Stellar Populations of the Globular Cluster M53

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Fig. 1. Color-magnitude diagram (CMD) for M53 based on the 21413 stars at radii r > 1 . The CMD shows the classic pattern of the metal-poor globular clusters, with the horizontal-branch predominantly blueward of the RR Lyrae instability strip and t he relatively steep red giant-branch (RGB). The CMD also reveals a rich population of blue straggler stars (BSSs). And the separation between the asymptotic giant branch ( AGB)and the RGB is clear.
II. OBSERVATIONS AND DATA REDUCTIONS
The observations were carried out on the two nights 1995 March 30/31 and March 31/April 1 at Mauna Kea using the University of Hawaii 2.2 m telescope with the f /10 secondary and the Tektronix 2K CCD. 40 CCD frames per lter in a grid of four partially overlapping elds (NE, NW, SE, SW) of the cluster center were taken. In total, these elds completely cover the cluster out to a radius of 7 from the center. The instrumental signature of each cluster frame was removed and calibrated by combination of overscan regions and use of twilight sky ats and zero exposure bias frames acquired during each observing run, with the aid of the CCDRED package within IRAF. Photometry of stars on the frames was performed using the multiplestar, point- spread-function (PSF) tting packages of second-generation DAOPHOT II, and ALLSTAR 6,7]. The instrumental magnitudes were transformed to the
Journal of the Korean Physical Society, Vol. 33, December 1998, pp. S545 S548
Stellar Populations of the ቤተ መጻሕፍቲ ባይዱlobular Cluster M53
Soo-Chang REY, Young-Wook LEE and Yong-Ik BYUN Center for Space Astrophysics & Department of Astronomy Yonsei University, Shinchon 134, Seoul 120-749 Mun-Suk CHUN Department of Astronomy, Yonsei University, Shinchon 134, Seoul 120-749
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I. INTRODUCTION
Stellar population studies of Galactic globular clusters have played a critical role of e orts to understand stellar evolution for low-mass stars in the metal-poor regime. The availability of globular cluster color-magnitude diagrams (CMDs) obtained with high photometric accuracy from statistically populous and homogeneous samples of stars still represents the most powerful tool to provide tests for stellar evolution of low-metallicity, lowmass stars. Given the tremendous progress in the theoretical understanding of stellar evolution of globular clusters, it is now clear that only big and very accurate samples may allow to achieve a decisive improvement in the comparison between theoretical models and observations 1,2]. M53 (NGC 5024 = C1310+184) is a moderately compact metal-poor ( Fe/H] = ? 2.04) 3] outer halo globular cluster that is rich in RR Lyraes. But, M53 is one of the poorly studied Galactic globular clusters. There are only two previous CMD studies for this cluster. The rst photometric study of M53 on the BV system was made by Cu ey 4], who obtained a photographic CMD for 569 stars that reaches only 1.5 magnitudes below the horizontal-branch (HB). More recently, Heasley & Christian 5] presented a new BV CMD, based on the CCD photometry of 392 stars, that reaches approximately two magnitudes below the main-sequence (MS) turno . However, only a small sample of red-giant branch (RGB) and
HB stars were measured in their photometry due to the small physical size of the CCD they used, an usual result before the advent of the large-format CCD. This situation has motivated us to observe M53 with the large-format CCD camera in order to obtain the high quality homogeneous color-magnitude (CM) data for more than 20000 stars in M53 that span from the tip of the RGB to some two magnitudes below the MS turno .
The rst large-format CCD color-magnitude diagram (CMD) in the B and V passbands is presented for the Galactic globular cluster M53 (NGC 5024). The observations consist of 40 pairs of B and V CCD frames with FWHM <1 .2. These frames were used to produce color-magnitude data for more than 20000 stars from the tip of the red giant-branch (RGB) to about two magnitudes below the main-sequence turno (V 22). The CMD reveals a classic example of a metal-poor stellar system, with the horizontal-branch (HB) predominantly blueward of the RR Lyrae instability strip and the relatively steep RGB. We have discovered 117 new blue straggler stars (BSSs) in the eld of M53. The analysis of bright (V <19.39) BSSs clearly shows a bimodal radial distribution, with a high frequency in the inner and outer regions, but a distinct dip in the intermediate region. The distribution is similar to that found in M3, a globular cluster with similar central density and concentration. The relative age dating based on the color di erence between the turno and the base of RGB reveals no signi cant age di erence ( t <1 Gyr) between M53 and M92.