1995A&A...300..726O

Astron. Astrophys. 300, 726-73 I ( 1995) ASTRONOMY AND

ASTROPHYSICS

BH 176 and AM-2: globular or open clusters?* S. Ortolani 1, E. Bica2 , and B. Barbuy3

1 Universita di Padova, Dipartimento di Astronomia, Vicolo dell'Osservatorio 5, 1-35123 Padova, Italy 2 Universidade Federal do Rio Grande do Sul, Depto. de Astronomia, CP 15051, Porto Alegre, 91500-970, Brazil 3 Universidade de Sao Paulo, lAG, Depto. de Astronomia, CP 9638, Sao Paulo 01065-970, Brazil

Received 7 September 1994/ Accepted 25 October 1994

Abstract. We have obtained VI photometry for two low Galac­tic latitude star clusters: BH 176 and AM-2, using the 2.2 m and the 3.5 m NTI telescopes at ESO. Their VI colour-magnitude diagrams reveal that: BH 176 may be a globular cluster, or a borderline object between a globular cluster and a disk clus­ter, showing a red horizontal branch and an extended red giant branch. We estimate E(B - V) = 0.77 and d0 = 13.4kpc. AM-2 appears to be an intermediate age open cluster, for which a reddening E(B- V) = 0.44 and d0 = 12.4 kpc are estimated. It is located in the outer regions of the Galactic disk.

Key words: HR diagram- globular clusters. BH 176- open clusters: AM-2

1. Introduction

Globular clusters belong to the oldest stellar population in our Galaxy. On the other hand, open clusters are considerably younger and only a few have been identified to be in the age interval of 6 to 10 Gyr, e.g., NGC 188, NGC 6791 (Demarque et al. 1992 and references therein).

The problem of the classification of open (globular) clusters in our Galaxy is based mainly on the colour magnitude dia­gram (CMD) properties, even if structurally the indications are contradictory (e.g., most of the Palomar clusters).

Interesting borderline objects have been recently studied in detail through CMDs, as Pal12, Rup 106 and Lynga 7 (Gratton & Ortolani 1988a; Buonanno et al. 1990; Ortolani et al. 1993). The aim of ~his paper is to study two additional borderline star clusters at low Galactic latitude, which are BH 176 and AM-2.

BH 176 was discovered in a survey of star clusters in the Southern Milky Way by van den Bergh & Hagen (1975). BH 176, also designated as ESO 224-SC8 and IAU 1535-499 is located at o:1950 = 15h35m29', 8 1950 = -49°53'18", and

Send offprint requests to: B. Barbuy * Observations collected at the European Southern Observatory, La Silla, Chile. Tables 2 and 3 are available at CDS, via anonymous ftp 130.79.128.5

l = 328.42°, b = +4.34 o. It is a loose cluster with very little information in the literature. Webbink (1985), in his compila­tion of globular clusters, gives some preliminary estimates of the cluster properties.

AM-2 was discovered by Madore & Arp (1979) on the UK­SRC blue sky survey. AM-2 is also designated as ESO 368-SC7 and IAU 0737-337 and its coordinates are o:1950 = 07h37m18', 81950 = -33°44'00", and l = 248.17°, b = -5.80°. A BV CMD study by Gratton & Ortolani (1988b) indicated that AM-2 could be a very old open cluster. Structurally, it was classified as a new loose globular cluster with a concentration parameter c = 0.50 (Trager et al. 1993).

In Sect. 2 we present the observations and reduction proce­dures. In Sect. 3 we discuss the CMDs of BH 176 and in Sect. 4 those of AM-2. The concluding remarks of this work are pro­vided in Sect. 5.

2. Observations

The observations of BH 176 and AM-2 were obtained in two different runs at the European Southern Observatory (ESO), La Silla, using Johnson-Cousins V ,I filters.

2.1. Observations and reductions of BH 176

BH 176 was observed in 1993 with the 3.5 m NTI telescope equipped with the ESO Multimode Imager (EMMI) used as di­rect imager/focal reducer through the red path. The CCD detec­tor employed was a thick, UV coated LORAL ESO No. 34 with pixel size of 15f.Lm (0.35" on the sky). The total size of the field corresponding to 2048x2048 pixels is of about 11.5' x 11.5'. An I image of BH 176 is shown in Fig. 1.

The reduction procedures follow those described in Or­tolani, Bica & Barbuy (1992, OBB92), using Midas and Daophot II at the ESO Garching computing center.

