.06 1.12 7 1817.09 42.58 20.04 0.86 20 - 8 653.33 45.23 17.20 1.13 9 207.33 51.78 18.30 1.05 10 1422.30 53.30 19.43 1.23 11 961.72 56.55 18.17 1.04 1.0 1.5 12 949.11 58.72 18.25 1.18 B-V 13 1431.35 59.90 16.76 1.12 14 1922.83 60.89 17.29 1.08 15 1314.38 67.49 17.83 1.12 Fig. 3— Color-magnitude diagram for a field region centered 10' south 16 724.37 68.17 15.34 1.08 of NGC 6535. Only stars with at least two detections, V errors less than 17 1950.74 72.38 17.72 1.16 0.050 mag, and B—V errors less than 0.071 mag are shown. 18 1559.11 72.93 19.04 1.04 19 1573.03 74.67 16.91 1.01 20 558.02 87.59 18.18 1.09 explored in Sec. 3.3. There is also a prominent blue- 21 1150.38 88.11 17.30 0.97 22 971.75 90.37 18.34 1.08 straggler sequence extending blueward and brightward of 23 555.70 93.03 18.12 0.84 the main-sequence tumofF. 24 1023.13 99.68 17.70 0.98 Although the features of a typical globular-cluster 25 1556.83 99.82 18.18 1.46 CMD are evident in Fig. 2, the signature of field-star con- 26 395.15 107.37 18.79 1.15 tamination is also quite obvious. As shown in Fig. 3, the 27 1547.00 110.13 16.66 1.43 28 551.94 114.00 18.61 1.31 morphology of the noncluster CMD consists of a "col- 29 770.82 118.34 19.11 1.05 umn" of stars with F between ~1.0 and ^ 1.3 along 30 669.39 118.82 17.13 1.05 with scatter to redder colors. The four panels of Fig. 4 31 1036.87 121.61 19.81 1.11 show the cluster CMD divided into arbitrary radial bins 32 506.74 124.03 17.99 1.09 33 1905.06 124.80 17.41 1.51 based on the core radius given by Trager et al. ( 1993). The 34 170.31 148.73 17.81 1.01 dashed polygon is the adopted region of the blue stragglers 35 1870.30 151.29 19.27 0.97 36 1846.93 155.89 17.81 1.20 37 1984.01 159.34 17.13 1.04 38 72.61 164.02 17.69 1.29 39 1365.29 167.47 18.14 0.56 40 48.30 172.15 18.65 1.43 41 1680.38 173.81 18.18 1.12 42 1265.39 176.52 19.07 0.87 43 1397.09 176.69 19.02 0.91 44 719.31 176.83 19.89 1.01 45 716.37 184.45 19.36 1.03 46 1495.47 185.19 17.29 1.14 47 1635.87 185.99 19.42 1.14 48 427.71 186.75 19.23 1.32 49 1521.74 188.66 18.95 0.91 50 1669.13 194.81 15.97 1.19

Note: Table 2 will be included in its entirety in Vol. 2 of the AAS CD- ROM Series. The first page of the table is presented here to show its form and content.

3.1 Morphology As first pointed out by L80, the CMD of NGC 6535 (Fig. 2) shows a predominantly blue HB and a relatively Fig. 4— Color-magnitude diagrams for four different radial zones in steep red-giant branch (RGB) both similar to that of M13. NGC 6535 using the data in Table 2. The solid line is the fiducial se- In addition, there are two faint blue stars that may consti- quence from Table 3. The dashed region designates the location of the tute the faint extension of the HB. This possibility will be blue straggler stars.

