[Mesa-users] Possibly strange values of adiabatic compressability (Gamma1)

Bill Paxton paxton at kitp.ucsb.edu
Fri Apr 17 20:18:21 EDT 2020

Hi Earl,

Would you pick a particular location in a mesa model where you are getting a strange Gamma1 and report the values for logT, logRho, X, Z, abar, zbar, and Gamma1 (from a profile for example).   I’ll dig into the guts of the code a bit to see where the result originates in the sense of which of the many component eos’s making up the crazy quilt of the mesa eos is responsible.


> On Apr 17, 2020, at 4:46 PM, Earl Bellinger via Mesa-users <mesa-users at lists.mesastar.org> wrote:
> Dear mesa-users, 
> I am comparing MESA and ASTEC models for a 1.2 solar mass track that I have evolved to around the base of the red giant branch. 
> At this phase of evolution, the two codes disagree rather substantially in the central regions on the value of Gamma1 = (∂ ln P / ∂ ln ρ)_ad (also known as the adiabatic compressabiltiy or the first adiabatic exponent). 
> While ASTEC has Gamma1 ~= 5/3 = 1.666... (monatomic ideal gas), the MESA values rise up to about 1.73. These increased values of Gamma1 occur both in the core and in a shell outside the core. 
> What causes these differences between the two codes? Is MESA doing the right thing here? What physics could cause Gamma1 to rise so high? 
> I have attached several plots to illustrate this situation: 
> - hrd_MESA.pdf, hrd_ASTEC.pdf: HR diagrams showing the tracks from the two codes for reference 
> - Gamma1_9.pdf: a snapshot of the Gamma1 profile from one of the MESA models along the subgiant branch 
> - Gamma1.pdf: a Kippenhahn-like diagram showing the MESA evolution of Gamma1 along the subgiant branch (the x-axis is age in megayears) 
> - Gamma1_max_MESA.pdf: evolution of the max value of Gamma1 in the innermost 10% by radius for the MESA models 
> - Gamma1_max_ASTEC.pdf: evolution of the max value of Gamma1 in the innermost 10% by radius for the ASTEC models 
> - Gamma1_T.pdf, Gamma1_rho.pdf: the maximum of Gamma1 against the the temperature and the density of the point where Gamma1 is maximum for the MESA models. This indicates what EOS is being used; it seems to be the OPAL EOS, although it is somewhat near to HELMS.  For comparison, the ASTEC models strictly use OPAL. 
> I have also attached the inlists for the MESA track (I think it is probably fairly innocuous). I am using mesa-r12115. 
> Thanks in advance for any insights. 
> Best regards, 
> Earl 
> Med venlig hilsen/Kind regards 
> Earl Patrick Bellinger 
> Postdoctoral Research Fellow 
> Stellar Astrophysics Centre 
> https://earlbellinger.com <https://earlbellinger.com/> 
> <hrd_MESA.pdf><hrd_ASTEC.pdf><Gamma1_9.pdf><Gamma1.pdf><Gamma1_max_MESA.pdf><Gamma1_max_ASTEC.pdf><Gamma1_inlists-r12115.tar.gz>_______________________________________________
> mesa-users at lists.mesastar.org
> https://lists.mesastar.org/mailman/listinfo/mesa-users

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