Probing the evolution of stellar systems

1
Probing the evolution of stellar systems

• Andreas Zezas
• Harvard-Smithsonian
• Center for Astrophysics

2
The lives of stars fighting against gravity

• Defining parameter
• Mass
• To avoid implosion stars must generate energy
  from fusion reactions
• The stages of stars are determined by the type of
  fuel left
• hydrogen, helium, carbon etc

3
The complicated lives of stars
but, after some time they run out of fuel
White dwarf (8 M?)
Supernova (8 M?)
4
Basic tool Photometry
5
Basic tool Photometry
6
(No Transcript)
7
Horizontal Branch
Red Giant Branch
Main Sequence
8
CMDs Simple cases
9
The tools Stellar Tracks
(ltmgt age, Mass, Z, stellar evolution)
10
The tools Isochrones
The tools Isochrones
11
The tools Isochrones
12
The tools Isochrones
13
The recipe

• Assume a star-formation scenario (model)
• Take some isochrones (calibration)
• Weight them by the IMF N ? M-?
• Mix them
• Simulate (include observational biases)
• Compare with observed CMD

14
Analogy with X-rays

• Isochrones ? RMF
• Detection probability ? ARF

15
The standard method
16
The standard method
17
The standard method
18
The standard method
19

Complications

• Incompleteness
• Multiple populations
• Uncertainties on isochrones

20
CMDs complications
21

CMDs complications
22
CMDs complications
23
CMDs more complications
Detection probability
24
Problems with standard method

• Gaussian statistics (as usually)
• ?2 fits
• All stars have equal weight
• Complex mixture problem with several free
  correlated parameters
• Several different sets of isochrones

25
The link with cal. uncertainties
26
A new method

• Estimate the likelihood that each data point
  comes from a given isochrone
• Find the likelihood for all points
• Advantages
• Easily generalized to n-dimensions
• Easily estimate effect of different isochrones
• Proper statistical treatment of uncertainties
• Can include correlated uncertainties and data
  augmentation for missing data
• Can treat as mixture problem

27
The test case The SMC
28
The test case

• Nearby galaxy (60 kpc)
• Recent star-formation (a bit complex)
• but can observe very deep and set good
  constraints
• on star-formation
• also can obtain additional constrains from
  spectroscopy
• .useful for testing the results and setting
  priors