New Photometric Data of Old Open Clusters in the Anti-Galactic Center Region

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1 PASJ: Publ. Astron. Soc. Japan 56, , 2004 April 25 c Astronomical Society of Japan. New Photometric Data of Old Open Clusters in the Anti-Galactic Center Region Takashi HASEGAWA,HakimL.MALASAN, Hideyo KAWAKITA, Hitoshi OBAYASHI, and Tsutomu KURABAYASHI Gunma Astronomical Observatory, Nakayama, Takayama, Agatsuma, Gunma hasegawa@astron.pref.gunma.jp Tatsuji NAKAI Sanken Electric Co., Ltd., Kitano, Niiza, Saitama Masaaki HYAKKAI Gunma University, 4-2 Aramaki, Maebashi, Gunma and Nobuo ARIMOTO National Astronomical Observatory, Osawa, Mitaka, Tokyo (Received 2002 April 25; accepted 2004 February 25) Abstract We present new photometric data for 14 galactic open clusters taken by the 65 cm telescope at Gunma Astronomical Observatory. They were in the anti-galactic center region selected from the Catalog of Open Cluster Data (Lyngå 1987). We estimated the parameters of the clusters, i.e., age, metallicity, distance, and reddening, by fitting Padova isochrones to the color magnitude diagram. While no clusters were dated to be as old as young globular clusters, 13 clusters out of 14 are older than 1 Gyr, ranging up to 3.6 Gyr. Bearing in mind that out of approximately 500 clusters dated so far, only 40 clusters are older than 1 Gyr, our sample is exclusively dominated by old clusters. Four clusters were found away from the metallicity gradient curve and age metallicity relation so far delineated. Especially, 3 metal-rich clusters in the outer disk (Berkeley 36, Biurakan 11, and Biurakan 13) provide evidence against the picture advocated by Twarog et al. (1997, AJ, 114, 2556) that there is a break in the metallicity distribution at r GC = 10kpc and that the outer disk is chemically less evolved than in the inner disk. Key words: Galaxy: evolution Galaxy: open clusters and associations: general Galaxy: structure ISM: extinction 1. Introduction Old open clusters are often used as a good probe of the formation, structure, dynamics, and chemical evolution of the galactic disk spanning over a longer period than the predominant young open clusters and the old globular clusters. By old open clusters we herein mean clusters older than Hyades ( 0.6 Gyr; Perryman et al. 1998), three-times as old as the destruction time scale of clusters in the inner galactic disk (Spitzer 1958). From an observational point of view, open clusters are relatively easy to be dated and located in the Galaxy, because the red-giant branch and red-clump stars in old clusters are numerous and luminous. A list of old open clusters was complied by Friel (1995) based on a recent intensive survey by Phelps et al. (1994) and includes 74 clusters as old as, or older than, Hyades. Twarog et al. (1997) successively measured the metallicities of 76 clusters with 32 clusters overlapped with the list of Friel (1995). More recent studies include that of Kaluzny (1998, and references therein). A significant number of old open clusters have been found: about half of the 74 clusters in the sample by Friel (1995) are younger than 2 Gyr, and 19 clusters are older than M 67 On leave of absence from the Department of Astronomy, Institut Teknologi Bandung, Jl.Ganesa 10, Bandung 40132, Indonesia. ( 5 Gyr, Dinescu et al. 1995). Several oldest open clusters, including NGC 6791 (Carraro et al. 1994; Chaboyer et al. 1999), Berkeley 17 (Phelps 1997), and Collinder 261 (Gozzoli et al. 1996), are claimed to be as old as the young globular clusters. These clusters cast doubt on a clear separation of age between globular and open clusters. Old open clusters are intermediate not only in their ages, but also in their spatial locations; the scale height of young open clusters is 55 pc, while that of old open clusters is 375 pc (Janes, Phelps 1994), considerably thicker than that of the young open cluster population. This number implies that old open clusters share the common space with thick disk field stars. The most distant old open cluster so far found in the sample of Friel (1995) in the radial and tangential directions are Berkeley 29 at 18.7 kpc and Berkeley 20 at 2.4 kpc, respectively, which is in a marked contrast with the typical site of star formation in the galactic disk. The distribution of the old clusters provides important information about their origin and star-formation history in the outer galactic disk. Mostly based on metallicities from spectroscopy, Friel (1995) found a significant amount of radial gradient in the metallicity of old clusters, 0.07 to 0.10dexkpc 1.Twarog et al. (1997) interpreted the radial distribution of the metallicity as a set of two homogeneous areas with regard to the metallicity. The boundary of these two areas is located at 10 kpc from the galactic center. Friel (1995) found no vertical

2 296 T. Hasegawa et al. [Vol. 56, Table 1. Frequency of the COCD morphological parameters of the old clusters listed in Friel (1995). Detachment I II III IV Contrast Richness p m r Two clusters are not listed in COCD, and 5 are listed without morphological parameters. For example, 12 clusters are classified as II, 2, and r in detachment, contrast, and richness, respectively. gradient in the metallicity, but Piatti et al. (1995) found a gradient of 0.34 dex kpc 1 for a sample of 63 open clusters based on the DDO abundance (they found a radial gradient of 0.07dexkpc 1, consistent with the studies mentioned above). While Friel and Janes (1993) reported no correlation between age and metallicity, Carraro et al. (1998) claimed that metalrich clusters are absent for 3 5 Gyr clusters, while they are abundant in the older era (5 9 Gyr). Carraro et al. (1998) also claimed that stars and clusters follow the same age metallicity relation (AMR). In this context, accurate and abundant metallicity data of old open clusters are apparently crucial to investigate the chemical evolution of the galactic disk. As described later, a large number of clusters without photometric studies have morphological parameters that are consistent with old clusters, and hence it is worth focusing on such clusters to identify the really old ones. In this paper we present a set of cluster parameters for 14 old cluster candidates. Taking into account the aforementioned pictures of old open clusters, our primary interests in this study are: 1) to investigate the distribution of ages, with particular interest in whether our sample includes clusters as old as a young globular cluster or not, and 