Science with the Hubble Space Telescope -- II
Book Editors: P. Benvenuti, F. D. Macchetto, and E. J. Schreier
Electronic Editor: H. Payne

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Combined IRAC2 vs HST Color-Magnitude Diagrams of Bulge Globular Clusters.

M.D. Guarnieri
Dept. of Mathematical Physic, Torino, Italy

S. Ortolani
Padua Astronomical Observatory, Italy

A. Renzini and P. Montegriffo
Dept. of Astronomy, Bologna, Italy

B. Barbuy
Univ. de Sao Paulo, Brazil

E. Bica
Univ. do Rio Grande do Sul, Brazil

Abstract:

Since 1993, we have started a long-term program aimed at obtaining near-infrared images for a wide sample of highly reddened globular clusters and the related stellar fields in the galactic bulge, from the brightest part of the CMD down to the main sequence. The main goal was to complement the high quality optical data coming, at the beginning, from various ESO observing runs and then from Hubble Space Telescope with all the advantages related with infrared imaging. Accurate photometry of bulge clusters, both in near-infrared and optical, is crucial in order to derive their location in the galaxy (most of the times not available), estimate their metallicity, reddening, age, the determination of the actual extent in luminosity and, above all, a more precise location in temperature of their giant branches. This last parameter is fundamental for the comparison between metal-rich populations models and their observations. Here we present combined ESO/IRAC-2 and HST/WFPC2 observations for the two metal-rich globular cluster NGC 6528 () and NGC 6553 (). All this information will help in understanding both the proto-Galaxy conditions after the initial collapse of the spheroid, and the age, dynamical and chemical conditions of the bulge.

Keywords: globular clusters, infrared, galactic bulge

Observations and Data Reductions

For both clusters, infrared observations have been carried out the night of June 11, 1992, using the ESO Rockwell NICMOS-3 (HgCdTe, pixels) infrared camera IRAC-2 (cutoff wavelength at 2.5micron Moorwood et al. 1992a, 1992b) mounted on the MPI/ESO 2.2m telescope at La Silla (Chile), at image scales of and for both standard J () and K () filters. Through each filter, we obtained a high resolution image (first magnification value) of the cluster center, and four partially overlapping images covering a square region centered on the cluster core (second magnification) resulting in a total field coverage of arcmin. The whole set of data has been analyzed using the package for crowded fields photometry ROMAFOT (Buonanno et al. 1979,1983).

Separate sky frames located at from the cluster center were also observed, in each filter and magnification, for sky subtraction purpose. All images are the average of 60 frames of 1 sec integration time. The observations were carried with seeing .

Eight SAAO standards (kindly provided by Dr. Ian Glass) were observed during the run and most of them on at least two positions of the array. See Ferraro et al. (1993) for the details regarding the calibration curve.

The optical VI sample, coming from HST observations (Ortolani et al., 1995), has been tied to the IR coordinate system using a linear interpolation, and a combined catalogue of 1426 and 961 VIJK stars has been created, for NGC6553 and NGC6528, respectively.

Common Considerations

Figures 1 and 2 present the composite NGC6553 and NGC6528 CMDs for all the stars in common between the infrared and optical sample. The main features can be summarized as follow:

 
Figure: NGC6553

 
Figure: NGC6528

• Both infrared (K,J-K) CMDs, confirm the peculiar nature of the clusters with a very red, compact horizontal branch while in the (V,V-K) CMDs it appears strongly tilted due to the higher blanketing effect in the V photometric data.
• The giant branch is well defined in the whole extension up to K 6.4 (NGC6553) and K 7.3 (NGC6528). Both figures show a turnover of the (V,V-K) CMDs due to the same blanketing effects of the cool stars.
• In both cases, a redder, lower and almost parallel sequence is also visible in all the diagrams. At the moment we are investigating if this feature is probably to a background bulge population.

NGC6553

• ;

• Located at a very low galactic latitude ( )

The main characteristics, derived from figure 1, are:

• HB is well defined in all the diagrams at about K = 12.28 mag, J-K = 0.96 and V-K = 4.64.

