THE 2MASS COLOR-MAGNITUDE DIAGRAM OF THE GLOBULAR CLUSTER LYNG8Å7 1

The Astronomical Journal, 1281228 1232, 2004 September # 2004. The American Astronomical Society. All rights reserved. Printed in U.S.A. THE 2MASS COLOR-MAGNITUDE DIAGRAM OF THE GLOBULAR CLUSTER LYNG8Å7 1 Ata Sarajedini Department of Astronomy, University of Florida, P.O. Box 112055, Gainesville, FL 32611 Received 2004 March 11; accepted 2004 May 20 ABSTRACT We have extracted JHK S photometry from the All-Sky Point Source Catalog of the Two Micron All Sky Survey (2MASS) in the vicinity of the globular cluster Lyngå 7. The cluster color-magnitude diagram (CMD) extends from the tip of the red giant branch to approximately 1 mag below the core helium burning red clump stars. The reddening-independent red giant branch slope in the 2MASS photometric system is measured to be (J K S )=K S ¼ 0104 0004, which implies a metal abundance of ½Fe=HŠ ¼ 076 006 on the Carretta & Gratton scale. Adopting a reddening of E(B V ) ¼ 073 012 as the average of previously published values, we use the inferred absolute K-band magnitude of the red clump ½M K (RC) ¼ 128 007Š to calculate a distance of (m M ) 0 ¼ 1433 010 (73 03 kpc). Finally, we have reanalyzed the previously published optical CMD of Lyngå 7 from Ortolani and coworkers along with the 2MASS near-ir photometry and conclude that Lyngå 7 is close in age to 47 Tucanae, if not slightly (2 Gyr) younger. Key words globular clusters individual (Lyngå 7) 1. INTRODUCTION Lyngå 7 ( 20000 ¼ 16 h 11 m 3 s 0, 20000 ¼ 55 18 0 52 00, l ¼ 328N77, b ¼ 2N79) was a relatively obscure star cluster until the work of Ortolani et al. (1993) showed that it may be a globular cluster belonging to the thick disk of the Galaxy. Their BVI color-magnitude diagrams (CMDs) revealed the cluster to have a predominantly red horizontal branch (i.e., red clump) and a metallicity between those of 47 Tucanae and NGC 6553/6528. In spite of the heavy field-star contamination in their CMDs, Ortolani et al. (1993) were able to conclude that Lyngå 7 is likely to be younger than the classical halo globular clusters but older than the most ancient open clusters like NGC 188 and NGC 6791. The question of which cluster population Lyngå 7 belongs to (old open cluster or young globular cluster) was addressed by Tavarez & Friel (1995). They obtained moderate-resolution spectra for stars in the direction of Lyngå 7.Basedonthe results for four of the cluster members, they find an average metallicity of ½Fe=HŠ ¼ 062 015. Tavarez & Friel (1995) also use the cluster s location and kinematics to argue that Lyngå 7 most likely belongs to the thick disk globular cluster system. In this paper, we use near-infrared JHK S photometry from the Two Micron All Sky Survey (2MASS) 2 in order to study the properties of Lyngå 7. 3 The next section (x 2) presents the CMD and its general properties. The subsequent subsections (xx 2.1 2.3) discuss the determination of the cluster s properties metal abundance, reddening, distance, and age. 1 This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. 2 See http//irsa.ipac.caltech.edu. 3 We also extracted 2MASS point source photometry around the globular clusters BH176, E3, and ESO452-SC11, but the principal CMD sequences are not adequately defined to allow fruitful analysis. 1228 2. COLOR-MAGNITUDE DIAGRAM Figure 1 shows the radial CMDs for the region centered on Lyngå 7. Following Grocholski & Sarajedini (2002, hereafter GS02), the 2MASS JHK S photometry has been transformed to the system of Bessell & Brett (1988) using equations (1) and (2) in GS02. The innermost diagram clearly shows features belonging to Lyngå 7 a red clump at K 135 and J K 10 along with a well-defined red giant branch (RGB) extending to brighter magnitudes. The other CMDs in Figure 1 show the effects of increasing field contamination at larger distances away from Lyngå 7. It should be noted that we also produced a (V; V K ) CMD for Lyngå 7usingtheBV data of Ortolani et al. (1993); however, because the optical data only cover 1 mag of the upper RGB, the utility of the resulting optical/ir CMD was limited. 