2) to find distant old clusters beyond r GC 15 kpc to probe past star formation in the outer disk. We are also interested in the metallicities of the clusters, and inspect whether our sample includes clusters that deviate from the canonical picture of the metallicity gradient and the age metallicity relations. Our observations are summarized in section 2, and photometric data are inspected in section 3 to fix the cluster parameters. The properties of our 14 sample clusters are compared in section 4 in light of the sample by Friel (1995). A summary is given in section Observation and Data Analysis 2.1. Sample Selection The largest catalog of open clusters is the Catalog of Open Cluster Data compiled by Lyngå (1987, hereafter COCD). COCD collected 1154 clusters found during an inspection of the Schmidt plates of the entire sky several times. The COCD provides three morphological parameters: the detachment, indicating the detachedness of the members running from I for the most crowded clusters to IV for the most detached clusters; the contrast, showing the range of the magnitude among the members from 1 for the small range to 3 for the large range (presumably including OB stars); and the richness, standing for the number of members with parameters p, m, and r, indicating poor, medium, and rich, respectively. These parameters were originally provided by Trumpler (1930) and re-defined in the COCD by Lyngå. In order to extract old clusters effectively from a catalog with a predominant population of young clusters, we refer to these morphological parameters. The old clusters listed in Friel (1995) are better delineated with the COCD parameters than with old Trumpler s parameters. The frequency of the COCD parameters of Friel (1995) sample is given in table 1. Very few old clusters are classified to have IV in detachment, 3 in contrast, and p in richness. Hence, old clusters are rich clusters and are dominated by a substantial number of crowded stars of similar luminosity. Following table 1, we have placed the selection criteria as follows: I, II, and III in detachment; 1 or 2 in contrast; and m or r in richness. Our selection yielded a list of 147 clusters (without measured ages). Here, we remember that a catalog by Collinder, which is dominated by sparse and poor clusters, judged from the morphological COCD parameters, does include a very important 10 Gyr old cluster, Collinder 261 (Gozzoli et al. 1996). This fact prompted us to include some additional clusters by inspecting the Digitized Sky Survey. For this purpose, we searched for the catalogs that appeared in the list of Friel (1995), and then had a look of all of the clusters in the catalogs to include those with similar appearance to the clusters in Friel (1995). This selection supplemented 50 more clusters. Clusters far below the celestial equator (δ < 15 ) were removed from our sample because of a poor point-spread function and difficulty in the photometric calibration. In this paper, as a step to increase the cluster sample, we report on the properties of 14 clusters, as listed in table 2. The observations were made during 2000 November and 2001 March and thus the clusters are predominantly distributed in the anti-galactic center region Observation The data were collected with the 65 cm telescope at the Gunma Astronomical Observatory, equipped with a CCD camera (AP7) assembled by Apogee Instruments Inc.. This camera installs a SITe blue-sensitized back-illuminated detector of pixels. Attached to the 65 cm telescope (F/12, f = 7800 mm), the pixel size of this detector, 24 µm, gives a pixel scale of 0. 6 in the sky and a field of view (FOV) of Most of the clusters in our sample have a small angular extension, and the FOV is eventually sufficient for the identification of main-sequence (MS) stars. Cooled with a Peltier cooler cycle, the detector temperature changed from 45 Cto

3 No. 2] Old Open Clusters in the Anti-Galactic Center Region 297 Table 2. List of targets and a summary of our observations. ID α δ l b COCD FWHM T Cam Exp [min] Limiting mag V -band C B V I B V I Be 8 2 h 01 m 06 s IIm Be 12 4 h 44 m 42 s IIm Be 23 6 h 33 m 30 s IIIm Be 36 7 h 16 m 06 s IIIm Be h 55 m 18 s IIm Be 64 2 h 21 m 00 s IIm Be 70 5 h 25 m 48 s IIIm Biu 7 6 h 57 m 36 s IIm Biu 8 6 h 58 m 06 s IIr Biu 9 6 h 57 m 42 s IIm Biu 11 6 h 51 m 18 s IIIp Biu 13 7 h 00 m 24 s IVp Rup 7 6 h 57 m 42 s IIm Czer 19 4 h 57 m 00 s IIm Included in this table are the targets, coordinates, morphological parameters (COCD), full width at half maximum (FWHM) of the stellar image, operating temperature of the camera (T Cam ), total exposure time, and the limiting magnitudes as measured where measurement error exceeds 0.1 mag. We note that Biu 7 and Biu 8 are identified as Be 31 and Be 32 in the catalog by Berkeley, respectively. 60 C, as shown in table 2, depending on the ambient temperature. Standard Bessell B, V,andI filters (Bessell 1990) were used, and the magnitudes of the stars were transformed into the Johnson Cousins system. Images obtained by the AP7 camera suffered from the so-called memory effect of incident light in the preceding exposures. In order to circumvent this, dark images were collected before the exposures for the objects and flat-fielding. Images of the clusters and standard stars were obtained at dithered positions by 10. The exposure time of the target clusters was s for short exposures and 3 or 5 min for long ones, and a cluster was observed at 4 dithered positions, at least. On each night, 6 or 7 standard stars were observed 5 7 times, selected from the catalog of Landolt (1992) in the field of SA 98. Observed in the winter season, the typical seeing size at V was 2. 5, and the best width in FWHM that we experienced was 1. 