恒星物理研究组知识库

We identify the location of the RGB bump of M4 (NGC 6121) using the available photometric data, and measure VRGBB (similar to 13.57 +/- 0.04 mag). According to the observed [Fe/H], we compute theoretical isochrones with the scaled-solar composition in the MESA model and alpha-enhanced composition in the BaSTI model, respectively. Using a color-magnitude diagram, we compare these isochrones with the observed data to obtain the magnitude difference delta V-bump between the predicted and the observed bump magnitude. Based on the best fit of the MESA model with [Fe/H] = -1.05, [alpha/Fe] =0, alpha(MLT) = 2.0, and t = 13.3 Gyr, the bump magnitude difference delta V-bump is about 0.46 mag. Based on the best fit of the BaSTI model with alpha(MLT) = 2.2 similar to 2.4, [Fe/H] = -1.05, [alpha/Fe] = 0.4, and t = 12.5 Gyr, the bump magnitude difference delta V-bump is about 0.2 mag. To try and explain the bump magnitude differences, the metal mass fraction Z is enhanced as 0.0037, 0.00591, 0.0076, and 0.0085. Accordingly, delta V-RGBB vertical bar(t=12.0,alpha MLT =2.2, [alpha/Fe] = 0,Z=0.0076), delta V-RGBB vertical bar(t=12.0,alpha MLT) (=2.2, [alpha/Fe]=0, Z=0.0085), and delta V-RGBB vertical bar(t=11.0,alpha MLT=2.4,[alpha/Fe]=0,Z=0.0085) by the MESA model are 0, -0.01, and 0 mag, respectively. Increasing the assumed metallicity can decrease the discrepancy between the predicted and observed bump magnitudes. However, a metallicity increase large enough to resolve the offset, roughly similar to 0.60 dex for the MESA models, is ruled out by the spectroscopic data.