The thin and thick disks of our galaxy may seem a bit arbitrary in their definition but there is more than just the number density of stars that separates them. The chemical composition of the stars is quite different in both. And all of that may be attributed to the age as well as the metallicity (the relative metal content) in the stars.
And if we are defining this relation on the basis of the relative metal content in stars, it would imply that there is a classification of stars on the basis of their metallicity. So, before diving into the Age-Metallicity relation, let us take a quick look at the different types of stars.
Different Populations of stars
Before we begin, remember - every element heavier than Hydrogen and Helium is a metal in astronomy!
You must be wondering how the metal abundances in stars are calculated. The answer lies is the spectra of stars. We can learn about the age of a star, what element it is burning, and even what stage of life it is in, among other things by understanding the spectra of stars. This is also how the mass fraction of different elements in the stars are calculated.
Suppose X, Y, and Z are the mass fractions of hydrogen, helium, and metals. Then we can broadly separate stars as Population 1 (Z=0.02, metal-rich), Population 2 (Z=0.001, metal-poor), and Population 3 (Z=0, hypothetical). In other words, for Population 1, the metals only make 0.02 parts whereas Hydrogen and Helium combined make 0.98 parts. Similarly, Population 2 is only made of 0.001 part metal and the hypothetical type, Population 3, should have 0 metal content (if it even exists!).
Now let us begin with the real question - what is the age-metallicity relation?
Assumptions for the relation
Studies of star clusters have revealed that for a particular spectral type, metal-rich stars are usually younger than their metal-poor counterparts. Meaning that the younger stars are more metal enriched and the older ones have the least metal content. This is the simplest way to state the age-metallicity relation.
But to understand this relation in detail, we will have to understand the assumptions we make for this relation.
Assumption 1:
The very first assumption is that the stars that the galaxy began with were made of simple elements such as Hydrogen and Helium. This means that the metal content was low and so low metallicity.
Do you think this assumption makes sense? Let’s see. The most abundantly available element in the universe is Hydrogen. All the other elements in the universe were produced inside the stars in various stages of their life depending on the mass of the star. So, assuming that our galaxy began with stars that were metal-poor makes sense.
Assumption 2:
Now, onto the second assumption. The newer stars are more metal-rich than the older stars.
Does this make sense? Well, the metals produced inside a star are ejected out via Supernovae (SNe) explosions becoming the seed for the newer stars. So, logically the new stars will be more metal enriched than their predecessors.
The assumptions we made draw a picture that the supernovae explosions are responsible for the metal enrichment of the galaxies. Since different components of galaxies house different generations of stars, we expect a different chemical composition of each part of the galaxy. But what metal exactly do we use for our comparison? I mean, shouldn’t there be a reference metal for our comparisons?
The metallicity index
Case 1: Iron
Let us take SNe Type Ia (read as supernovae type one 'a') as our guide because it is found in all galaxies, unlike the other types of SNe. Since the explosion gives us 0.6 Solar Masses of Iron made in its core, it is understandable if we use Iron as a metallicity index i.e. [Fe/H]. The index compares the iron abundance with respect to hydrogen in the star with the iron abundance in our sun as a logarithm. Here’s how it works:
[Fe/H] = 1 means ten times or 10^1 times the iron as compared to the sun
[Fe/H] = -1 means 1/10 or 10^-1 times the iron as compared to the sun
[Fe/H] = 0 means 10^0 or exactly the same as the sun
What happens when we test this index against our assumptions? Let’s see.
The moment of truth! The index says [Fe/H]=-4.5 for the extremely old star population and 1 for the younger. This translates into 3 x 10^-5 part of the iron abundance of our Sun for the older population and 3 x 10^1 part for the younger. Clearly, the younger population is more metal-rich than the older.
In the thin disk of our galaxy, we see stars with [Fe/H] from -0.5 to 0.3 (agreeable as the sun also lies in the thin disk), whereas for thick disk it usually goes from -0.4 to -0.6. (revealing that these are older stars). (Remember!! We are looking at a lot of negative values in this paragraph!)
But [Fe/H] as a metallicity index is not a very good option. And why is that?
First, an SN of type Ia happens more than 10^9 years after the star formation with no exact time span. Why? Because for an SN of this type to happen it will require the white dwarf to accumulate mass from its partner. This rate is never constant. Also, it can happen in case of a collision with another white dwarf as well, the time of which is again not defined. The other reason is that the metal enrichment due to this SN happens pretty locally so how can we expect homogeneous metallicity in that case?
So, what element do we use as our index then?
Case 2: Oxygen
First of all, we need that element to be a part of the SN explosion of the heavier stars so the metals can spread far and wide. Also because the explosions of massive stars happen just after 10^7 years so comparatively it is quicker. Next, we need the element to be produced in abundance in the cores of these dying massive stars. Keeping it all in mind it is obvious that our choice of element is Oxygen. It is produced inside the massive stars which die via SN explosion.
And this is how Oxygen becomes our reference metal and is used as a metallicity index ([O/H]) in the present-day research on the metallicity of galaxies. But truth be told, since Iron has been in use as index for so long, it is still accepted wherever we can work with it.
So, this was all one needed to know to understand the age-metallicity. This is a relation that one must take with a grain of salt. Because even though it seems to be applicable in many instances, there are still a few where it doesn’t. As is everything is life!
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