6forI band. Flat-fielding was made mostly with a sky-flat, since the consistency of a dome-flat and a sky-flat are approximately 0.5%, 1%, and 0.5% for B,V, and I bands, respectively. It was found from exposures of a stabilized lamp that the linearity of the detector was excellent between 300 ADU and ADU within 0.2%, and the AD conversion factor was 0.277e /ADU (independent measurements by TH and HLM) Measurements The standard procedures for dark-subtraction, flat-fielding, and sky-subtraction were subsequently performed with IRAF. Images were registered with each other and combined using a sigma clipping algorithm given in the IRAF. This clipping algorithm worked well, and any spurious traces from the memory effect were virtually eliminated. The seeing condition and crowded target field made the aperture photometry practically impossible, and we employed a point-spread function fitting with DAOPHOT (Stetson 1987) to find the instrumental magnitudes. An inspection of the images that were subtracted point-spread functions of all of the detected stars indicated that our detection was limited by the noise source (dark current and sky background), rather than the confusion. The instrumental magnitudes were then transformed to the magnitudes in the Johnson Cousins system night by night. Our photometric calibrations during our run may be summarized as: B = b c secz (±0.010)(b v) 4.16(±0.12) ± 0.027, (1) V = v c secz 0.089(±0.008)(b v) 4.14(±0.11) ± 0.016, (2) V = v c secz 0.127(±0.008)(v r) 4.10(±0.09) ± 0.015, (3) V = v c secz 0.062(±0.005)(v i) 4.15(±0.09) ± 0.015, (4) R = r c secz 0.135(±0.010)(v r) 4.14(±0.06) ± 0.013, (5) R = r c secz 0.126(±0.029)(r i) 4.22(±0.12) ± 0.021, (6) I = i c secz (±0.008)(r i) 4.66(±0.05) ± 0.038, (7) I = i c secz (±0.007)(v i) 4.64(±0.08) ± 0.023, (8) where b, v, r, andi stand for the instrumental magnitudes and B, V, R, andi for the magnitudes in the standard system, and

4 298 T. Hasegawa et al. [Vol. 56, Table 3. First 10 lines of the photometric data of Berkeley 12 sorted by V magnitude. ID X Y m B m V m I σ B σ V σ I A sample to show the contents and layout of all data which are available in the electronic version. z for the zenith distance, respectively. The day-by-day values of coefficient c of the airmass and the color term are given in appendix 3.2. Two error numbers in the coefficients indicate the day-by-day dispersion of the calibration, and the last one the typical residual of the standard star magnitudes from the catalog value. Since the magnitudes of the standard stars were measured with aperture photometry, our photometry may suffer from a small offset of a few percent due to an aperture correction. The magnitudes are correct relative to each other, and the ages and metallicities are basically robust to this error. The magnitudes from short and long exposures were, after a correction for the exposure time, merged together. The error in the magnitudes is quoted from the mag error parameter of DAOPHOT, which is statistically reliable when compared with two different exposures. The magnitudes are reliable down to V 19 with S/N=10 (see table 2 for each cluster), as measured by mag error = 0.1 mag from DAOPHOT. The first 10 lines of the data of Berkeley 12 are given in table 3 to illustrate the contents and the data format. All of the data are is accessible electronically, in which photometric data of stars with V<18.0 and σ V < 0.03 are included. 3. The Parameters of the Clusters Figure 1 shows the area for a photometric study of 14 clusters in pseudo-color. Color magnitude diagrams (CMDs) in (V I,V)[hereafter(V I,V)CMD]andin(B V,V) [hereafter (B V,V) CMD] are presented in figures 2 and 3. Two best-fit isochrones in our selection from the dataset of Padova group (Bertelli et al. 1994) are superposed to find the age, metallicity, color excess, and distance of the clusters, as indicated in the CMDs and summarized in table Isochrone Fitting Since our estimate of the parameters was not objective, we describe the way to fix the parameters and the factors that may lead to ambiguities. Our age estimates may be classified into two groups: isochrone fitting supplemented by a morphological inspection of CMD, and semi-quantitative measurements. The former is to find the best-fit isochrones to the CMD by trial and error, with free parameters of the age, metallicity, reddening E(B V )ande(v I), and (m M). Important morphological signs for the fitting include: 1) the slope (curvature) of the upper main-sequence (MS), 2) the gap (and/or hook) just below the blue turn-off, 3) the MS blue turnoff (MSTO), 4) the slope of the red-giant branch (RGB), 5) the luminosity of He-burning red clump stars (RC), which are the young metal-rich analog of horizontal branch in globular clusters, and 6) the ratio of the number of giants to the mainsequence stars (Castellani et al. 1992). The slope of the RGB is an indicator of the metallicity; it is shallower for a metal-rich system and steeper for a metalpoor system. Zinn and West (1984) evaluated the metallicity of young globular clusters from RGB; technically the same way was applied to the near-ir CMD of old open clusters Be 17 and 18 (Tiede et al. 1997). Judging from the Padova isochrones, this is also applicable to the old open clusters. For metallicity measurements, the slope and the curvature of the MS is rather reliable when numerous and clearly delineated MS dominate the CMD, especially for the old clusters (e.g. Biu 11 is typical in our sample). RC stars are numerous in old clusters (Cannon 1970). They are more quantitatively measured by Castellani, Chieffi, and Straniero (1992), who reported that RC stars are the most numerous for clusters of 1 2 Gyr, while subgiant and faint red giant stars become dominant ( 50%) in cluster older than 2.5 Gyr. Our semi-quantitative age measurements includes the so-called morphological age indicator, which is an analog of the δv method often applied to globular clusters, in which we measure the difference of magnitudes, δv, between the RC and MSTO. The magnitude difference, δv, basically depends on the cluster age, but for clusters younger than 3 Gyr this indicator is also slightly dependent on the metallicity, because of the morphology of MSTO. The δv method applied to our sample is described in subsection 3.3. We paid the attention to three points in the fitting. First, binary stars broaden the MS toward a more luminous direction, and we fit the fainter ridge in the CMD. Binary stars also work so that MSTO looks brighter; Anthony-Twarog and Twarog (1985) and Carraro and Chiosi (1994) applied a correction that a real MSTO would be fainter by 0.25 mag (in V ), which is not followed in this paper. The ages given with this correction would be longer than that without the correction. Second, blue stragglers also blur the MSTO to a brighter magnitude, and hence conspire to give a better fit with younger

5 No. 2] Old Open Clusters in the Anti-Galactic Center Region 299 Fig. 1. Area for a photometric study of the 14 clusters in pseudo-color. Blue, green, and red of the chart correspond to B, V, and I -band images, respectively. The FOV is approximately 5. Identification of the clusters is shown in the upper-right corner of each panel. The area adopted for the isochrone analysis is shown by thick yellow boxes.