Table 3 NGC 6535 Fiducial Sequence 0.4 -

V B-V 0.2 - < 0.0 . Λ 13.20 1.638 13.40 1.587 13.60 1.539 -0.2 ■ 13.80 1.493 14.00 1.450 14.20 1.410 14.40 1.372 I " ι I ι . ι ι . I . I I I I 14.60 1.336 14.80 1.303 15.00 1.273 15.20 1.245 15.40 1.218 15.60 1.195 Μ 0.0 ■ 15.80 1.175 < . : · · f.· s 16.00 1.158 16.20 1.145 -0.2 - 16.40 1.131 16.60 1.117 -0.4 - 16.80 1.105 17.00 1.093 17.20 1.082 17.40 1.072 17.60 1.062 17.80 1.052 Fig. 5— A comparison of the CCD photometry presented in this study 18.00 1.042 with the Video Camera data of Anthony-Twarog and Twarog (1985, 18.10 1.036 hereafter referred to as ATT). The two panels show Vccd~ ^att (t0P) 18.20 1.030 18.30 1.022 and (B— K)CCD—(i?— V)att (bottom) both as a function of FCCD. 18.40 1.010 18.50 0.995 18.60 0.973 mag and =0.00±0.01 mag. If we consider 18.70 0.940 only stars with FCCD<19.0, then 16.2. mediate metallicity cluster M79 with [Fe/H] = -1.69 ±0.09 (ZW84). The top panel of Fig. 6 shows the HB of Ml5 from Buonanno et al. (1985) shifted by +0.35 in 3.2 Comparison with Anthony-Twarog and Twarog (1985) Β— V and —0.10 in V along with the NGC 6535 blue HB stars from Fig. 2 and the fiducial from Table 3. These There are 91 stars in common between the present study offsets were chosen so as to match the two HBs. The and that of ATT. The two panels of Fig. 5 show dashed box represents the region of the Ml5 RR Lyrae

^ccd νatt (top) and (B — V)CCD — (B—V)ATT (bot- variables from the work of Sandage (1990). Finding the tom) both as a function of FCCD. The mean offsets for all magnitude at each end of the box and computing the mean stars plotted in Fig. 5 are as follows: (ΔΚ) =0.03 ±0.01 yields ( FRR) = 15.72 ±0.15, where the error represents the

15 stars with colors as blue as these. One reason we selected M79 to compare with NGC 6535 is that its blue HB extends to remarkably faint magnitudes. In fact, the HB fi- 16 ducial of Ferraro et al. ( 1992) indicates a difference in V of > 2.8 mag between the level of the RR Lyraes and the faint end of the blue HB. There is also a handful of stars even 17 fainter than this point by up to ~ 1 mag. This implies that the two faint blue stars in NGC 6535 may constitute an extension of the HB. If so, we will need to explain the gap 15 of 1.5 mag along the NGC 6535 HB, which is not seen in that of M79.

