Publications of the Korean Astronomical Society pISSN: 1225-153430: 265 ∼ 266, 2015 September eISSN: 2287-6936c©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 M22USING HST ACS PHOTOMETRIC DATA

Dong-Hwan Cho1, Tae Seog Yoon†1, Sang-Gak Lee2,3, and Hyun-Il Sung4

1Department of Astronomy and Atmospheric Sciences, Kyungpook National University, Daegu 702-701, Korea2Astronomy Program, Department of Physics and Astronomy, Seoul National University, Seoul 151-742, Korea

3National Youth Space Center, Goheung, Jeonnam 548-951, Korea4Korea Astronomy and Space Science Institute, 776 Daedeok-daero, Yuseong-gu, Daejeon 305-348, Korea

E-mail: yoots@knu.ac.kr

(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 carriedout using the deep and homogeneous photometric catalog of Galactic GCs taken with the ACS/WFCaboard the Hubble Space Telescope (HST) by Sarajedini et al. and Anderson et al.. It resulted in theidentification of luminous WD candidates in the GCs M13 (NGC 6205) and M22 (NGC 6656). Thepurpose of the present study is to identify luminous WDs in the deep and homogeneous V versus V − Icolor-magnitude diagrams (CMDs) of several nearby Galactic GCs taken with the ACS/WFC aboard theHST. Using photometric data for the GCs M13 and M22 that are now in the public domain, the V versusV − I CMDs of the GCs M13 and M22 were constructed. Many spurious detections in the CMDs wereremoved using the photometric quality parameters qfit(V ) and qfit(I), and a radial restriction was appliedto the CMDs to remove the central stars with higher photometric errors due to central crowding. Fromeach resultant V versus V − I CMD of the GCs M13 and M22, a dozen or so luminous WD candidateswere identified. They were confirmed as stellar objects in the accompanying ACS/WFC images and theirpositions in the CMDs were in the bright part of the DA WD cooling curve. Therefore, the luminous WDcandidates in the GCs M13 and M22 seem to be true luminous WDs, and spectroscopic observations areneeded 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

The end products of ∼97% of normal stars undergo-ing stable nuclear burning whose masses are below M∼ 8–9 M� (except for brown dwarfs) are white dwarfs(WDs) (Fontaine et al., 2001). Although the search forWDs in Galactic globular clusters (GCs) began in 1978by Richer (1978) in the GC NGC 6752, the first clearidentifications of WDs in GCs was made in 1995 in theGCs NGC 6397 (Paresce et al., 1995b), 47 Tuc (NGC104) (Paresce et al., 1995a), ω Cen (NGC 5139) (El-son et al., 1995), and M4 (NGC 6121) (Richer et al.,1995) using the Hubble Space Telescope (HST). WDswere discovered in less than ten GCs (NGC 104, 5139,6121, 6397, and 6752 etc.) before now. The search forhot and luminous WDs in several nearby Galactic GCs,using the deep and homogeneous HST Advanced Cam-era for Surveys (ACS) photometric catalog of Anderson

†Correspanding author : T. S. Woon

http://pkas.kas.org

et al. (2008) and Sarajedini et al. (2007), has been madeby the authors of the present study. Here, the identifi-cation of hot and luminous WD candidates in GCs M13(NGC 6205) and M22 (NGC 6656) is briefly reported.

2. COLOR-MAGNITUDE DIAGRAMS AND RESULTS

The source of the color-magnitude diagrams (CMDs)of the present study was the HST ACS photometriccatalog of Anderson et al. (2008) and Sarajedini etal. (2007), now open to the public through the STScIMAST archive. V versus V − I photometric data of theGCs M13 and M22 in the Johnson-Cousins photomet-ric system defined by Landolt (1992) were taken fromthe catalog and transformed from mF606W and mF814W

in the Vega magnitude system according to the trans-formation equations of Sirianni et al. (2005) and thephotometric zero points of Mack et al. (2007). In theconstructed V versus V − I CMDs of M13 and M22,many spurious detections were removed with photomet-ric quality parameters qfit(V ) > 0.30 and qfit(I) > 0.30for fainter regions. However, saturated bright stars in

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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), onlythe stars satisfying the condition, r ≥ 0.62′, are shown for M13.

the long exposure frames with qfit(V ) = 2.50 and qfit(I)= 2.50 were retained. In the case of M13, a furtherradial restriction was applied to remove central starswith higher photometric errors due to central crowding,whose distances from the cluster center (r) were smallerthan 0.62′ (r < 0.62′).

The resultant V versus V − I CMDs of M13 and M22are shown in Figure 1. In Figure 1, small open cir-cles with a dot are the WD candidates for M13 andM22, respectively. The WD candidates for M13 andM22 were all confirmed as point sources in the accom-panying ACS/WFC images using IRAF routines. Thenumber of WD candidates for M13 is sixteen and thatfor M22 is thirteen. They are randomly distributed overthe ACS/WFC images, suggesting that they are reallyhot and luminous WDs.

ACKNOWLEDGMENTS

This research was supported by Basic Science Re-search Program through the National Research Foun-dation of Korea (NRF) funded by the Ministry of Edu-cation, Science and Technology (No. 2011-0026126 andNo. 2012R1A1A2006759).

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