6 300 T. Hasegawa et al. [Vol. 56, Fig. 2. CMDs of the observed clusters in (V I,V) CMD. Identification of the clusters is the same as figure 1. The filled and open circles stand for the stars inside and outside of the cluster area (yellow boxes in figure 1). The parameters of the isochrones are indicated. The size of photometric error is shown on the left side of the panel at intervals of V magnitude.

7 No. 2] Old Open Clusters in the Anti-Galactic Center Region 301 Fig. 3. Same as figure 2, but for the (B V,V)CMD.

8 302 T. Hasegawa et al. [Vol. 56, Table 4. Parameters of open clusters, as found from isochrone fits. ID Parameters from isochrone fit f 100 Age Z (m M) E(V I) E(B V ) (m M) 0 r GC z Gyr kpc pc MJyst 1 Czer Biu : Be Rup Be : Be Be : Biu Biu Be Biu Be Be Biu Columns (2 9) Parameters from isochrone fitting, including the age (Gyr), metallicity Z, (m M), color excesses in (V I,V)CMDand(B V,V) CMD, the distance modulus (m M) 0, the galactocentric distance r GC (kpc), and the distance from the galactic plane z (pc). Column (10) ISSA flux density at 100 µm. The galactocentric distance is given assuming r =8.5kpc. Colon in column (3) indicates that the value is poorly fixed. isochrones by 2 3 Gyr. We also need to discriminate the gap from the contamination of blue stragglers and binaries for younger clusters. Third, convective overshooting considerably changes the estimate of the ages; ages without overshooting would be underestimated by 30% for younger clusters of 1 2 Gyr (Maeder, Meynet 1991; Bertelli et al. 1992; Carraro, Chiosi 1994). Two estimations of age for clusters older than 4 Gyr, with and without overshooting are coincident within 0.5 Gyr, as is evidenced in the case of NGC 6791 (8.0 Gyr with or without overshooting by Carraro et al. 1994). It is interesting to note that mass segregation would be at work for the oldest open clusters, as confirmed by Sarajedini et al. (1999) for NGC 188 and M 67, and that Hawarden (1975) reported a deconcentration of red giants and clump stars for 6 old clusters. In our sample, Be 57, Biu 7, Biu 8, and Biu 13 are for the case of deconcentrated massive stars. For these clusters, it is difficult to fix the cluster area so that the cluster giants member are included as many as possible and, simultaneously, the CMDs are the least contaminated from the field stars. We specified a rectangular area as narrow as possible for the fitting of MS. However, since red giants and red clump stars are useful for guessing the cluster metallicity, and are rarely contaminated from background, we included bright red stars to look for the best-fit isochrones to the CMD, even if they are not included in the cluster area, only if these stars are not located unacceptably far from the best-fit track based on the MS stars. The field contamination may be evaluated as the surface number density of stars multiplied by our field of view. As an upper limit, we could refer to the star-count data given in Gilmore and Zeilik (2000); the contamination would be 0.39, 1.31, 3.55, and 8.84 stars for V =12.0, 13.0, 14.0, and 15.0, respectively. This should be an overestimate; the star count given in Gilmore and Zeilik (2000) is calculated at the galactic equator, averaged over the whole galactic longitude (including the galactic center), and no color selection is placed to select red stars in the RGB A Summary of Isochrone Fitting The best-fit isochrone is shown in the CMDs (figures 2 and 3). For clarity, the number of isochrones superposed in each CMD is limited to 2. Differential reddening is within the nominal DAOPHOT photometric error of MS stars for all of our CMD, as is described in subsection 3.4. Therefore, the parameters of the age, metallicity, and (m M) areset tobecommontothe(b V,V)and(V I,V)CMDs,but the reddening is determined independently in the 2 CMDs. Both (B V,V)CMD and(v I,V) CMD (figures 2 and 3) are fitted reasonably well with the same age, metallicity, and distance. The resultant best-fit parameters are given in table 4. Our judgment is that the ages are determined within 0.05 dex and the metallicities within one-step among the available metallicity scale in the Padova track ( 0.3 dex); they are inferred as being the nominal internal accuracy based on the CMD. A cluster-by-cluster description of an isochrone fitting is given in appendix 1. An independent check of the metallicity by means of the reddening distance relation is described in appendix δv Method (Morphological Age Indicator) Apart from isochrone ages, many authors have devised ways to evaluate ages for old open clusters; the most notable way among them is the δv method, or the Morphological Age Indicator advocated by Janes and Phelps (1994) and Carraro and Chiosi (1994). This is an open-cluster analog of the δv method for globular clusters. Although the spread in the

9 No. 2] Old Open Clusters in the Anti-Galactic Center Region 303 Table 5. Parameters of open clusters as found from δv method. ID δv Age JP94 CC94 mag Gyr Gyr Czer 19 Biu Be Rup Be Be Be Biu Biu Be Biu Be Be Biu Columns 2 4: δv value and ages from two calibrations by JP94 and CC94. magnitudes at RC gives an ambiguity of the method, in this study we applied δv method to 13 clusters, for which the probable RC were identified (except for the youngest cluster Czer 19). The δv method depends on the calibration from δv to the age which is so far given as an equation in Janes and Phelps (1994, hereafter JP94) and equation (3) in Carraro and Chiosi (1994 hereafter CC94). Columns (2), (3), and (4) in table 5 correspond to the measurement of δv in our CMDs and the ages following the calibrations given in JP94 and equation (3) in CC94. The calibration by JP94 shows a considerable overestimate if we directly measure the δv of the isochrone and convert this value to age, which is shown as three thin lines in figure 4 for three metallicities. This is also the case for the clusters in this study, for which the ages from isochrone and a JP94 calibration are given in table 5. This means that a significant number of clusters in Friel (1995), which are dated with JP94 calibration, may be overestimated. This is especially the case for clusters older than 4 Gyr, and it may be less reliable to rely on this calibration for the clusters of this range of age. CC94 presented another calibration based on the synthetic CMD to the typical old clusters. This calibration took into account the spectroscopic metallicities as input, and thus would be very accurate in the case where the metallicities are known. We have also converted the δv of our sample clusters (dots) as well as that of Padova isochrones (thick lines) based on the CC94 calibration, as shown in figure 4. Now, the derived ages from CC94 calibration are in a much better agreement with the ages from isochrone-fitting of the clusters, as well as (true) ages of the Padova isochrone. We conclude herein that our cluster ages are, as expected, consistent with those from the CC94 calibration. It is noted that the three relations between the CC94 calibration and the ages of isochrone are