2.1. Metallicity To estimate the metal abundance of Lyngå 7, we rely on the slope of the RGB. This feature has historically been used with great utility for this purpose. However, in the near-ir passbands, the calibration of metal abundance with RGB slope was first characterized relatively recently in a series of papers by Kuchinski et al. (1995). As the metallicity of a cluster increases, the slope of its RGB becomes shallower. Two of the more recent near-ir RGB slope studies are those of Ivanov & Borissova (2002) and Valenti et al. (2004a). The former is based on JK S photometry for 27 Galactic globular clusters from the Second Incremental Release of the 2MASS Point Source Catalog. The clusters span a range of abundances from the most metal-poor regimes up to ½Fe=HŠ CG 06, where CG represents the metallicity scale of Carretta & Gratton (1997). Based on these data, Ivanov & Borissova (2002) find RGB Slope ¼ 0158( 0010) 0058( 0007)½Fe=HŠ CG The work of Valenti et al. (2004a) builds on the results of Valenti et al. (2004b), Sollima et al. (2004), and Ferraro ð1þ

2MASS CMD OF LYNG8Å7 1229 Fig. 1. Radial CMDs for the region around Lyngå 7constructedusing2MASSJK S photometry transformed to the JK system of Bessell & Brett (1988). The inner region shows the clear presence of features belonging to Lyngå 7 red clump and red giant branch. et al. (2000). They use near-ir photometry transformed to the 2MASS system of 24 Galactic globular clusters, some as metal rich as ½Fe=HŠ CG 04. They find RGB Slope ¼ 0126 0045½Fe=HŠ CG Valenti et al. (2004a) also compare their relation to that of Ivanov & Borissova (2002) and discuss possible reasons for the difference in slope and zero point. In order to apply these relations to Lyngå 7, we need to measure the slope of its RGB. The vast majority of metallicity- RGB slope calibrations in the literature (e.g., Kuchinski et al. 1995; Ferraro et al. 2000; Ivanov & Borissova 2002; Valenti et al. 2004a) define the slope using RGB stars between 0.5 and 5.0 mag above the horizontal branch (HB). In the case of Lyngå 7, we have used the RGB fiducials of Ferraro et al. (2000) as a guide and extracted what appear to be firstascent RGB stars in this magnitude range from the 2MASS photometry inside 2 0 of the cluster center. These are shown as the filled symbols in the JK S CMD of Figure 2. The solid line in this figure is our least-squares fit to these stars and has the form J K S ¼1000(0009) 0104(0004)(K S 13) From this, we calculate a metal abundance of ½Fe=HŠ ¼ 093 007 based on the Ivanov & Borissova (2002) relation and ½Fe=HŠ ¼ 049 009 from the Valenti et al. (2004a) relation. Taking the weighted mean of these values yields ½Fe=HŠ ¼ 076 006. Given the uncertainties, this is consistent with the assertion of Ortolani et al. (1993) that the abundance of Lyngå 7 is between than of 47 Tuc and NGC 6553/6528, which have ½Fe=HŠ 01 (Harris 1996). Furthermore, our metallicity is slightly lower but still statistically identical to the spectroscopically determined value of ½Fe=HŠ ¼ 062 015 from Tavarez & Friel (1995). ð2þ ð3þ 2.2. Reddeninggand Distance The two previous studies of Lyngå 7 are in good agreement as far as the cluster s reddening is concerned. For example, Ortolani et al. (1993) estimated E(B V ) ¼ 072 012 using the magnitude and color of the red HB as compared with 47 Tuc and NGC 6553. Tavarez & Friel (1995) measured the reddening via a variety of techniques, finding a value of E(B V ) ¼ 073 012. As a result, for the remainder of this paper, we adopt E(B V ) ¼ 073 012, which translates to E(J K ) ¼ 039 006, using A V ¼ 31E(B V ), A K ¼ 011A V,andA J ¼ 028A V from GS02. To check this value using the 2MASS photometry of Lyngå 7, we rely on the work of Ivanov & Borissova (2002), which provides a method by which reddening can be estimated using the parameters of the JK S RGB. Their equation (7) is E(J K S ) ¼ RGB app ZP RGBabs ZP þ RGB slope (m M ) KS