Publications of the Korean Astronomical Society pISSN: 1225-1534 30: 265 ∼ 266, 2015 September eISSN: 2287-6936 c 2015. The Korean Astronomical Society. All rights reserved. http://dx.doi.org/10.5303/PKAS.2015.30.2.265 IDENTIFICATION OF LUMINOUS WHITE DWARF CANDIDATES IN THE GLOBULAR CLUSTERS M13 AND M22 USING HST ACS PHOTOMETRIC DATA Dong-Hwan Cho1, Tae Seog Yoony1, Sang-Gak Lee2,3, and Hyun-Il Sung4 1Department of Astronomy and Atmospheric Sciences, Kyungpook National University, Daegu 702-701, Korea 2Astronomy Program, Department of Physics and Astronomy, Seoul National University, Seoul 151-742, Korea 3National Youth Space Center, Goheung, Jeonnam 548-951, Korea 4Korea Astronomy and Space Science Institute, 776 Daedeok-daero, Yuseong-gu, Daejeon 305-348, Korea E-mail: [email protected] (Received November 30, 2014; Reviced May 31, 2015; Aaccepted June 30, 2015) ABSTRACT A search for luminous white dwarfs (WDs) in several nearby Galactic globular clusters (GCs) was carried out using the deep and homogeneous photometric catalog of Galactic GCs taken with the ACS/WFC aboard the Hubble Space Telescope (HST) by Sarajedini et al. and Anderson et al.. It resulted in the identification of luminous WD candidates in the GCs M13 (NGC 6205) and M22 (NGC 6656). The purpose of the present study is to identify luminous WDs in the deep and homogeneous V versus V − I color-magnitude diagrams (CMDs) of several nearby Galactic GCs taken with the ACS/WFC aboard the HST. Using photometric data for the GCs M13 and M22 that are now in the public domain, the V versus V − I CMDs of the GCs M13 and M22 were constructed. Many spurious detections in the CMDs were removed using the photometric quality parameters qfit(V ) and qfit(I), and a radial restriction was applied to the CMDs to remove the central stars with higher photometric errors due to central crowding. From each resultant V versus V − I CMD of the GCs M13 and M22, a dozen or so luminous WD candidates were identified. They were confirmed as stellar objects in the accompanying ACS/WFC images and their positions in the CMDs were in the bright part of the DA WD cooling curve. Therefore, the luminous WD candidates in the GCs M13 and M22 seem to be true luminous WDs, and spectroscopic observations are needed to confirm their true identity. Key words: globular clusters: individual: M13, M22 | Hertzsprung{Russell and C{M diagrams | stars: Population II | white dwarfs 1. INTRODUCTION AND STUDY PURPOSE et al. (2008) and Sarajedini et al. (2007), has been made by the authors of the present study. Here, the identifi- The end products of ∼97% of normal stars undergo- cation of hot and luminous WD candidates in GCs M13 ing stable nuclear burning whose masses are below M (NGC 6205) and M22 (NGC 6656) is briefly reported. ∼ 8{9 M (except for brown dwarfs) are white dwarfs (WDs) (Fontaine et al., 2001). Although the search for 2. COLOR-MAGNITUDE DIAGRAMS AND RESULTS WDs in Galactic globular clusters (GCs) began in 1978 by Richer (1978) in the GC NGC 6752, the first clear The source of the color-magnitude diagrams (CMDs) identifications of WDs in GCs was made in 1995 in the of the present study was the HST ACS photometric GCs NGC 6397 (Paresce et al., 1995b), 47 Tuc (NGC catalog of Anderson et al. (2008) and Sarajedini et 104) (Paresce et al., 1995a), ! Cen (NGC 5139) (El- al. (2007), now open to the public through the STScI son et al., 1995), and M4 (NGC 6121) (Richer et al., MAST archive. V versus V − I photometric data of the 1995) using the Hubble Space Telescope (HST). WDs GCs M13 and M22 in the Johnson-Cousins photomet- were discovered in less than