# What does the abbreviation YP stand for?

1. Apr 14, 2015

### Buzz Bloom

I have been looking up information about the promordial abundances of isotopes on the internet, and I find many of the papers discussing this use the notation YP (where P is usually a subscript). Can anyone tell me what this YP stands for? It appently means the primoridal abundance of the particular isotope being discussed, but what do the letters Y and P stand for?

2. Apr 14, 2015

### bapowell

Mass fraction of Helium 4

3. Apr 14, 2015

### Buzz Bloom

Thank you for responding bapowell, but you misunderstood what I was asking. The context value told me that it was for He 4 , but what do the letters "Y" and "P" stand for?

4. Apr 14, 2015

### marcus

http://www.astro.umd.edu/~miller/teaching/astr422/lecture25.pdf
==quote page 1 of lecture 25 of an astro course by Prof. Coleman Miller at University of Maryland==
Observations of Light Element Abundances

Last time we discussed the expectations of Big Bang Nucleosynthesis (BBN to its friends). Here we talk about the observations. For a bit of change of pace, and to give some appreciation of the difficulties and the care needed, we’re going to go the sausage-making route here: lots of gory details. In some cases, we will use the standard nomenclature that X is the mass fraction of hydrogen, Y is the mass fraction of helium, and Z is the combined mass fraction of everything else.
==endquote==
The notation seems to be traditional, somebody (Peebles1966? earlier?) found it convenient to divide stuff up into three categories (H, He, Other) and to study the probable division into 3 mass fractions.
The subscript "p" as you guessed stands for "primordial".

It sometimes happens that a notation goes back to early papers on the topic. Somebody made a (possibly arbitrary) choice and it stuck---becoming traditional.
Here is some historical background:
https://software.rc.fas.harvard.edu/pairitel/talks/PrimordialFireball.pdf
But I don't know which authors first made that division into X + Y + Z

Last edited: Apr 14, 2015
5. Apr 15, 2015

### Buzz Bloom

Hi marcus:

Thanks very much for the history. It was exactly what I was looking for.