Different Characteristics of the Bright Branches of the Globular Clusters M3 and M13

• Published in: The Astronomical journal 2005-04, Vol.129 (4), p.1922-1933
• Main Authors: Cho, Dong-Hwan, Lee, Sang-Gak, Jeon, Young-Beom, Sim, Kyung Jin
• Format: Article
• Language: English
• ISSN: 1538-3881; 0004-6256; 1538-3881
• DOI: 10.1086/428369

We carried out wide-field BVI CCD photometric observations of the globular clusters M3 (NGC 5272) and M13 (NGC 6205) using the Bohyun Optical Astronomy Observatory 1.8 m telescope equipped with a SITe 2K CCD. We present color-magnitude diagrams (V vs. B - V, V vs. V - I, and V vs. B - I) of M3 and M13. We have found asymptotic giant branch (AGB) bumps at V = 14.85 ± 0.05 mag for M3 and at V = 14.25 ± 0.05 mag for M13. We also found that AGB stars in M3 are more concentrated near the bump, while those in M13 are scattered along the AGB sequence. We identified the red giant branch (RGB) bump of M3 at V = 15.50 ± 0.05 mag and that of M13 at V = 14.80 ± 0.05 mag through luminosity functions and slope changes of the integrated luminosity functions of M3 and M13. We have estimated the ratios R and R2 for M3 and M13 and found that the value of R for M3 is larger than that for M13, while values of R2 for M3 and M13 are similar and compatible with the value expected from evolutionary theory when only normal horizontal-branch (HB) stars are used for estimation of R and R2 for M13. However, we found that values of R for M3 and M13 are similar, while the value of R2 for M3 is larger than that for M13 when all the HB stars are included for estimation of R and R2 for M13. We have compared the observed RGB luminosity functions of M3 and M13 with the theoretical RGB luminosity function of Bergbusch & VandenBerg at the same radial distances from the cluster centers as used in the estimation of the ratios R and R2 for M3 and M13. We found 'extra stars' belonging to M13 in the comparison of the observed RGB luminosity function of M13 and the theoretical RGB luminosity function of Bergbusch & VandenBerg and even in the comparison of the observed RGB luminosity functions of M3 and M13. In the original definition of the ratio R of Buzzoni et al., NHB corresponds to the lifetime of HB stars in the RR Lyrae instability strip at log Teff = 3.85. Thus, the smaller R value resulting for M13 compared with that for M3 in the case in which only normal HB stars are included in the estimation of R and R2 for M13 may be partially caused by extra stars, and the similar R values for M3 and M13 in the case in which all the HB stars are included in the estimation of R and R2 for M13 may be caused by extra stars in the upper RGB of M13. If extra stars in the upper RGB of M13 are caused by an effective 'deep mixing,' these facts support the contention that an effective deep mixing could lead to