[Mesa-users] Artificially inflating a star

Mon Mar 15 16:40:32 UTC 2021

Hi James,

My apologies if I misled.  My mention of other_jdot_mb() was simply to 
provide an example of an other_ hook that you might be familiar with.  
My intended suggestion was to use other_rlo_mdot() while leaving the 
angular momentum loss unchanged from MESA's default.

The essence of my suggestion was to change

Mdot_RLOF = -Mdot_0 * exp ( (R_1-R_RL1) / (H_P/gamma) )

to

Mdot_RLOF = -Mdot_0 * exp ( (f*R_1-R_RL1) / (H_P/gamma) )

where f is like 1.02-1.04. i.e. just apply an "inflation factor" to the 
star's radius.

I think this is analogous to what was done in Knigge et al. (2011) 
(though I may have misinterpreted).  So I thought that's what you were 
looking for.

If you introduce some new physics in order to expand the radius of the 
star, you will first have to go through the process of figuring out how 
that physics changes the stellar state for different masses and at 
different points in the evolution.  That's a worthy goal, but would 
involve a whole separate process before you could get to evolving 
binaries, and so seemed more indirect than what you were looking for.

Anyhow, mine was intended to be a very simple suggestion.  But if your 
are interested in adding or changing physics in the star, that is a 
whole different can of worms and I'm sure you will learn a lot.  Looks 
like Matt Browning has given some good pointers for one way to do that.