separated to a degree, but the estimated ages are consistent within 1 Gyr difference from metal solar to 20% solar metallicity. Fig. 4. Comparison of cluster ages from an isochrone-fit and the δv method with the CC94 calibration (filled circle). Ages of Padova tracks are correlated with the CC94 calibration, as shown by thick lines and with JP94 calibration by thin lines. The full, dashed, and dotted lines correspond to the Padova isochrones of three metallicities as indicated in the lower-right corner The Galactic Extinction Consistency of the color excesses from (B V, V) CMD and (V I,V) CMD The zero-point of photometry is not guaranteed apriori. One of the test to check this is to see the consistency of color excesses in E(B V )ande(v I) that are determined independently in the CMDs. This is shown in figure 5, and a tight correlation is found, except for Biu 11. This suggests that: 1) our photometric zero-point is calibrated reasonably well and we may infer the reddening and the distance of clusters based on our photometric data, and that 2) the reddening distance relation would be applicable to check the cluster metallicity, as described in appendix 2. Three relations are superposed in figure 5; the full line stands for E(V I) =1.25 E(B V ) from Munari and Carraro (1996), long-dashed line for Rieke and Lebofsky (1985), and dot-dashed line for Cardelli et al. (1989), respectively. In our case, a relation by Munari and Carraro (1996) represents our data best Correlation of color excess with ISSA 100 µm flux density With a successful estimate of the galactic absorption, even in the galactic center region (Hasegawa et al. 2000), we may refer to the 100 µm flux density of the IRAS Sky Survey Atlas (hereafter ISSA) to investigate the color excess of the clusters, partly because ISSA 100 µm flux density has a better angular resolution than the H I column density, and is emitted by the dust grains by themselves. To confirm that this effort is also reasonable in the anti-galactic center region, we compared the color excess estimated in our cluster region (as described in the subsection 3.2) with the ISSA 100 µm flux density (by

10 304 T. Hasegawa et al. [Vol. 56, Fig. 6. Correlation between ISSA 100 µm flux density and color excess from (V I,V) CMD indicated with filled circle and from (B V,V) CMD indicated with the open circle. The least-squares fit is shown for E(V I) with the full line and for E(B V ) with the dashed line, respectively. Fig. 5. Comparison of color excesses from (V I, V) CMDand (B V,V) CMD. The cross indicates the typical error size in color excess. The lines indicate the correlation between the two color excesses expected from various dust properties (see text for details). averaging the area of 5 square centered at clusters) in figure 6. A bright point source is apparent in the ISSA map of Be 57, and we have omitted this cluster from this comparison. A welldefined linear correspondence is found, rendering the ISSA 100 µm flux density to estimate the galactic extinction. The regressions are E(V I)=0.038 (f ) (9) and E(B V )=0.029 (f ). (10) The two coefficients of f 100 are in a good agreement with the relation of Munari and Carraro (1996). Taking equation (9) and assuming R V =3.1, we obtain A V =0.118 (f ). (11) The constant term (3.84 MJy str 1 ) is obviously contributed from stellar emissions and the interstellar medium. It should be noted that while the ISSA flux density traces the full column density throughout the disk, the color excess found from clusters is due to dust in front of the cluster; this is partly responsible for the scatter in the correlation Differential reddening CMDs inevitably suffer from differential reddening due to a fluctuation of the dust column density across the FOV. With the positive correlation of the ISSA flux density and the galactic absorption in equation (11), we next consider the differential reddening in the cluster region. The cluster for which differential reddening is the most significant is Be 70. However, even in this case, the fluctuation in the ISSA 100 µm flux density is about 3 MJy sr 1, corresponding to 0.11 mag in E(V I) when we apply equation (11); fortunately, the parameters of this cluster is well-defined as described in appendix 1. For other clusters, the peak-to-peak amplitude of the fluctuation in the detector FOV is as small as 0.5 MJy sr 1, corresponding to 0.02 mag in E(V I). This fluctuation is smaller than the photometric error of the CMD, suggesting that the differential reddening is not significant in our case. We note that an inspection of the ISSA flux density over a wider area (15 square) suggests that the fields of Be 64 and Be 70 may be located close to the diffuse interstellar medium. 4. Properties of Clusters in This Study It is apparently very difficult to construct a complete sample of old open clusters because they are mostly embedded deeply in the galactic plane, and also COCD is by nature heterogeneous. However, although our sample of clusters are not complete, it is interesting to see the properties of the clusters observed in this study. It is of particular interest to see if our sample includes any clusters that may be atypical in the current view of old open clusters General Properties Figure 7 is the distribution of the ages of the clusters in our sample, shown together with the old open clusters and globular clusters appeared in Friel (1995). Our cluster sample does not include any young globular clusters, nor clusters as old as M 67. However, except for Czer 19, the clusters are old, or at least the intermediate-age in the definition given in Carraro et al. (1999) that they are older than Hyades, ( 0.6Gyr, Perryman 1998). This age roughly corresponds to the lifetime of the cluster mentioned by Janes and Adler (1982). Our result would indicate that the selection criteria that we used were effective in the anti-galactic center region to exclude the predominant population of young clusters in the COCD, and is useful to understand the formation history of old open clusters. If we take the ages given in Friel (1995) at face value, then approximately half (32/74) of their sample are younger

