User:Tohline/Appendix/Ramblings/Radiation/InitialTemperatures
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Initial Temperature Distributions
In an accompanying Wiki page we've discussed in detail (or see the summary page) how to transform back and forth between cgs units and the dimensionless code units that have been adopted by Dominic Marcello in his radiationhydro simulations of binary masstransfer. Here we want to probe in more depth what temperature distributions are obtained from the initial polytropic structure once Dominic chooses particular values of the four scaling parameters: , , , and .
Our derivation of the temperature distribution will center around the following ideas. First, the initial binary model that Dominic obtains from Wes Even's selfconsistentfield (SCF) code obeys a polytropic equation of state (EOS), namely,
with an adopted polytropic index = 3 / 2. Hence, at any point inside either star, the pressure (in code units), P_{code}, can be obtained from knowledge of the massdensity (in code units), ρ_{code}, and the polytropic constant, K_{code}, via the relation,
Second, Dominic's models are evolved assuming a more realistic EOS. Specifically, he assumes that the total pressure is given by the expression,
where mathematical expressions for the ideal gas pressure, P_{gas}, the electron degeneracy pressure, P_{deg}, and the photon radiation pressure, P_{rad}, are provided in an accompanying discussion of analytically prescribed equations of state. (Actually, Dominic is presently ignoring the effects of P_{deg}, but because it allows for a more general treatment at some later date, we will assume the more general expression for P_{total} and set P_{deg} = 0 near the end of our discussion.)
Various Scalings
Pressure
Now, realizing that pressure has units of energy per unit volume, we conclude that in order to transform between cgs units and code units, Dominic must adopt the relation,

= 


= 


= 


= 

where , and (the characteristic Fermi pressure) are physical constants defined in our accompanying variables appendix. (Numerical values of these constants can be obtained by scrolling the cursor over the symbols for the constants in this last sentence.) This relation also means that, generally,
and, specifically when P_{cgs} = P_{total}, we have,
Density
In a similar manner we recognize that the density transformation must be governed by the relation ... (express this in terms of χ^{3} so that it is obvious how to introduce ρ_{code} into the quartic equation, below).

= 


= 


= 


= 

Hence,
Temperature
Also, the temperature scaling can be rewritten as follows.

= 


= 

Hence,
Total Pressure Relation
In an accompanying page of our Wikibased H_Book, we show that, when normalized to , the analytic expression for the dimensionless total pressure takes the form,
Based on the abovederived relations, this can now be rewritten in terms of the variables used in Dominic's simulations as follows:

= 





= 


= 

EOS Quartic Solution
We can view this last expression as having the form,
where,
a_{4} 


a_{1} 


a_{0} 


that is, it is a quartic equation describing the relationship between T_{code} and ρ_{code}. Following our accompanying H_Book Wiki discussion, the solution to this quartic equation is,
where,
θ 


λ 





φ 


Application to Dominic Marcello's RadHydro Models
Temperature
Currently Dominic is ignoring the effects of electron degeneracy pressure, so in applying the above T(ρ) solution to his models we can set F(χ) = 0 in the definition of a_{0}. Doing this, we find that,
θ 
= 

λ 
= 

For purposes of discussion, we will define ρ_{1} as the value of ρ_{code} when λ = 1, that is,
As the accompanying discussion points out, the limiting behavior of the quartic solution is as follows:
For 


= 

For 


= 

Now let's plug in numerical values for the two stars in Dominic's Case A, q_{0} = 0.7 model evolution, as drawn from the accompanying properties table.
Case A: ; ; ; ; ; 

Star 
K_{code} 
θ 
λ 
ρ_{1} 
T_{code} (for ) 
P_{rad} / P_{gas} (for ) 
Accretor 
0.2571 


15.86 


Donor 
0.2364 


20.4 


Other Physical Variables
The ratio of radiation pressure to gas pressure (see the last column of the above table) is calculated via the relation,
Also note that,
In order to avoid establishing stellar structures that are convectively unstable, Dominic also needs to choose an evolutionary ratio of specific heats, γ, such that its value is everywhere greater than a critical value, γ_{c}, established at the center of the accretor. From Equation (131) in Chapter II of Chandrasekhar (1967) we see that γ_{c} depends on each star's central value of β — that is, it depends on β_{c} — and on each star's structural (which is 5 / 3 for our two = 3 / 2 polytropic stars) in the following way:
Plugging into these expressions lets us tabulate various properties at the center of both stars.
Central Stellar Values 


Approximations 
From Quartic Solution 

Star 



β_{c} 


β_{c} 
γ_{c} 

Accretor 
1.0000 
0.5843 

0.99339 
0.5805 

0.99352 
1.67765 

Donor 
0.6077 
0.3854 

0.99687 
0.3842 

0.99690 
1.67188 

The central values of P_{rad} / P_{gas} obtained via the quartic solution match exactly the values that Dominic read straight from the radhydrocode, namely, (for the accretor) and (for the donor). (See email from Dominic to Joel dated 8/4/2010.) In this email, Dominic also stated that he chose γ = 1.67114094; I'm not quite sure how he derived this value.
© 2014  2019 by Joel E. Tohline 