Each term is described more fully below, but for now, we note that this equation requires knowledge of the cluster s distance modulus. Ivanov & Borissova (2002) use the K-band magnitude of the RGB tip to estimate the distance. This approach is not practical in the case of Lyngå 7 because of the sparsely populated RGB. As a result, we use the luminosity of the red clump as calibrated by GS02 for this purpose. We begin with the assumption that the age of Lyngå 7is similartothatof47tucandngc362(seex 2.3) the only two red HB globular clusters in GS02 s calibration. Using the empirical result from GS02 that M K =½Fe=HŠ ¼119 for these two globular clusters along with the value of ½Fe=HŠ ¼ 076 006, we find M K (RC) ¼ 128 007 for Lyngå 7. The median magnitude of the Lyngå 7 red clump on the Bessell & Brett (1988) system is measured using a box size identical to that employed by GS02 as shown in Figure 2. This process yields K(RC) ¼ 1331 005, which implies an apparent distance modulus of (m M ) K ¼ 1459 009. ð4þ

1230 SARAJEDINI Vol. 128 Fig. 2. Left CMD for the inner region from Fig. 1 along with the fitted points ( filled circles) and solid line representing the red giant branch of Lyngå 7. Right Variation of measurement error with K magnitude. The other inputs into equation (3) are the values of RGB app ZP, RGB abs ZP,andRGB slope. The first quantity (RGB app ZP ¼ 2350 0041) is derived by redoing our RGB fit (eq. [2]) without subtracting an offset of 13 from K S. The second input (RGB abs ZP ¼ 0348 0006) is calculated from Table 4 of Ivanov & Borissova (2002) using the metallicity value determined from the application of equation (1). The third number (RGB slope ¼ 0104 0004) is given in equation (3) above. Inserting all of these values into equation (4) and noting that (m M ) K ¼ (m M ) KS, 4 we find that E(J K S ) ¼ E(J K ) ¼ 048 007½E(B V ) ¼ 091 013Š. Towithinthecombined errors, this is consistent with the values derived by Ortolani et al. (1993) and Tavarez & Friel (1995) noted above. The apparent distance modulus coupled with our adopted reddening leads to an absolute distance modulus of (m M ) 0 ¼ 1433 010 (73 03 kpc). This is somewhat larger than the distance derived by Ortolani et al. (1993), who find (m M ) 0 ¼ 1413, but is consistent with the Tavarez & Friel value of (m M ) 0 ¼ 143. 2.3. Cluster Agge Ortolani et al. (1993) used their BVI CMD to estimate the difference in magnitude between the HB and main-sequence turnoff (TO) of Lyngå 7tobeV(TO HB) 31. The mean 4 The transformation equation from the 2MASS K S magnitude to the Bessell & Brett (1988) K magnitude contains no color dependence. value for the majority of Galactic globular clusters is V (TO HB) 35 (Rosenberg et al. 1999); based on this difference, Ortolani et al. (1993) suggested that Lyngå 7is probably younger than the bulk of these clusters. In Figure 3, we show the Ortolani et al. (1993) BV photometry for Lyngå 7 offset using the distance and reddening we have determined herein. The dashed lines show the difference in magnitude between the HB and the TO [i.e., V (TO HB) 31] given by Ortolani et al. (1993). The solid line in the left panel of Figure 3 is the fiducial sequence of 47 Tuc derived from the photometric data of Kaluzny et al. (1998) adjusted by (m M ) V ¼ 1345 and E(B V ) ¼ 004 (GS02). The solid lines in the right panel of Figure 3 are the theoretical isochrones of Girardi et al. (2002) for Z ¼ 0004 and log t(yr) ¼ 66, 9.8, 10.0, and 10.2. Based on the appearance of Figure 3, we draw the following inferences. First, the left panel suggests that Lyngå 7 is about the same age, if not slightly younger, than 47 Tuc. In particular, the isochrones in the right panel favor an age of 12 Gyr for 47 Tuc and 10 Gyr for Lyngå 7. In light of the fact that the TO is the bluest point on the main sequence, it seems that the estimated value of V(TO HB) given by Ortolani et al. (1993) is too small by perhaps a few tenths of a magnitude. We can perform a consistency check of the cluster s age by examining the predicted dereddened color of the red HB as a function of metal abundance and age. We have replotted Figure 8 of GS02 in Figure 4 of the present paper. This shows the median red clump (J K ) 0 value for 14 open clusters and

