[mesa-users] Core hydrogen mass fraction increases in early evolution of 1.5 Msun star

[Evan O'Connor]

Hi again,

Ok, I'll retract my ^3He explanation, on further thought its seems 
unlikely especially given the temperature.   From a look at the 
evolution it looks like the ZAMS model already has CNO burning started, 
which lowers X, then and Conny and Bill point out, the convection brings 
it back up.

Evan


On 01/14/2013 12:44 PM, Bill Wolf wrote:
> I shared Conny's suspicion and just ran it with pgstar. There is 
> indeed some convection going on at that point, so I would guess that 
> it's just the introduction of fresh hydrogen to the core.
>
> Bill
>
> On Jan 14, 2013, at 9:40 AM, Conny Aerts <conny at ster.kuleuven.be 
> <mailto:conny at ster.kuleuven.be>> wrote:
>
>>
>> Hi Rich,
>>
>> without having computed MESA models in this regime, I would tell my
>> students that this is due to the increase of the convective core during
>> the early MS for such a mass, such that fresh hydrogen fuel enters the
>> core and is instantaneously mixed. This increase in convective core is
>> due to the relative increase of importance of the H burning by CN cycle
>> versus  pp chains (different dependencies of opacity: pp only on X and
>> CN on X and Z with X decreasing).  In fact, this is about the mass where
>> this effect is largest (the convective core starts shrinking as you move
>> above 1.6 solar masses birth mass). This is beautifully illustrated in
>> Fig.3.6 of the book "Asteroseismology"...;-)
>>
>> Hope this is a valid explanation,
>> Conny
>>
>>
>> On 01/14/2013 06:21 PM, Richard Townsend wrote:
>>> Hi folks --
>>>
>>> I'm running a vanilla MESA (4740) calculation for a 1.5 Msun star, 
>>> from MS to He flash. Surprisingly (to me), the core hydrogen mass 
>>> fraction *rises* slightly during the early evolution (~timestep 
>>> 170). Can anyone explain why this might be so? I attach the inlist.
>>>
>>> cheers,
>>>
>>> Rich
>>>
>>>
>>>
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>>
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> ________________________
> William Wolf
> wmwolf at physics.ucsb.edu <mailto:wmwolf at physics.ucsb.edu>
>
> UCSB Department of Physics
>
>
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