16 3.4 Reddening There are several reddening values for NGC 6535 in the 17 literature. Webbink (1985) lists E{B— V) =0.33±0.04 > based on the color of the blue edge of the instability strip. From a comparison of the cluster-integrated color with 18 intrinsic colors derived from the cluster spectral type, Bica and Pastoriza ( 1983) give E{B—V) =0Al and Reed et al. (1988) derive EiB-V)=0.20. Zinn (1985) lists 19 E(B—V)=0.32 based on a relation between intrinsic color and the line-blanketing index Ô39. The reddening maps of Burstein and Heiles (1982) yield a value of E{Β-V) =0.37±0.04 (see ATT). It is also possible to derive a reddening through the use of Fig. 6 and the reddenings of Ml5 and M79. For the Fig. 6— (a) The superposition of the blue horizontal-branch (HB) stars in NGC 6535 and the HB fiducial of Ml5, which has been shifted by former, we adopt ^(5-K) =0.10±0.02 (Sandage 1990; +0.35 in 5— F and —0.10 in V. The dashed box represents the location Zinn 1985) and for the latter, ii(i?—F) =0.01 ±0.02 of the RR Lyraes in Ml5 while the dashed line is the NGC 6535 fiducial (Ferraro et al. 1992, and references therein; Zinn 1985). from Table 3. (b) Same as in (a) except that the HB fiducial of M79 has Using these reddenings along with the color offsets needed been plotted oiFset by +0.42 in Β— V and —0.45 in V. Because the M79 RR Lyrae photometry is not well determined, the dashed box indicates to superpose the HBs in Fig. 6, we find reddenings for the location of the RR Lyrae variables in M3, which has a metallicity NGC 6535 of E{B—V)= 0.45 ±0.03 from M15 and similar to that of M79. ii(i?—F) =0.43 ±0.03 from M79. The quoted errors as- sume an error in the color offsets of ±0.02 mag. Taking the mean of these values yields a reddening of E( B—V) half-height of the dashed box. The bottom panel of Fig. 6 =0.44 ±0.02. As a check of this quantity, we can compare shows the same comparison with the HB of M79 from the observed blue-edge color of the NGC 6535 HB with the Ferraro et al. ( 1992), which has been shifted by +0.42 in intrinsic color given by Sandage ( 1990) of ( i?— F)o=0.18. B—V and —0.45 in V. Unlike those in M15, the photom- The dashed box in Fig. 6(a) has a blue-edge color of B—V etry of the RR Lyraes in M79 is not well determined. As a =0.62 implying a reddening of E(B—V) =0.44 while the result, the dashed box in Fig. 6(b) represents the region of same box in Fig. 6(b) has a blue-edge color of i?— F=0.61 the M3 ([Fe/H]=-1.66±0.06, ZW84) RR Lyrae vari- leading to a reddening of E{B—V)=0A3. Both of these ables from the work of Sandage (1990). In this case, we values are in accord with our derived reddening. find (KRR) = 15.74±0.15. Taking the mean of these quan- With such a high reddening value, one might suspect tities results in our adopted value of (FRR) = 15.73±0.11. that some amount of differential reddening exists across This compares favorably with ΚΗΒ=15.8±0.1 derived by the face of NGC 6535. Using the formalism described by L80. We note in passing that the NGC 6535 star in the Cudworth and Rees (1990), we find a gradient of +0.005 dashed boxes of Fig. 6 should be an RR Lyrae based on its mag/arcmin in the north direction and —0.003 mag/ location. However, we cannot check this possibility be- arcmin in the east direction. Within 5 core radii, where the cause it was only detected twice on our frames and over a cluster CMD is best defined (see Fig. 4), differential red- short time baseline (^30 min). dening accounts for only ±0.01 mag dispersion in the Turning to a discussion of the two faint blue stars in the B—V color. It is for this reason that these effects have been cluster CMD of Fig. 2, we pose the question: are these ignored in the present discussion. genuine HB stars? The brighter one (#270 in Table 2) is at λ:=673, ^=826 in Fig. 1, which is an uncrowded region 3.5 Metallicity ~ 100 arcsec (^4Äcore) from the cluster center. The fainter star (#404 in Table 2) is at λ: =1011, j>=993, The metallicity of NGC 6535 has been studied via inte- which is a relatively crowded region near the cluster cen- grated-light techniques. Using DDO photometry, Bica and ter. First, note that the off-cluster CMD in Fig. 3 shows no Pastoriza (1983) derive [Fe/H] = —1.73 ±0.45, while