The VI calibrations were obtained using Landolt (1992) star fields. The derived calibration equations are:

V = v + 0.02(V - I)+ 23.86 ± 0.015

I= i- 0.015(V- I)+ 23.58 ± O.D15

© European Southern Observatory • Provided by the NASA Astrophysics Data System

1995A&A...300..726O S. Ortolani et al.: BH 176 and AM-2: globular or open clusters? 727

N

w

Fig. 1. NTI' 4 min. exposure in I of BH 176. The dimensions of the field are 9.9' x 8.0'

where v and i are the instrumental magnitudes for 15 s expo­sures, at airmass of 1.2. Crowding conditions in such low lat­itude fields are important and the main source of error in the absolute calibration comes from the transfer of aperture mag­nitudes to the convoluted ones. In order to minimize the effect we calibrated the deep exposures, containing several saturated stars, by means of short exposures. We estimate the total zero point uncertainties to ±0.04, taking also into account the shutter delay problems. The details of the observations are summarized in the log-book presented in Table 1. VI data for BH 176 are given in Table 2, available at CDS.

2.2. Observations and reductions of AM-2

AM-2 was observed in 1993 with the 2.2m telescope equipped with a Thompson UV coated 1000 x 1000 pixel ESO CCD No.19 with pixel size 19J.Lm x 19J.Lm (0.33" on the sky). The EFOSC camera in the focal reducer mode was employed giving a final field of 5.5' x 5.5' . A V image of AM-2 is provided in Fig.2.

The reduction of the AM-2 frames were carried out as in Sect. 2. 1. Details on the calibration procedures are given in Car­rare & Ortolani (1994). The observations obtained are reported in Table 1. Table 3, reporting VI data for AM-2, is available at CDS.

© European Southern Observatory • Provided by the NASA Astrophysics Data System

1995A&A...300..726O

728 S. Ortolani eta!. : BH 176 and AM-2: globular or open clusters?

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22

Fig. 2. V image of AM-2. The field dimensions are 5.5' x 5.5'.

3. The star cluster BH 176

A preliminary BV CMD of BH 176 by Gratton & Ortolani (un­published) revealed that BH 176 is not a young cluster, with the presence of a very red horizontal branch (HB). We present in Figs. 3a,b the V vs. (V - I) and I vs. (V - I) CMDs from a circular extraction of r < 53".

In Fig. 4 a coordinate map was produced for stars with I < 17, where different symbols are used for those of (V-I) > 2.3 belonging to the cluster, in a concentrated area, and (V - I) < 2.3 belonging to the field, these latter ones uniformly distributed in the frame. Prominent features in Figs. 3a,b are:

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1032

(i) on the blue side the presence of disk field main sequence (MS) which considerably contaminates the CMD of the cluster.

(ii) the bright red sequence of the cluster including an ex­tended red giant branch (RGB), a red HB, and a subgiant branch (SGB) which merges with the field MS. The RGB curvature (Or­tolani et a!. 1991) indicates a high metallicity similar to that of NGC 6528 (OBB92), i.e., basically solar. The RGB extension spans .6.(V- I)= 3.0 at least, which is absent in the surrounding field (Fig. 5). A very extended RGB has been detected in other clusters such as NGC 6528 (OBB92) and NGC 6791 (Garnavich eta!. 1994).

The HB is located at V ~ 19.0±0.15 and (V-I)= 2.15 ± 0.10. We derive the reddening from the colour of the RGB at

© European Southern Observatory • Provided by the NASA Astrophysics Data System

1995A&A...300..726O

S. Ortolani et al.: BH 176 and AM-2: globular or open clusters? 729

Table 1. Log-book of the observations

Filter Date UT Exposure Seeing time (s) (")

NIT:BH176

v 14.06.93 1h12m 15 1.2 I 1 18 8 1.0 v 1 24 300 1.4 v 1 33 300 1.4 I 2 02 240 1.0

2.2m:AM-2

v 21.11.93 6hsom 60 0.9 v 7 16 60 0.9 v 7 19 600 0.9 I 7 32 60 0.85 I 7 36 600 0.7 I 7 48 600 0.7 v 8 01 600 0.9

the level of the HB, at (V - I) ~ 2.25 ± 0.1 0, together with the same locus for an old cluster RGB at solar metallicity from the new Padova group isochrones (Bertelli et al. 1994), where (V- I)o = 1.23, and we derive for BH 176 E(V- I)= 1.02. This converts to E(B - V) = 0.77 ± 0.10, using Dean et al. (1978)'s ratio E(V - I)/ E(B - V) = 1.33.

For the distance derivation we assume Buonanno et al. (1989) value of M~B = 1.06 for solar metallicity (which basi­cally coincides with the value from the isochrones). We obtain a true distance modulus of (m- M)o = 15.63 and a distance from the sun of d0 = 13.4 kpc. Adopting a distance for the Galactic center of 8 kpc (Reid 1993), the Galactocentric coordinates are X = -3.4 kpc, Y = -7 kpc, Z = 1 kpc (X < 0 means the other side of the Galaxy).