11 No. 2] Old Open Clusters in the Anti-Galactic Center Region 305 Fig. 7. Distribution of the ages of our sample clusters dated with isochrones (thick line), Friel (1995) sample (dashed line), and globular clusters (thin line). than 1.5 Gyr, while a significant fraction (9/13) of our sample clusters are older than this. The distribution of clusters in the Galaxy is shown in figure 8. The distance modulus (m M) 0 to the clusters is derived from the (m M) ande(v I) in table 4, assuming E(V I) = 1.25 E(B V ) (Munari, Carraro 1996) and A V = R V E(B V ) with R V =3.1. Our sample clusters are located up to 1kpc away from the galactic plane, unambiguously coming from a population different from young clusters having a scale height of 55 pc (JP94; Phelps et al. 1994). The distribution of clusters in our sample is consistent with the sample by Friel (1995), but our sample increased the sample toward the outer disk. The galactocentric distance of the clusters is plotted against the age for the list of Friel (1995) and our sample in figure 9. This is analogous to figure 12 of JP94. Young clusters are distributed only in the inner disk (r GC 12kpc), and no clusters younger than 2 Gyr are found in the outer disk, consistent with the sample by Friel (1995). Biu 11 may be an exception, but we do not take this case very seriously, since this cluster suffers from some ambiguity of photometric calibration, and then our data follow the tendency that very few clusters are found in the outer disk during 1 2 Gyr, although clusters of such ages are observed frequently in the range of r GC = 8 12kpc. The distribution of clusters becomes more important when they are related with their metallicities (figure 10). The metallicity of our clusters is scaled into [Fe/H] with the simple relation of [Fe/H] = log(z/z ), (12) with Z = Since our cluster parameters are dependent on the Padova track (Bertelli et al., 1994), our metallicity scale is rather coarse. Our judgment is that they are measured within one step difference in the metallicity grid of Padova track. This is not so radical; a simple comparison of the spectroscopic metallicities given in Friel and Janes (1993) and the metallicities from isochrone-fitting on the broad-band CMD in the literature suggests that metallicity from the Padova track is on the accuracy of the one-step difference. From an inspection of figure 10, it is noteworthy that our sample does include 5 clusters, (Be 64, Be 70, Be 23, Be 57, and Biu 9), which prefer the metal-poorest scale of Padova track of Z =0.004, or equivalently [Fe/H] 0.7. Including these clusters, 9 out of 13 old clusters in our sample (shown with filled circle in figure 10) follow the radial metallicity distribution found in Friel (1995) (open circle). Contrarily, our sample includes 4 clusters in the outer disk (r GC 12kpc), Be 36, Biu 7, Biu 11, and Biu 13, which are considerably more metal-rich than that expected from the canonical metallicity gradient given by Friel (1995). Also, three of these clusters conflict with the picture claimed by Twarog et al. (1997), that the radial distribution of metallicity has a break at 10 kpc and clusters inside or outside that radius are homogeneous by means of metallicity. Taking into account the accuracy of our metallicity scale, the significance level is marginal but these clusters are worth being observed spectroscopically for further confirmation. Figure 11 is the age metallicity relation (AMR) after scaling the metallicity of clusters to the solar circle (assuming r GC = 8.5 kpc) with a radial gradient of 0.07 dex kpc 1. Bulk of clusters in our sample are sub-solar and most of our clusters are in the scatter of AMR in Friel (1995). Among the 5 abovementioned clusters with the metal-poorest scale in the Padova track, 3 younger clusters (Be 57, Be 64, and Biu 9) (1 1.5Gyr) still remain at the metal-poorest level, even if they are referred to the sample by Friel (1995). Four clusters (Be 36, Biu 7, Fig. 8. Distribution of open clusters (older than 0.5 Gyr) of our sample (filled symbol) and Friel (1995) sample (open symbol) in the distance from the galactic center and from the galactic plane. The symbol corresponds to the age of the clusters, as indicated in the upper-left corner.

12 306 T. Hasegawa et al. [Vol. 56, Fig. 9. Relation of age and the galactocentric distance. The open circles stand for clusters listed in Friel (1995) and the filled circles for our sample. Fig. 11. Age metallicity relation for our sample (filled circle) and the Friel (1995) sample (open circle). The metallicity values were corrected to the solar circle. The step size of metallicity scale in the Padova track is indicated in the upper-right position in the panel. is the third-distant cluster from the galactic disk among the 265 clusters younger than 0.15 Gyr in the WEBDA database. Also, most of the young clusters so far located are distributed inside 10 kpc from the galactic center, and Czer 19 is atypical in this context, and interesting object for further study. 5. Summary Fig. 10. Radial metallicity gradient of our sample (filled circle) and the sample by Friel (1995) sample (open circle). The step size of the metallicity scale in the Padova track is indicated in the lower-left position in the panel. Biu 11, and Biu 13), which are only marginally consistent with the metallicity gradient, are again metal-rich in the bulk of the Friel (1995) sample when corrected for the radial gradient scaled to r GC =8.5kpc ([Fe/H] corr > 0.3) Clusters of Interest With our estimate, the cluster Biu 7 is the second-distant cluster so far known located at r GC =17.5kpc and separated from the galactic plane by 800 pc. Since the most distant cluster so far located as an open cluster, Be 29, has no estimate on the metallicity, Biu 7 is, with a metallicity of [Fe/H]= 0.4, an extremely metal-rich cluster so far known at a distance of r GC 14kpc. This argument is still marginal, allowing for the one-step ambiguity in the metallicity measurements; this is a precious cluster for further confirmation and a detailed study. The youngest cluster in our sample (Czer 19 of 0.1Gyr) is located at 11 kpc away from the galactic center and away from the galactic plane by 400 pc, that is somewhat atypical as a young open cluster (with a scale height of 55 pc). In fact, this We present a new set of photometric data of 14 galactic open clusters in the anti-galactic center region, increasing the sample of old open clusters. From the Catalog of Open Cluster Data (Lyngå 1987) we selected a sample having the Trumpler parameters that are consistent with the old clusters identified in Friel (1995), with a small supplementary sample to this by eyeinspection of the Digitized Sky Survey. Photometric calibration was good enough to derive the cluster parameters: the age, metallicity, distance, and the reddening were found by fitting Padova isochrones in the color magnitude diagram. Our results are: 1. While no clusters were estimated to be as old as young globular clusters, out of 14 are older than Hyades, ranging up to 3.6 Gyr. Bearing in mind that only 80 clusters were found to be older than Hyades and 40 clusters older than 1 Gyr among 500 clusters so far dated, our sample is exclusively dominated by old open clusters. Our selection described in the text would be efficient to search for old clusters which are important for studying the history of cluster formation. 2. From the perpendicular distribution of the sample in the Galaxy, clusters in our sample are not consistent with the young open clusters at a scale height of 55 pc. 3. Four clusters in the outer disk are found to be metalrich, and are marginally consistent with the metallicity gradient and age metallicity relation so far delineated by Friel (1995), allowing for a coarse metallicity grid. Among these four, 3 clusters are against the picture advocated by Twarog et al. (1997): that there is a break at 10 kpc in the metallicity distribution in the galactic disk.

