Steinn Sigurðsson - Stellar Populations, I, II, III?

Looking around, there are two different types of stars out there:
imaginatively named Population I and Population II
The Sun, of course, defines Population I.

Originally defined in terms of colour and kinematics, but really we are looking at metallicity - fraction of elements with atomic mass of carbon or higher.
Population I is corotating with Sun and has redder colour.
Population II has higher velocity dispersion and is bluer.
define [Z] = log10(ratio of element Z/H relative to Solar composition)

Fe often used as proxy for mean metallicity.
Can have large variations in metallicity for different elements for the same star - which is important.
NB: this is a LOG scale!!

Metallicity as an age proxy?
Compare CMDs/H-R diagrams for populations with well defined distances.
Population I is young metal rich stars, Population II is old metal poor stars.
Here there ought to be most excellent links and pretty pictures.

Population I has [Z] ~> -0.5 or -0.3 -- so factor of 2-3 less than solar, up to [Z] ~ +0.5

NGC 1818

Open Cluster

Orion Nebula

Population II has [Z] < -0.5 down to maybe -3 or -5 depending on how and what you measure.

M22

Why two populations and not a continuum?
Local peculiarity?
Why?

Z vs v

Is there now, or has there ever been a Population III?
We'll get back to that.

So, what's up with that?

Last updated 08/11

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