Comparison with Theoretical CM diagram

1
Comparison with Theoretical CM diagram

• Galactic Astronomy 6.1.3
• Jae Gyu Byeon

2
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
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
BASTI Isochrones
a Enhanced Models Canonical Models Z 0.01 Y
0.259 Fe/H -0.60 M/H -0.25 ? 0.4
5
BASTI Isochrones
a Enhanced Models Canonical Models Z 0.001 Y
0.246 Fe/H -1.62 M/H -1.27 ? 0.4
6
BASTI Isochrones
a Enhanced Models Canonical Models ? 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
Y2 Isochrones
Scaled Solar Models Z 0.02 Y 0.27 Fe/H
0.046 a/H 0.0
8
Y2 Isochrones
Scaled Solar Models Z 0.001 Y 0.232 Fe/H
-1.289 a/H 0.0
9
Y2 Isochrones
a Enhanced Models Z 0.001 Y 0.232 Fe/H
-1.758 a/H 0.6
10
Y2 BASTI Isochrones
Scaled solar Models a/H 0.0 Age 0.1, 0.7,
2.0, 10 Gyr Y2 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
Y2 BASTI Isochrones
Scaled solar Models a/H 0.0 Age 0.1, 0.7,
2.0, 10 Gyr Y2 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
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
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.
• MRG Mass of tip of RGB
• Mc Mass of hydrogen-depleted helium
• ?M lost a mass from its atmosphere
• MHB MRG - ?M
• A spred in values of ?M
• Probability distribution of MHB (Eq. 6.1)

14
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