13 No. 2] Old Open Clusters in the Anti-Galactic Center Region The cluster Biurakan 7 is the second-most distant cluster so far known with a moderate metallicity. The cluster Czer 19 is the third most distant cluster from the galactic plane so far known among clusters younger than 0.15 Gyr in the outer disk. These two clusters are important for understanding their nature and origin. This observing program was carried out in part as a public school of astronomy given in the Gunma Astronomical Observatory in 2000 and 2001, and thanks are to the relevant observatory staff for the support. TH would greatly appreciate the encouragement of Y. Nakada and B. Hidayat. Discussions with K. Ogura, K. Yoshioka, and H. Okuda were helpful to promote this observational program. T. Kinoshiro kindly made available their data for us on the standard stars taken after mid- March in Appendix 1. Cluster by Cluster Description of the Isochrone Fit In this section, isochrone-fitting to the clusters is described in chronological order within our estimate. A.1.1. Czer 19 The morphology of MS stars definitely indicates that this cluster is young. Provided that a blue excursion star at (V I, V)=(1.74, 11.62) does belong to this cluster, this picture is strengthened, because a younger isochrone must be invoked to fit this star consistently with MS. A.1.2. Biu 9 The presence of RC and small δv suggest that this cluster is not very old. If a star at (V I,V)=(1.70,13.24), which could not be fitted within our effort to any isochrones, is omitted, there is no RGB star in our field, and it is difficult to state anything about the metallicity of this cluster. It was not an easy task to fit the curvature in the MS nicely, with our best effort, as shown in the CMD. Some inconsistency in the RC stars is found in (B V,V) CMD. The higher metallicity isochrone (Z =0.008) may fit these RC stars better, but could not fit stars at other positions in the CMD. Ann et al. (1999) found a younger age for this cluster based on a similar CMD. Their fit (figure 6 of their paper) could reproduce the RC well, while their isochrone seems to pass through above the TO. A.1.3. Be 64 Since this cluster is poorly populated in our FOV and the MS is not sharply delineated in the CMD, it is hard to constrain the parameters accurately. However, RC is amply populated and asmallδv would indicate that this cluster would be as old as 1 2 Gyr. Isochrones of solar-metallicity poorly fit the MS. One star at (V I,V)=(2.59, 13.63) is well explained as a RGB (or AGB) star if we take the metal-poor isochrones (Z =0.004). RGB (or AGB) stars are also fitted in the (B V,V)CMD and favor the lower metallicity. Our CMD and the isochrone fit are almost identical with those obtained in Ann et al. (2002). A.1.4. Rup 7 This was the most difficult cluster in our sample to fix the cluster parameters. The star density blue-ward of a point (V I,V)=(0.8, 16.5) is poorly populated, and we took this position as the MSTO. In this respect, several stars brighter than MSTO would be blue stragglers. A large separation in the color between RC and MSTO would favor younger and metalpoor isochrones, and the slope of the MS also favors metal-poor isochrones. However, unfortunately, our efforts could not give a convincing fit. A.1.5. Be 57 In this case, the blue straggler and field star contamination can be clearly discriminated. The gap just below the MSTO is discernible at (V I,V)=(1.1, 18.0). The rich population of RC and the small δv would favor an age of a few Gyr, which is supported by the isochrone fit. Without RGB stars, it is not easy to fix the metallicity. While metal poor isochrones fit the lower MS nicely, those fit the RC stars less satisfactory. MS stars are too red in the (B V,V)CMD. A.1.6. Be 12 It would be appropriate to take the stars around (V I,V)= (1.4,17.2) as the MSTO. The morphology that RC stars around (V I,V)=(2.1,16.0) are richly populated and that RGB stars are not observed suggests that the cluster would be as young as 2 3 Gyr. Our best-fit isochrone, as well as a small δv,is consistent with this picture. Ann et al. (2002) derived a similar CMD but found an age of 4 Gyr, that is much older than our estimate. However, their best-fit isochrone could not fit the lower part of the RC appropriately. A.1.7. Be 23 The rich red clump at (V I,V)=(1.2,15.0) and the small δv suggest that this cluster is not very old. Although it is difficult to discriminate whether there are blue stragglers or not, this ambiguity only leads to a difference of 0.2 Gyr in the resultant age. The difference among the two groups of isochrones is 0.2 Gyr. Ann et al. (2002) found a younger age of 0.8 Gyr than our estimate of 1.8 Gyr. While their isochrone does fit the more clearly delineated upper MS better, their isochrone could not fit the RC as nicely as our fit. A.1.8. Biu 11 The scarcely populated RC stars and not a small δv suggest that this is an old cluster. Two stars visible around (V I,V)= (1.8,15.0) are not RGB members, in our opinion, although they fit some of isochrones in the phase from tip-rgb to RC. A star at (V I,V)=(1.83,13.67) would suggest that the metallicity is as high as solar or more. A.1.9. Biu 7 (Be 31) Only a handful of RC stars could be identified in the CMD, supporting that Biu 7 would be a cluster as old as 3Gyr. This is consistent with the best-fit isochrone totally basedon MS and a possible RGBat (V I,V)=(1.9,13.2). This RGB candidate

