1. Dept. of Astronmy Comparison with Theoretical CM diagram Galactic Astronomy #6.1.3 Jae Gyu Byeon

2. Dept. of Astronmy Isochrone The Morphology of G.C. CMD : All star formed single epoch Choose initial abundances for the chemical element For heavy element, initial helium abundance Evolve the population forward in time Solving the stellar structure equation Keeping track of the chemical evolution For each step, calculate the luminosities and colors The curve connecting all the stars in the CMD is called an “Isochrone” from the Greek for “same time”

3. Dept. of Astronmy Isochrone If assumed initial chemical composition and stellar structure calculations is correct, Comparing isochrones to observed sequences : Powerful tool For measuring the ages of G.C For testing our understanding of the basic physics of stellar structure. Isochrones have been calculation for a wide range of different metallicities, age, and physical assumptions Yonsei-Yale group, BASTI group, Victoria group

4. Dept. of Astronmy BASTI Isochrones α Enhanced Models Canonical Models Z = 0.01 Y = 0.259 [Fe/H] = -0.60 [M/H] = -0.25 η = 0.4

5. Dept. of Astronmy BASTI Isochrones α Enhanced Models Canonical Models Z = 0.001 Y = 0.246 [Fe/H] = -1.62 [M/H] = -1.27 η = 0.4

6. Dept. of Astronmy BASTI Isochrones α Enhanced Models Canonical Models η = 0.4 Red solid lines Z = 0.01 Y = 0.259 [Fe/H] = -0.60 [M/H] = -0.25 Blue solid lines Z = 0.001 Y = 0.246 [Fe/H] = -1.62 [M/H] = -1.27

7. Dept. of Astronmy Y 2 Isochrones Scaled Solar Models Z = 0.02 Y = 0.27 [Fe/H] = 0.046 [α/H] = 0.0

8. Dept. of Astronmy Y 2 Isochrones Scaled Solar Models Z = 0.001 Y = 0.232 [Fe/H] = -1.289 [α/H] = 0.0

9. Dept. of Astronmy Y 2 Isochrones α Enhanced Models Z = 0.001 Y = 0.232 [Fe/H] = -1.758 [α/H] = 0.6

10. Dept. of Astronmy Y 2 & BASTI Isochrones Scaled solar Models [α/H] = 0.0 Age = 0.1, 0.7, 2.0, 10 Gyr Y 2 Isochrones Z = 0.02 Y = 0.27 [Fe/H] = 0.046 Red solid lines BASTI Isochrones Z = 0.0198 Y = 0.2743 [Fe/H] = 0.06 Blue solid lines

11. Dept. of Astronmy Y 2 & BASTI Isochrones Scaled solar Models [α/H] = 0.0 Age = 0.1, 0.7, 2.0, 10 Gyr Y 2 Isochrones Z = 0.001 Y = 0.232 [Fe/H] = -1.288 Red solid lines BASTI Isochrones Z = 0.001 Y = 0.246 [Fe/H] = -1.27 Blue solid lines

12. Dept. of Astronmy Comparision between Isochrone and RGB, HB Unable to model accurately the deep convective layers and mass loss in giant star The match between calculation and observation becomes rather poor Vertical RGB to depend on metallicity Most metal-poor clusters have the bluest RGBs The line blanketing effects of heavy elements Poorer for the RGB than it is for the MS and SGB Still-later stages of stellar evolution : discrepancies grow

13. Dept. of Astronmy Comparision between Isochrone and RGB, HB The Helium Flash Instantaneous mass loss and a rearrangement of the structure Not possible to follow the evolution of a star from the RGB on to the HB This transition is treated in a semi-empirical manner. M RG : Mass of tip of RGB M c : Mass of hydrogen-depleted helium ΔM : lost a mass from its atmosphere M HB = M RG - ΔM A spred in values of ΔM Probability distribution of M HB (Eq. 6.1)

14. Dept. of Astronmy Comparision between Isochrone and RGB, HB These HB calculations explain the variation in HB color with cluster metallicity This semi-empirical approach allows us to understand many of the features of the HB The simple mass-loss model also fails to explain the differences in HB color seen in clusters with identical metallicities