3.6 The Age of NGC 6535 -2 It is clear from panels (a), (b), and (c) of Fig. 4 that the photometry of NGC 6535 extends to ^2 mag below the tumoff (TO). This makes it possible to study the clus- an ter age via AFXo_hb d Δ (5 — F)SGB_T0. After combin- ^ 0 ing the data in panels (a) and (b) of Fig. 4, we fit a cubic polynomial to the turnoff region (i.e., 18.7< V<20.2 and 0.73 < 5—F<0.93) using the iterative 2σ rejection algorithm described by Sarajedini and Norris ( 1994). From the resulting relation, we derive (i?— F)Xo=0.831 ±0.018 and 2 Ftq- 19.39 ±0.15, which, along with the value of ^RR. yields ΔΚΧΟ_ΗΒ=3.66±0.19. This is essentially identical to the AFXo_hb of NGC 6397 (3.64±0.14, Chaboyer et (B-V)o al. 1992), which has a metal abundance of [Fe/H] = -1.91 ±0.14 (ZW84). Thus, the close agreement of the metal- Fig. 7— The principal sequences of M15 (solid line) and M79 (dotted licities and ΔΚΤο_ηβ values of NGC 6397 and 6535 im- line) placed in the HR diagram (see text) along with the giant branch plies that their ages are also similar. fiducial of NGC 6535 and its blue horizontal-branch stars. Since these two clusters have similar metallicities, we can also compare them via the A(i?—F)SGB_Xo method ZW84 quote a value of [Fe/H] = —1.75±0.15 through the (Sarajedini and Demarque 1990; VandenBerg et al. 1990; application of their index. Note that based simply on hereafter referred to as VBS). Simply stated, this technique the morphology of the NGC 6535 CMD, blue HB and no relies on the difference in color between the turnoff and the RR Lyraes, it appears that the cluster metal abundance is subgiant branch as an indicator of age with probably between [Fe/H] = — 1.5 and 2 (Lee et al. Δ (5 — K)SGB_Xo becoming smaller for older clusters. The 1990). At the level of the HB ( < FRR) = 15.73 ± 0.11 ), the comparison is usually performed between two clusters of RGB fiducial in Table 3, which was derived via a polyno- similar metal abundance to minimize uncertainties in the mial fit, gives a value oí {B— F)g= 1.18±0.02, where the theoretical colors used to interpret the observed difference error incorporates the uncertainty in (^rr) and an esti- in Δ (5— F)sgb_Xo in terms of an actual age difference. In mated uncertainty of ±0.02 mag in the photometric ze- the present case, adequate photometry is not yet available ropoint. When the apparent RGB color is combined with in the literature for NGC 6397. However, because the prin- the cluster reddening, we find (i?—0.74±0.03. If cipal sequences of NGC 6397 and M92 are a close match the relation between [Fe/H] and {B— V)og given by ZW84 as shown in Fig. 4 of VBS, we will use M92 ([Fe/H] is used, we derive a metal abundance of [Fe/H] =-1.82 = -2.24±0.08, ZW84) in the comparison presented ±0.13. If instead, we make use of the relation derived by below. Sarajedini and Norris (1994), The A(B— F)sgb_Xo method as implemented by VBS involves the registration of cluster sequences by the tumoff [Fe/H] =5.37(5- F)^-5.84, (7) color (B—V)TO and the magnitude F+005 of the main then [Fe/H] = —1.87±0.16. Equation (5) is on the abun- sequence at the point 0.05 mag redder than the tumoff. dance scale of ZW84, but the fit has been performed When registered in this manner, older clusters have bluer slightly differently and incorporates somewhat different giant branches and vice versa. Figure 8 illustrates this nor- photometric data than the relation derived by ZW84. Com- malization using the NGC 6535 photometry (open circles) bining these two metallicity measures yields [Fe/H] from panels (a) and (b) of Fig. 4 and the fiducial sequence = —1.85 ±0.10, which is in reasonable agreement with the of M92 (Stetson and Harris 1988, solid line). In the case of results of the integrated light studies mentioned above. NGC 6535, the V+o.os value has been determined via the Finally, to check the self-consistency of these cluster 2σ rejection algorithm applied to stars with 20.0 < V< 20.8 parameters, the HBs and RGBs of M15, M79, and NGC and 0.8 < 5—F< 1.0. The magnitude and color offsets 6535 can be placed in the HR Diagram. Adopting given in Fig. 8 are in the sense M92—NGC 6535, and the Fhb(M15) = 15.86 (Buonanno et al. 1985), dashed lines indicate the RGB locations for a difference in FHB(M79) = 16.15 (Ferraro et al. 1992), the reddenings age of ±2 Gyr (see VBS). It is apparent that the ages of and metallicities given above, and the HB luminosity- NGC 6535 and M92 are identical to within ±2 Gyr. metallicity relation of Lee et al. (1990), we construct the The method as devised by Sarajedini and Demarque diagram shown in Fig. 7. The solid lines represent the Ml5 ( 1990) involves subtracting the color of the SGB from that sequences and the dotted lines are the M79 sequences both of the main-sequence turnoff thus forming the quantity taken from the studies cited previously. The dashed line is A(i?—F)sgb_Xo. Needless to say, Fig. 8 shows that the the NGC 6535 RGB from Table 3 and the filled circles SGBs of NGC 6535 and M92 have similar if not identical constitute the brighter HB stars from Table 2. As is evident B—V colors leading to similar A(i?—F)SGB_Xo values. from Fig. 7, the relative locations of all three cluster se- Based on ΔFxo_HB and the two incamations of quences are consistent with their metal abundances. A(B— F)sgb_Xo, we conclude that no significant age dif-