14 308 T. Hasegawa et al. [Vol. 56, strongly suggests that the cluster would have a metallicity of Z = Our parameter also fits the brightest RGB stars nicely in the (B V,V)CMD. Several observations have been made concerning this cluster. Guetter (1993) obtained CMD of a similar quality with ours (figures 6a and 7a in their paper) 1 to claim a very old age of 8.0 Gyr with a best-fit Vandenberg (1985) isochrone extending to be no more massive than a subgiant. Subsequently, Carraro et al. (1998) found 4.0 Gyr from their synthetic CMD based on Padova track, which naturally matches better with our age. More problematic is the distance to this cluster; Guetter (1993) found a distance of 5.2 kpc, almost half of our estimate with a consistent color excess of E(V I) =0.16. This may be due to the ages and the model used for the fit; younger isochrones with convective overshooting would predict a longer distance. Carraro et al. (1998) found a galactocentric distance of 12.7 kpc, again shorter than our estimate. Having no description of color excess data in Carraro et al. (1998), this value may be responsible. We note that Guetter (1993) referred to the two color diagram to infer the metallicity of the cluster, but for this purpose we have presented a more sensitive way based on the reddening distance relation that is described in section 2. Both estimates of the metallicity by Guetter (1993), [Fe/H] = 0.4, as well as 0.5 by Carraro et al. (1998), are consistent with our measurement. A Be 70 Blue straggler/field star contamination is not considerable, and the best-fit parameters are well constrained for this cluster. AlargeδV of MSTO and RC unambiguously indicates that this is an old cluster. The rather scarcely populated RC stars (when compared with the number in MS) and well-delineated RGB also support this picture. If the star at (V I,V)=(2.20,12.70) is a member RGB, this suggests a lower metallicity of this cluster. It is noted that only a lower metallicity isochrone could fit the brightest RGB stars in the (B V,V)CMD. Ann et al. (2002) found a older age of 4.0 Gyr and a higher metallicity for this cluster. This is consistent within error, but our fit could reproduce both the RC and the RGB more properly. A Biu 13 Because the MS stars are so faint, this cluster may be a distant cluster. Blue straggler/field star contamination is significant, but we identified that the MSTO is around (V I,V)=(1.2,18.5). Since stars around RC are not sharply delineated and sparse, this cluster is expected to be old, but estimates of precise age is very difficult. Clumps of the stars around (V I, V) =(1.8, 17.0), (1.6, 16.5), and (1.8, 16.0) look equally like the RC. However, it is not favored to 1 The zero-point of Guetter (1993) photometry is different by 0.1 mag in the V I color, but their V magnitude and B V color are consistent with ours. What we may be careful is that the color scale may be inconsistent between the two CMD for V I; while the color difference between MSTO and RC is read as 0.55 mag for Guetter (1993), it is 0.45 mag in our CMD. This may be due to the fact that RC stars are not cleanly delineated in both of CMDs but possibly to the non-linearity in the detector of either one of the two group. take the clump at (V I,V)=(1.6, 16.5) as RC according to the (B V,V) CMD. Within our effort, a RGB star at (V I,V)=(2.2,15.1) would also exclude the isochrone with RC at (V I,V)=(1.6, 16.5), as well as constrain the metallicity of this cluster more than solar. If we take the stars around (V I,V)=(1.8,16.0) as RC stars, it is difficult to fit the star at (V I,V)=(2.2,15.1) as a RGB star. Thus, we take the clump at (V I,V)=(1.8,17.0) as RC stars, and the ages would be estimated to be 2.8 Gyr with a moderately good fit also in (B V,V)CMD. A Be 8 This cluster is an example of severe contamination of blue stragglers. If we take the stars around (V I,V)=(1.4,17.5) as the MSTO and the stars around (V I,V)=(1.45,18.0) as the gap of the MS, we would invoke a rather younger isochrones with 2.3 Gyr, yielding a less satisfactory fit to the lower MS. Contrarily, by fitting with older isochrones, as indicated in the figure 2, we could simultaneously give a good fit to the MS and the sparsely populated RGB stars at the cost of a poor fit to the clump of stars (with much smaller number density than MS) around (V I,V)=(1.4,17.5), which we could take as field and possible blue stragglers in the outer field of this cluster. The slope of the probable RGB stars is rather shallow and prompts us to adopt metal-poor isochrones, say, Z =0.008 or A Be 36 This is another example of blue straggler and/or field star contamination in the MSTO. The CMD is blurred because this cluster is dominated by faint stars and this clusters is located far below the equator. Thus, it was important to rely on the highly likely members for a consideration of the fitting, and we restricted the cluster area to be as small as possible. As a result, we could notice a paucity of stars above the top of the MS bluer than V I 1.0 and more luminous than V 18.0 in the cluster area. We thought that this paucity would imply the position of MSTO. We are aware that the adopted isochrones poorly reproduce the RC if they are located at (V I,V)=(1.7,17.0), but the isochrones with our parameters could explain RGB that is delineated by more than 5 stars. We have fitted with isochrones that is so adjusted that the possible red clump at (V I,V)=(1.7, 17.0) would be best reproduced, but such isochrones fit the MS unacceptably bluer and runs into the CMD area where MSTO stars are quite sparsely observed. What should also be noted concerning this isochrone is that the isochrones fit the RGB (if they are indeed the RGB in this cluster) much less appropriately when compared with the isochrone with a metal-solar 3.2 Gyr isochrone. The lower clump is more loosely distributed in the (B V,V)CMDthan in (V I,V) CMD, suggesting that these clump stars are not RC, but RGB stars. It is unfortunate that the star at the tip of RGB is not fitted in the (B V,V) CMD nicely with any isochrones. A Biu 8 (Be 32) Because the morphology of CMD that δv is large, several RGB stars are visible, and RC stars are scarcely populated (in comparison with MS), it is likely that Biu 8 is also an old cluster. Again, the RGB stars strongly indicate a metallicity