Since we cannot distinguish the cluster turn-off of BH 176, the precise determination of the cluster age remains open. The global morphology of the observed CMD is however compatible with one of a metal-rich globular cluster such as NGC 6528 and NGC 6553, but it still could be a borderline case or even a very old disk open cluster, similar to NGC 6791 (Garnavich et al. 1994). Nevertheless, structurally, it looks rather like a globular cluster.

We show in Fig. 5 the V vs. (V-I) field CMD surrounding the cluster BH 176 corresponding to the stars in the frame for r > 2.9'. The MS is considerably wide (~(V - I) ~ 1.3 mag) and presents internal structure, which can be interpreted as the line of sight crossing different spiral arms, differentially red­dened. We note that the cluster is located slightly above the Galactic plane, at Galactocentric radius similar to that of the Sun, at a direction~ 30° with respect to the center of the Galaxy.

4. The star cluster AM-2

The brighter part of the CMD of AM-2 was studied in B, V by Gratton & Ortolani (1988b). The aim of the present work

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Fig. 3a and b. CMDs for BH 176 for r <53": a V vs. V-I; b I vs. V-I

is to analyse in more detail the HB and tum-off regions of the cluster based on deep V, I CMDs. We show in Fig. 6 a V vs. (V-I) CMD of AM-2 for an extraction r < 100". In Fig. 7 we provide the whole frame V vs. (V - I) diagram, where the field contamination becomes more important. The V, I features are considerably deep so that the upper parts (V ~ 17.0) of the diagrams in Figs. 6, 7 are affected by saturation. In Fig. 5 the cluster morphology in terms of MS, turnoff, subgiant branch and HB are well defined.

In Fig. 8 a pixel coordinate map for stars ofV < 19 is shown, where different symbols are adopted for those of (V - I) < 1.5 and (V - I) > 1.5: the bluer stars have a uniform distribution, while redder stars, more probably belonging to the cluster, are more concentrated around the cluster area.

TheclusterHBislocatedatV = 17.75±0.10and(V -I)~ 1.69 ± 0.05. The tum-off is located at V ~ 20.0 ± 0.15.

© European Southern Observatory • Provided by the NASA Astrophysics Data System

1995A&A...300..726O

730 S. Ortolani eta!.: BH 176 and AM-2: globular or open clusters?

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Fig. 4. BH 176: a coordinate map was produced for stars with I < 17, Fig. 6. V vs V - I CMD of AM-2 for an extraction of r < 1 00". where open circles: V - I > 2.3 (belonging to the cluster), and dots: V - I < 2.3 (belonging to the field).

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The magnitude difference between the HB and turn-off is ~ V(TO - HB) = 2.25 ± 0.18. This age indicator reveals that AM -2 is an intermediate age open cluster similar to M67. Indeed M67 presents ~V(TO- HB) = 2.20, whereas for the oldest disk clusters NGC 188 and NGC 6791 these values are consider­ably larger (e.g., Barbuy et al. 1993). As the RGB ofM67 is red­der than that of AM-2 (Gratton & Ortolani 1988b), we conclude that AM-2 is less metallic, of the order of [Fe/H] ::::::: -0.3. The HB locus of AM-2 together with that in the new isochrones of the Padova group for intermediate age clusters of nearly solar metal­licity ((V- l)o = 1.10) leads to a reddening of E(V- I) = 0.59, which converts to E(B - V) = 0.44, compatible with the es­timates in Gratton & Ortolani (1988b). Adopting for the ab-

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solute HB level Mv = 0.97 for a cluster of [Fe/H] = -0.3 (Buonanno et al. 1989), we derive a true distance modulus of (m- M)o = 15.46 and a distance from the Sun d0 = 12.4 kpc. The Galactocentric coordinates are X = + 3.42, Y = -11.45, Z = -1.25 kpc. The cluster is located in the outer regions of the Galactic disk.

5. Conclusions

TheCMD characteristics ofBH 176 and AM-2reveal thatAM-2 is an intermediate age open cluster, whereas for BH 176 we could not decide, but it should be a true globular cluster or a very old open cluster such as NGC 6791.

© European Southern Observatory • Provided by the NASA Astrophysics Data System

1995A&A...300..726O

S. Ortolani eta!.: BH 176 and AM-2: globular or open clusters? 731

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Acknowledgements. BB and EB acknowledge the Brazilian institutions CNPq and Finep. BB also acknowledges Fapesp.

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