Dept. of AstronmyDept. of Astronmy

Comparison with Theoretical CM diagramComparison with Theoretical CM diagram

Galactic Astronomy #6.1.3Galactic Astronomy #6.1.3

Jae Gyu Byeon

Dept. of AstronmyDept. of Astronmy

IsochroneIsochrone

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”

Dept. of AstronmyDept. of Astronmy

IsochroneIsochrone

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 struc

ture.

Isochrones have been calculation for a wide range of different metallicities, age, and physical assumptions

Yonsei-Yale group, BASTI group, Victoria group

Dept. of AstronmyDept. of Astronmy

BASTI IsochronesBASTI Isochrones

α Enhanced Models

Canonical Models

Z = 0.01

Y = 0.259

[Fe/H] = -0.60

[M/H] = -0.25

η = 0.4

Dept. of AstronmyDept. of Astronmy

BASTI IsochronesBASTI Isochrones

α Enhanced Models

Canonical Models

Z = 0.001

Y = 0.246

[Fe/H] = -1.62

[M/H] = -1.27

η = 0.4

Dept. of AstronmyDept. of Astronmy

BASTI IsochronesBASTI Isochrones

α Enhanced Models Canonical Modelsη = 0.4

Red solid lines Z = 0.01Y = 0.259[Fe/H] = -0.60[M/H] = -0.25

Blue solid linesZ = 0.001Y = 0.246[Fe/H] = -1.62[M/H] = -1.27

Dept. of AstronmyDept. of Astronmy

YY22 Isochrones Isochrones

Scaled Solar Models

Z = 0.02

Y = 0.27

[Fe/H] = 0.046

[α/H] = 0.0

Dept. of AstronmyDept. of Astronmy

YY22 Isochrones Isochrones

Scaled Solar Models

Z = 0.001

Y = 0.232

[Fe/H] = -1.289

[α/H] = 0.0

Dept. of AstronmyDept. of Astronmy

YY22 Isochrones Isochrones

α Enhanced Models

Z = 0.001

Y = 0.232

[Fe/H] = -1.758

[α/H] = 0.6

Dept. of AstronmyDept. of Astronmy

YY22 & BASTI Isochrones & BASTI Isochrones

Scaled solar Models [α/H] = 0.0Age = 0.1, 0.7, 2.0, 10 Gyr

Y2 IsochronesZ = 0.02Y = 0.27[Fe/H] = 0.046Red solid lines

BASTI IsochronesZ = 0.0198Y = 0.2743[Fe/H] = 0.06Blue solid lines

Dept. of AstronmyDept. of Astronmy

YY22 & BASTI Isochrones & BASTI Isochrones

Scaled solar Models [α/H] = 0.0Age = 0.1, 0.7, 2.0, 10 Gyr

Y2 IsochronesZ = 0.001Y = 0.232[Fe/H] = -1.288Red solid lines

BASTI IsochronesZ = 0.001Y = 0.246[Fe/H] = -1.27Blue solid lines

Dept. of AstronmyDept. of Astronmy

Comparision Comparision between Isochrone and RGB, HB 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

Dept. of AstronmyDept. of Astronmy

Comparision Comparision between Isochrone and RGB, HB 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 t

o the HB This transition is treated in a semi-empirical manner.

MRG : Mass of tip of RGB Mc : Mass of hydrogen-depleted helium ΔM : lost a mass from its atmosphere MHB = MRG - ΔM

A spred in values of ΔM

Probability distribution of MHB (Eq. 6.1)

Dept. of AstronmyDept. of Astronmy

Comparision Comparision between Isochrone and RGB, HB 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