• This measure is much more accurate than the luminosity level obtained using only optical CMDs where it appears broadened and strongly tilted. From these values is deduced.

• A clump of points appears in the SGB at and at . This could be identified with the so-called ``RGB-bump''. The possibility, however, that it is due simply to a blend with the field, background HB cannot be ruled out.

• Assuming a reddening of E(B-V)= 0.8, the calculated distance modulus is giving an heliocentric distance of 5.38 Kpc. This distance is slightly higher than previously published data by Ortolani et al. (1990).

• The brightest giants have been detected at and that is and , which are exceptionally high values for a globular cluster.

• The metal abundance estimation has been derived, applying the GB slope method of Kuchinski et al. (1995), to the K vs. (J-K) CMD. The obtained value is (GB slope) = -0.1170.005, which corresponds to [Fe/H] = -0.180.17 in the Kuchinski et al. calibration. This value is consistent to () derived from the high-dispersion analysis of the giant III-17 by Barbuy et al. (1992).

• Finally, the estimate of the Helium abundance via the so-called method calculated on the final VIJK sample, yield a mean value of .

NGC6528

• ;

• Located at a very low galactic latitude ( )

The main characteristics, derived from figure 2, are:

• The HB is worse defined than NGC6553, but anyway still well locatable in all the diagrams at about K = 13.25 mag, J-K = 1.09 and V-K = 3.93

• As in the previous case, from these values is deduced.

• A clump of points appears in the SGB at and at . This could be identified with the so-called ``RGB-bump'' but still in this case we cannot ruled out the possibility of a blending with a field, background HB.

• Assuming an average reddening of E(B-V)= 0.55, the calculated distance modulus is giving an heliocentric distance of 8.53 Kpc. As in the previous case, this distance is slightly higher than previously published data by Ortolani et al. (1992).

• The brightest giants have been detected at and , that is, and .

• The metal abundance estimation has been derived as for NGC6553, applying the GB slope method of Kuchinski et al. (1995), to the K vs. (J-K) CMD. The resulting value is (GB slope) = -0.1210.005, which corresponds to [Fe/H] = -0.070.17. This value is consistent to () derived by Ortolani et al. (1995).

• Finally, the estimate of the Helium abundance via the so-called method calculated on the final VIJK sample, yield a mean value of .

Acknowledgments:

We are grateful to the ESO organization for the allocation of observing time. We also thank Hans Gemperlein for the help during the observing run. MDG acknowledges the Universitá and Osservatorio Astronomico of Torino for their support.

References:

Barbuy, B., Castro, S., Ortolani, S., & Bica, E. 1992, A&A 259, 607

Buonanno, R., Corsi, C.E., De Biase, G.A., & Ferraro, I. 1979, in Image Processing in Astronomy, ed, G. Sedmak, M. Capaccioli, & R.J. Allen, Trieste Obs., Trieste, 354

Buonanno, R., Buscema, G., Corsi, C.E., Ferraro, I., & Iannicola, G. 1983, A&A, 126, 278

Ferraro, F.R., Fusi Pecci, F., Guarnieri, M.D., Moneti, A., Origlia, L., & Testa, V. 1993, MNRAS, 266, 829

Kuchinski, L.E., Frogel, J.A., & Terndrup, D.M. 1995, A&A 109, 1131

Moorwood, A. & Finger, G. 1992a, The Messenger, 68, 21.

Moorwood, A., Finger, G., Biereichel, P., Delabre, B., Van Dijsseldonk, A., Huster, G., Lizon, J.L., Meyer, M., Gemperlein, H., & Moneti, A. 1992b, The Messenger, 69, 61.

Ortolani, S., Barbuy, B., & Bica, E. 1990, A&A, 236, 362

Ortolani, S., Bica, E., & Barbuy, B. 1992, A&AS, 92, 441

Ortolani, S., Barbuy, B., Bica, Marconi, G., & Renzini, A. 1995, in preparation

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