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What does the abbreviation YP stand for?

  1. Apr 14, 2015 #1
    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. jcsd
  3. Apr 14, 2015 #2

    bapowell

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    Mass fraction of Helium 4
     
  4. Apr 14, 2015 #3
    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?
     
  5. Apr 14, 2015 #4

    marcus

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    Dearly Missed

    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
  6. Apr 15, 2015 #5
    Hi marcus:

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