No. 3, 2004 2MASS CMD OF LYNG8Å7 1231 Fig. 3. Optical BV CMD of Lyngå 7 from Ortolani et al. (1993) adjusted for a distance modulus of (m M ) V ¼ 1659 and a reddening of E(B V ) ¼ 073. The solid line in the left panel is the 47 Tuc fiducial based on the photometry of Kaluzny et al. (1998) and adjusted by (m M ) V ¼ 1345 and E(B V ) ¼ 004 (GS02). The solid lines in the right panel are the theoretical isochrones of Girardi et al. (2002) for Z ¼ 0004 and log t(yr) ¼ 66, 9.8, 10.0, and 10.2. The dashed lines superposed on both panels represent a difference of 3.1 mag between the HB and turnoff as measured by Ortolani et al. (1993). 2 globular clusters compiled by GS02 using the 2MASS database. This diagram suggests that the (J K ) 0 color of the red clump is a good predictor of the metal abundance for stellar populations with ½Fe=HŠ P 05 and those with ages less than 10 Gyr. In the case of more metal-rich systems, (J K ) 0 loses sensitivity to abundance, and for populations older than 10 Gyr, the (J K ) 0 color appears to be so sensitive to age as to be impractical. The solid line in Figure 4 shows the observed value of (J K ) 0 for Lyngå 7of055 010 mag. The dashed lines Fig. 4. Variation of dereddened J K color of the red HB with (left) metal abundance and (right) age. The data points are taken from GS02. The solid line represents the value of (J K ) 0 ¼ 055 for Lyngå 7 while the dashed lines show the 1 error of 0.10 mag.

1232 SARAJEDINI indicate the 1 range of this value. For both the metallicity and age, the red clump of Lyngå 7 possesses an intrinsic (J K ) 0 color similar to that of 47 Tuc, although the errors are admittedly large. Of course, the color of the Lyngå 7 red clump is also consistent with some very young high-metallicity open clusters, but the morphology of other CMD regions rules out this possibility. For example, Lyngå 7 cannot be younger than 3 Gyr [log age(yr) ¼ 95], because its TO would be located at M V 30, which is clearly not the case. Similarly, a metallicity as high as solar is ruled out by the slope of the RGB analysis presented earlier. However, we must keep in mind that the (J K ) 0 color does become insensitive to abundance for ½Fe=HŠ k 05. In closing this section, we note that in the absence of information about the cluster age and/or the metal abundance, Figure 4 can provide an important means by which to constrain these quantities if the error on the dereddened color is sufficiently small. The author is grateful to Aaron Grocholski for comments on an early version of this manuscript. We also acknowledge the comments of the referee, which greatly enhanced the content of this paper. This research was supported by NSF Career grant AST 00-94048. Bessell, M., & Brett, J. M. 1988, PASP, 100, 1134 Carretta, E., & Gratton, R. 1997, A&AS, 121, 95 Ferraro, F. R., Montegriffo, P., Origlia, L., & Fusi Pecci, F. 2000, AJ, 119, 1282 Girardi, L., Bertelli, G., Bressan, A., Chiosi, C., Groenewegen, M. A. T., Marigo, P., Salasnich, B., & Weiss, A. 2002, A&A, 391, 195 Grocholski, A., & Sarajedini, A. 2002, AJ, 123, 1603 (GS02) Harris, W. E. 1996, AJ, 112, 1487 Ivanov, V., & Borissova, J. 2002, A&A, 390, 937 Kaluzny, J., Wysocka, A., Stanek, K. Z., & Krzeminski, W. 1998, Acta Astron., 48, 439 REFERENCES Kuchinski, L. E., Frogel, J. A., Terndrup, D. M., & Persson, S. E. 1995, AJ, 109, 1131 Ortolani, S., Bica, E., & Barbuy, B. 1993, A&A, 273, 415 Rosenberg, A., Saviane, I., Piotto, G., & Aparicio, A. 1999, AJ, 118, 2306 Sollima, A., Ferraro, F. R., Origlia, L., Pancino, E., & Bellazzini, M. 2004, A&A, submitted (astro-ph/0402100) Tavarez, M., & Friel, E. D. 1995, AJ, 110, 223 Valenti, E., Ferraro, F. R., & Origlia, L. 2004a, MNRAS, in press (astroph/0404403) Valenti, E., Ferraro, F. R., Perina, S., & Origlia, L. 2004b, A&A, 419, 139