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Homework Help: Initial Mass Function

  1. Mar 29, 2012 #1
    1. The problem statement, all variables and given/known data

    In a particular star forming cloud the initial mass function (IMF) is given by
    [tex] N(M_{*})=Cexp(-M_*^2) [/tex]
    where C is a normalisation constant. (The IMF describes the initial relative number of stars of different masses). Assuming that the number of hydrogen Lyman continuum photons created by stars of different mass, [tex] N_{Lyc}=10^{34}M_*^{32}[/tex] which mass stars dominate the volume of ionized natal star forming material?

    2. Relevant equations

    I've found this equation while I was searching for a lead:

    [tex]N=\int_{m_1}^{m_2} dN=\int_{m_1}^{m_2} \frac{dN}{dm}dm[/tex]

    but I'm not sure what values to use as integration limits.

    Also, the problem sheet gives the following formula (including gamma and Riemann zeta functions) which is probably useful at some point, though I don't know where or how.

    [tex] \int_0^\infty x^n\frac{1}{exp(x)-1}dx=\zeta(n+1)\Gamma(n+1)[/tex]

    3. The attempt at a solution

    My first step was differentiating the expression I was given to get
    which looks like something similar to the first equation, and also hints at the formula given in the problem sheet.
    But I'm really not sure where to go from here, and how to incorporate the value given for [tex]N_{Lyc}[/tex].
    Any help/pointers would be greatly appreciated.
  2. jcsd
  3. Mar 30, 2012 #2
    Looking at the expression I got for [tex]\frac{dN}{dM}[/tex], and assuming we're integrating from zero to infinity, the integral looks like
    [tex]-2C \int_0^{\infty}M_{*}\frac{1}{exp(M_{*}^2)}dM_{*}[/tex]
    which looks quite like the formula given,
    [tex]\int_0^\infty x^n\frac{1}{exp(x)-1}dx[/tex]
    except for the -1 on the bottom.

    Any ideas as to how to move forward?
    Last edited: Mar 30, 2012
  4. Mar 30, 2012 #3


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    Staff: Mentor

    Hmm. Given a formula for the number of stars produced of a given mass M, and a formula for the number of (particular) photons produced by a star of a given mass M, what would be an expression for the total number of such photons produced by all stars of a given mass M?
  5. Mar 30, 2012 #4
    Well, I'd say that would be
    (Total number of photons produced by all stars of given mass M) = (Number of photons produced by a star of mass M) x (Number of stars of mass M).
    Or in symbols
    [tex] N_{ph-tot}(M_*)=N_{Lyc}(M_*)*N(M_*)[/tex]
    And that would give
    [tex] N_{ph-tot}(M_*)=10^{34}CM_*^{32}exp(-M_*^2) [/tex]

    Is that right?

    If so, what's the next step, as it is still not in the form of the given integral?
  6. Mar 30, 2012 #5


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    Staff: Mentor

    If that is an expression for the total number of photons as a function of M, I'd look for the M that gives that expression a maximum value... I wouldn't presuppose that the problem has to involve an integral that happens to appear on the problem sheet.
  7. Mar 31, 2012 #6
    Okay, so differentiating and setting to zero gives
    which simplifies to

    So how do I interpret that? It only really makes sense if it's in units of Msun..

    While that method seems to make sense (and thanks for your help) it's quite unusual for a formula to appear that isn't necessary for the problem.

    Could there be an alternative method that requires the use of the given formula?
  8. Mar 31, 2012 #7


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    Staff: Mentor

    Presumably the "normalisation constant", C, takes care of the mass units. I wouldn't be at all surprised if it were to be chosen to produce units of Msun :wink:
    Nothing springs to mind.
  9. Mar 31, 2012 #8
    Thanks very much for your help :)
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