Cheers,

Dean

On 3/12/21 4:07 AM, James Wild wrote:
> We've spent a week ruminating on this, and I'm not convinced it's 
> quite what we're after. We are interested in trying to reproduce the 
> observed donor_mass vs orbital period data in order to measure the 
> required angular momentum loss (AML) rate. The end goal is an 
> empirically measured AML rate for CVs below the gap.
>
> Unless I've misunderstood, your approach would be analogous to adding 
> extra AML - increasing the mass loss rate at a given donor mass. But 
> this would have an inconsistent effect along the track. It would have 
> no impact on the orbital period at which the CV came back into contact 
> at the bottom of the gap, for example, since the stars in the gap are 
> in equilibrium and not experiencing any mass loss.
>
> Instead, we want to get the input stellar models as close to reality 
> as possible. This way, when we adjust the AML rate to fit the observed 
> data, we can be confident we are not adding extra AML to compensate 
> for the tendency for the models to underpredict main sequence radii.
>
> Ideally, we would tweak the models so that the MESA main-sequence 
> radii are around 2-4% larger than now - see the attached plot, which 
> shows the measured masses and radii of M-dwarfs from Parsons et al 
> (2018), where we have shown the percentage discrepancy between 
> measured radii and MESA main-sequence models with tau_100 boundary 
> conditions.
>
>
>
> I wonder if this might be done by introducing the effect of starspots 
> into MESA somehow. For example, we might make our own boundary 
> conditions table with a modified effective temperature, to account for 
> the fact that starspots reduce the effective temperature from the 
> pristine photospheric value. We are worried there are some subtle 
> implications we haven't thought through, and wondered if anyone has 
> any advice how to do this.
>
> On Thu, 4 Mar 2021 at 01:46, Dean Townsley <Dean.M.Townsley at ua.edu 
> <mailto:Dean.M.Townsley at ua.edu>> wrote:
>
>     Hi James,
>
>     While I think changing the size of the star that MESA produces is
>     difficult, since that is determined by the physical state of the
>     star, I think you can accomplish what you want in a different way.
>
>     The mass transfer rate between stars is usually determined by
>     comparing the radius of the donor star (R_1) and the Roche Lobe
>     radius (R_RL1).  A good example is equations 13 and just after in
>     Paxton et al (2015).  I think you can accomplish what you want
>     just by effectively changing how R_1 and R_RL are used to compute
>     the mass transfer rate.  e.g. use f*R_1 where f is some inflation
>     factor (like 1.02 for a 2% inflation), and/or something similar
>     applied to R_RL1.
>
>     To implement this, I think you would want to implement your own
>     subroutine to attach to the other_rlo_mdot() subroutine hook in
>     your run_binary_extras.f90 and have use_other_rlo_mdot set to
>     true.  You'll probably want to pattern your function after the
>     actual rlo_mdot() subroutine that is in
>     binary/private/binary_mdot.f90.  It looks like that implements
>     what is described in Paxton et al. (2015).
>
>     This is somewhat like what is done in the run_binary_extras.f90
>     that is included with the Pala et al (2017) paper, but there it
>     was done for the angular momentum loss rate using the
>     other_jdot_mb() hook.  So it might be good to use the
>     run_binary_extras.f90 there as an example of how to use one of
>     these hooks.  There is also some documentation about using the
>     _extras stuff on the mesa website I believe.
>
>     Hopefully that at least gets you started in the right direction.
>
>     Dean
>
>
>     On 3/3/21 2:00 AM, James Wild via Mesa-users wrote:
>>     Hi,
>>
>>     I'm trying to simulate a full CV evolutionary track, based on the
>>     work by Paxton et.al. (2015)
>>     <https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fui.adsabs.harvard.edu%2Fabs%2F2015ApJS..220...15P%2Fabstract&data=04%7C01%7Cdmtownsley%40ua.edu%7Cfc9d22bee14a46caf83008d8e53ec996%7C2a00728ef0d040b4a4e8ce433f3fbca7%7C0%7C0%7C637511405593976594%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=%2FxlQ%2B%2BBAFBKRumWAlSLLeoW1Hfh2Schfu8GYvHDmO5w%3D&reserved=0>,
>>     similar to this Pala (2017) paper
>>     <https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fui.adsabs.harvard.edu%2Fabs%2F2017MNRAS.466.2855P%2Fabstract&data=04%7C01%7Cdmtownsley%40ua.edu%7Cfc9d22bee14a46caf83008d8e53ec996%7C2a00728ef0d040b4a4e8ce433f3fbca7%7C0%7C0%7C637511405593986592%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=RkevcTWt8HbNaDtx%2BupLFYJ5pdpCLqXOyyfv0rz0lT0%3D&reserved=0>.
>>     Their inlists are provided on cococubed, so make for a good
>>     jumping off point. However, I'd like to make the empirical
>>     corrections that Knigge (2011)
>>     <https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Farxiv.org%2Fabs%2F1102.2440&data=04%7C01%7Cdmtownsley%40ua.edu%7Cfc9d22bee14a46caf83008d8e53ec996%7C2a00728ef0d040b4a4e8ce433f3fbca7%7C0%7C0%7C637511405593986592%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=vGB2QuU0XTh9CJmxuGV5%2BwJ9TBhWsoGJ%2F7Dz4N63368%3D&reserved=0> make
>>     in their simulations, which includes modifying the radius of the
>>     M dwarf donor star by about 10% to account for both the generally
>>     underestimated radii of low mass main sequence stars, and the
>>     non-spherical geometry of the Roche Lobes
>>
>>     From what I understand, the radius of a star in mesa is an
>>     observable quantity, calculated from other variables, rather than
>>     something that can be modified directly by the user, but I'm sure
>>     there will be some way to force the code to do this. I suspect
>>     that the non-spherical Roche geometry is already accounted for in
>>     the mass transfer code, but the underestimation of the main
>>     sequence radii can vary between 1-5%.
>>
>>     Unfortunately, I'm not familiar enough with the codebase to do
>>     this while being sure I'm not breaking anything important, what
>>     might the best approach be here?
>>
>>     Thanks for your time,
>>     James Wild.
>>
>>     _______________________________________________
>>     mesa-users at lists.mesastar.org  <mailto:mesa-users at lists.mesastar.org>
>>     https://lists.mesastar.org/mailman/listinfo/mesa-users  <https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flists.mesastar.org%2Fmailman%2Flistinfo%2Fmesa-users&data=04%7C01%7Cdmtownsley%40ua.edu%7Cfc9d22bee14a46caf83008d8e53ec996%7C2a00728ef0d040b4a4e8ce433f3fbca7%7C0%7C0%7C637511405593996583%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=eZcQDpcWlL5U2noJjF%2FZsSpgUxDzujDggnZpsz9RSnE%3D&reserved=0>
>>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.mesastar.org/pipermail/mesa-users/attachments/20210315/357474ee/attachment.htm>