Dong-Hwan Cho1, 2, Hyun-Il Sung2, Sang-Gak Lee3, and Tae Seog Yoon1 Relative Age Difference between the Metal-poor Globular Clusters M53 and M92 Dong-Hwan Cho1, 2, Hyun-Il Sung2, Sang-Gak Lee3, and Tae Seog Yoon1 1 Department of Astronomy and Atmospheric Sciences, Kyungpook National University 2 Korea Astronomy and Space Science Institute 3 Department of Astronomy, Seoul National University February 22, 2013

<Contents> I. Introduction II. Observations and Data Reduction 2.1 Observations 2.2 Data Reduction III. Color-Magnitude Diagrams IV. Relative Age Measurement Using the Δ(B-V) Method V. Conclusions

I. Introduction M53 (NGC 5024): [Fe/H] = -2.10 (Harris 1996 [2010 edition]) R⊙ = 17.9 kpc, Z = 17.6 kpc, (m-M)V =16.32, E(B-V) = 0.02 M92 (NGC 6341): [Fe/H] = -2.31 (Harris 1996 [2010 edition]) R⊙ = 8.3 kpc, Z = 4.7 kpc, (m-M)V = 14.65, E(B-V) = 0.02 <Age Debates on M53 and M92> 1) Heasley & Christian (1991): The ages of M53 and M92 are same within the error. M53: The Color-Magnitude Diagram reported by Heasley & Christian (1991) M92: The fiducial sequence reported by Stetson & Harris (1988) 2) Rey et al. (1998): No significant age difference between M53 and M92 (Δt < 1 Gyr) M53: The fiducial sequence reported by Rey et al. (1998) 3) VandenBerg (2000): ΔVTOHB age of 26 globular clusters including M53 and M92 → The age of M53 = 12.5 ± 1.0 Gyr; The age of M92 = 14.0 ± 0.8 Gyr

M53: V vs. B-V CMD reported by Rey et al. (1998) M92: V vs. B-V CMD reported by Stetson & Harris (1988) and V vs. V-I CMD reported by Johnson & Bolte (1998) 4) Marín-Franch et al. (2009): The Relative ages of 65 globular clusters including M53 and M92 → The relative ages of M53 and M92 are the same within the error. M53 and M92: The VI CCD photometric data of the HST ACS (Anderson et al. 2008) 5) Dotter et al. (2010): The absolute ages of 61 globular clusters including M53 and M92 by the isochrone fitting method The age of M53 = 13.25 ± 0.50 Gyr; The age of M92 = 13.25 ± 1.00 Gyr <Study Purpose> The age problem of M53 and M92 → Resolution by the accurate relative age measurement by the Δ(B-V) method reported by VandenBerg et al. (1990) (The same telescope, filter set, and CCD camera + the same observing sessions + the same reduction method) ==> The derivation of the homogeneous photometric data The BOAO 1.8-m telescope + 2K CCD observations

M92 Stetson & Harris (1988)

II. Observations and Data Reduction 2.1 Observations M53 and M92: BOAO 1.8-m telescope (f/8) + BVI filter set + SITe 2K CCD Observations on April 4, 2002 (Central 11.7 x 11.7 arcmin2 region of each cluster) :Short, medium, and long exposures Average seeing: M53(1.2″); M92(1.3″) ----------------------------------- Observations for standardization: Landolt’s (1992) standard regions and central region of each cluster (May 4, 2003) Average seeing: 1.2″ ∼ 1.9″ For the details of M92, refer to Cho & Lee (2007)

Observed region of M53 Observed region of M92

2.2 Data Reduction Preprocessing: The IRAF CCDRED package, PSF photometry: The IRAF DAOPHOT package, Aperture photometry: IRAF APPHOT package, Standardization: IRAF PHOTCAL package Applying the same reduction method to both M53 and M92 ⇒ The derivation of homogeneous photometric data For more details of the reduction method of M92, refer to Cho & Lee (2007): The same method was applied to M53.

M53 M92 M53 M92 The present study vs. data for standard stars reported by Stetson (2000)

<Residuals> 1) M53: The present study vs. Stetson (2000) Δ(B-V) = -0.054 ± 0.003 Δ(V-I) = -0.015 ± 0.002 2) M92: The present study vs. Stetson (2000) ΔV = -0.007 ± 0.001 Δ(B-V) = -0.020 ± 0.001 Δ(V-I) = -0.025 ± 0.001 <Trend in magnitude and color index> M53 and M92: V magnitude, V-I color: No significant trend B-V color: Slight but similar trend --> very weak effect on the relative age measurements of M53 and M92

III. Color-Magnitude Diagrams <The characteristics of CMDs of M53> 1) The BHB is predominant in the HB. 2) The AGB is well separated from the RGB. 3) The BSS sequence is well developed. CMDs of M53

<The characteristics of CMDs of M92> Refer to Cho & Lee (2007) CMDs of M92 (Cho & Lee 2007)

Relative Age measurement: V vs. B-V CMDs --> The derivation of the fiducial sequence of each cluster (additional radial restriction)

<The comparison of the fiducial sequences> The present study vs. Rey et al. (1998) ΔV = -0.023 ± 0.001 Δ(B-V) = -0.052 ± 0.001 ==> Well coincident The present study vs. Stetson & Harris (1988) ==> Not coincident at the lower RGB

Comparison of the CMD and the fiducial sequence of each cluster

IV. Relative Age Measurement Using the Δ(B-V) Method The merit of the relative age measurement techniques 1) Weakly affected by the uncertainties of the stellar evolution theory 2) Independent of distance modulus, interstellar reddening, and zero points of the photometric calibrations Δ(B-V) method reported by VandenBerg et al. (1990): use of the (B-V)TO and V+0.05 VandenBerg et al. (1990)

-4 < V – V+0.05 < -2.5 Color difference between M53 and M92 = Δ(B-V) ≈ 0.011-0.022 ± 0.009 -------------------------- The absolute age of M92 = 14 Gyr (VandenBerg 2000) --> Δ(B-V) = ±0.01 ⇒ ±1 Gyr (The isochrones of VandenBerg et al. 2006) ⇒ The relative age of M53 is smaller than that of M92 by ≈1.1-2.2 ± 0.9 Gyr (mean = 1.6 ± 0.9 Gyr). ⇒ Inconsistent with the previous studies

Rey et al. (1998)

Comparison of the isochrones reported by VandenBerg et al. (2006)

V. Conclusions 1) BVI CCD photometric study on M53: the same telescope, filter, & CCD camera and the same observing sessions and the same reduction method for M92 reported by Cho & Lee (2007) → Accurate relative age measurement of M53 and M92 2) Relative age measurements of M53 and M92 using the Δ(B-V) method of VandenBerg et al. (1990): The relative age of M53 is smaller than that of M92 by 1.6 ± 0.9 Gyr assuming the absolute age of M92 as 14 Gyr (VandenBerg 2000). 3) Age difference between M53 and M92 --> Slightly different HB morphology between M53 and M92 ((B-R)/(B+V+R) = 0.76 vs. 0.88 (M53 vs. M92)) ⇒ Larger age of M92 makes the HB morphology bluer than that of M53.

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