[mesa-users] A common stellar-structure myth

[RICHARD H D TOWNSEND]

Hi again —

Just run a MESA model at Z=1E-5 — I find that the central pressures for 1 and 10 Msun models are practically identical at the ZAMS (I prefer to stick to the ZAMS, since having to deal with non-uniform compositions further muddies any narrative one is trying to develop).

Going back to Tomek’s remarks, I’ve just run a gedankenexperiment where I calculate 1 and 10 Msun solar-metallicity ZAMS models using nuclear reaction rates that are artificially inflated by a factor 1E4. Comparing these against the models from my original email, I find the following result

1) For both 1Msun and 10Msun cases, the original and high-eps models have almost identical luminosities (differing by less than 0.03 dex)

2) The high-eps models have larger radii than the original models — by ~ 0.23 dex for 1Msun, 0.36 dex for 10Msun

3) The potential well (as measured by the central gravitational potential) is shallower in the high-eps models — by a factor ~0.25 for 1Msun, ~0.5 for 10Msun

Result (1) nicely highlights that the luminosity of a ZAMS star doesn’t depend on the mode of energy generation. Result (2) indicates that the increased energy production in the high-eps models is able to balance radiative surface losses earlier during the pre-MS contraction, resulting in a larger ZAMS star. Result (3) is a natural consequence of Result (2).

Given that the original vs. high-eps models have the same luminosities (for each mass), but quite different potential well depths, I don’t think it’s right to argue that the depth of the potential well has any direct control over the luminosity of a star.

cheers,

Rich

> On Aug 30, 2016, at 7:20 PM, RICHARD H D TOWNSEND <townsend at astro.wisc.edu> wrote:
> 
> Hi Alexander —
> 
> Interesting! I wonder whether this is a result of degeneracy behaving differently in the Pop I vs Pop III cases. What initial metallicity did you assume — zero, or something small?
> 
> cheers,
> 
> Rich
> 
>> On Aug 30, 2016, at 7:12 PM, Alexander Heger <alexander.heger at monash.edu> wrote:
>> 
>> Dear Richard,
>> 
>> I just looked at two stars, admittedly Pop III stars, and those, middle of H burn, pressure is 10x higher in 10 M_sun star compare to the 1 M_sun star.  Both half way through H burn.
>> 
>> Maybe an interesting difference to Pop I.  And related to the shape of the curve in Ibeling & Duligur (2013).
>> 
>> Never mind, gravitational acceleration is zero in the centre of both stars.
>> 
>> -Alexander
>> 
>> On 31 August 2016 at 09:56, RICHARD H D TOWNSEND <townsend at astro.wisc.edu> wrote:
>> Hi Alex —
>> 
>> In the context of an entry-level astronomy textbook, I think ‘gravity’ is intended to reference acceleration (which students have every-day experience of) rather than potential (which they don’t).
>> 
>> Regarding pressures — as you can see from the plot below, the pressure at the center of the massive star is about 0.5 dex smaller than that at the center of the solar-mass star. So, comparing pressures doesn’t help here.
>> 
>> Rich
>> 
>> 
>> 
>> 
>> -- 
>> Sent from gmail.
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