Table 4 NGC 6535 Blue Stragglers

star B-V

39 1365.29 167.47 18.14 0.56 302 1139.24 877.63 17.81 0.56 315 1091.49 900.64 17.71 0.47 384 200.94 971.25 18.90 0.72 420 1063.87 1007.16 17.74 0.76 505 814.66 1100.02 18.89 0.65 526 1653.70 1122.14 18.53 0.78 852 1099.97 1655.44 18.33 0.68 881 211.90 1750.70 18.55 0.73 0.0 0.1 0.2 (Β - V) - (Β - V)to cluster region (Fig. 3) is devoid of stars in the BSS region. Fig. 8— A comparison of the NGC 6535 photometry from panels (a) As a result, it is likely that the majority of the nine BSS and (b) of Fig. 4 (open circles) with the fiducial sequence of M92 (solid candidates are members of the cluster. line) both normalized by the color of the tumoff {{B— V)TO] and the V When placed in the BSS HR diagram of Paper 1, the magnitude on the main sequence at a point 0.05 mag redder than the BSSs of NGC 6535 occupy the same general region as tumofF ( ^+0.05)- When registered in this manner, older clusters have bluer giant branches and vice versa. The dashed line segments indicate the those in NGC 6101, which has a similar metallicity. Unlike RGB locations for a difference in age of ±2 Gyr. the case of NGC 6101, there are too few BSSs present in our sample to make meaningful conclusions regarding their radial distribution. However, it is interesting to note ference exists between NGC 6535, M92, and by implica- that the three brightest BSSs are located in the innermost tion NGC 6397. radial CMD plotted in Fig. 4. This phenomenon has also Before leaving this section, several further comments been detected in NGC 5053 (Nemec and Cohen 1989) and regarding the reddening and HB magnitude of NGC 6535 NGC 6101 (Paper 1). are in order. First, with regard to the reddenings, based on the theoretical isochrones of Chaboyer et al. (1992), a change in [Fe/H] from —2.30 to —1.80 causes a change in 4. SUMMARY (B—V)TO of -+0.03. Assuming a reddening of E(B — F)=0.02 for M92 (Zinn 1985; Stetson and Harris We have presented CCD photometry for the Galactic 1988) implies a value of E(B- V) =0.43 for NGC 6535. globular cluster NGC 6535. From the (V,B—V) color- This is in excellent agreement with the adopted reddening magnitude diagram, which extends below the main- of ^(5—F) =0.44±0.02 derived in Sec. 3.4. As for the sequence turnoff, we draw the following conclusions. HB magnitudes, based on the HB luminosity-metallicity (i) The horizontal branch (HB) is predominantly blue relation of Lee et al. ( 1990), a change in [Fe/H] of ^ +0.4 in nature with no RR Lyrae variables known to be cluster dex, as exists between M92 and NGC 6535, results in an members. However, based on a comparison with clusters HB luminosity that is fainter by 0.07 mag. Assuming an which have blue HBs and RR Lyraes (M15 and M79), we HB level of ΓΗΒ= 15.05 for M92 (Buonanno et al. 1985) infer a mean HB magnitude of (Κκκ) = 15.73 ±0.11 for implies a value of VHB= 15.70 for NGC 6535. Once again, NGC 6535. this is in excellent agreement with the adopted HB magni- (ii) Again, a direct comparison with the blue HBs of tude of (Κκκ) = 15.73±0.11. We take these internal con- Ml5 and M79 implies a reddening of E(B— F) =0.44 sistencies as evidence that the cluster parameters derived ±0.02. herein are robust and as a testament to the validity of the (iii) When combined with the apparent color of the red techniques and data used in their determination. giant branch at the level of the HB, {B—F)g= 1.18 ±0.02, this derived reddening yields a metal abundance of [Fe/H] = —1.85 ±0.10, similar to that of NGC 6397. 3.7 The Blue-Straggler Stars (iv) Application of the AFxo_HB and Δ (^- ^)sgb-to cluster dating techniques reveals no perceptible age differ- In the work of ATT on NGC 6535, the cluster CMD ence between NGC 6535 and NGC 6397. revealed two possible blue-straggler star (BSS) candidates. (ν) There is a significant population of nine blue- However, the photometry of these stars was sufficiently straggler stars detected in NGC 6535. However, this is too uncertain to lead ATT to speculate that NGC 6535 may few to facilitate a meaningful analysis of their radial dis- not have any BSSs. As is evident from Fig. 2, NGC 6535 tribution. does have a significant BSS population. There are nine such stars with 0.45 < (B— F) <0.80 and 17.5 < F< 19.0. These It is a great pleasure to thank the observing support staff are listed in Table 4 with the star numbers corresponding at CTIO for their tireless assistance. Travel support was to those in Table 2. Furthermore, the CMD of the off- provided by Yale University and KPNO.

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