ten GCs (NGC 104, 5139, ric system defined by Landolt (1992) were taken from 6121, 6397, and 6752 etc.) before now. The search for the catalog and transformed from mF606W and mF814W hot and luminous WDs in several nearby Galactic GCs, in the Vega magnitude system according to the trans- using the deep and homogeneous HST Advanced Cam- formation equations of Sirianni et al. (2005) and the era for Surveys (ACS) photometric catalog of Anderson photometric zero points of Mack et al. (2007). In the constructed V versus V − I CMDs of M13 and M22, yCorrespanding author : T. S. Woon many spurious detections were removed with photomet- ric quality parameters qfit(V ) > 0.30 and qfit(I) > 0.30 http://pkas.kas.org for fainter regions. However, saturated bright stars in 265 266 D.-H. CHO ET AL. Figure 1. V vs. V − I CMDs of M13 and M22. In each figure, small open circles with a dot are WD candidates. In (a), only the stars satisfying the condition, r ≥ 0.620, are shown for M13. the long exposure frames with qfit(V ) = 2.50 and qfit(I) Fontaine, G., Brassard, P., & Bergeron, P., 2001, The Po- = 2.50 were retained. In the case of M13, a further tential of White Dwarf Cosmochronology, PASP, 113, 409 radial restriction was applied to remove central stars Landolt, A. U., 1992, UBV RI Photometric Standard Stars with higher photometric errors due to central crowding, in the Magnitude Range 11.5 < V < 16.0 around the Ce- whose distances from the cluster center (r) were smaller lestial Equator, AJ, 104, 340 than 0.620 (r < 0.620). Mack, J., Gilliland, R. L., Anderson, J., & Sirianni, M., 2007, WFC Zeropoints at −81C, Instrument Science Re- The resultant V versus V − I CMDs of M13 and M22 port, ACS 2007-02 (Baltimore: STScI) are shown in Figure 1. In Figure 1, small open cir- Paresce, F., De Marchi, G., & Jedrzejewski, R., 1995a, White cles with a dot are the WD candidates for M13 and Dwarfs and Mass Segregation in the Core of 47 Tucanae, M22, respectively. The WD candidates for M13 and ApJL, 442, L57 M22 were all confirmed as point sources in the accom- Paresce, F., De Marchi, G., & Romaniello, M., 1995b, Very panying ACS/WFC images using IRAF routines. The Low Mass Stars and White Dwarfs in NGC 6397, ApJ, number of WD candidates for M13 is sixteen and that 440, 216 for M22 is thirteen. They are randomly distributed over Richer, H. B., 1978, Evidence for White Dwarfs in the Glob- the ACS/WFC images, suggesting that they are really ular Cluster NGC 6752, ApJL, 224, L9 Richer, H. B., Fahlman, G. G., & Ibata, R. A., et al., 1995, hot and luminous WDs. Hubble Space Telescope Observations of White Dwarfs in the Globular Cluster M4, ApJL, 451, L17 ACKNOWLEDGMENTS Sarajedini, A., Bedin, L. R., & Chaboyer, B., et al., 2007, The ACS Survey of Galactic Globular Clusters. I. This research was supported by Basic Science Re- Overview and Clusters without Previous Hubble Space search Program through the National Research Foun- Telescope Photometry, AJ, 133, 1658 dation of Korea (NRF) funded by the Ministry of Edu- Sirianni, M., Jee, M. J., & Ben´ıtez, N., et al., 2005, The cation, Science and Technology (No. 2011-0026126 and Photometric Performance and Calibration of the Hubble No. 2012R1A1A2006759). Space Telescope Advanced Camera for Surveys, PASP, 117, 1049 REFERENCES Anderson, J., Sarajedini, A., & Bedin, L. R., et al., 2008, The ACS Survey of Galactic Globular Clusters. V. Generating a Comprehensive Star Catalog for Each Cluster, AJ, 135, 2055 Elson, R. A. W., Gilmore, G. F., Santiago, B. X., & Caser- tano, S., 1995, HST Observations of the Stellar Population of the Globular Cluster ! Cen, AJ, 110, 682.