[Mesa-users] taking very small steps during accretion

Tue Aug 17 20:06:28 UTC 2021

Hi Harish (and Amber),

  1.  I don’t recall if cooling will continue, but it certainly doesn’t hurt to simply try it and see if the core temperature changes substantially over the relaxation process. Try it out and explore!

Unless the white dwarf is extremely hot, 100 years is an extremely short time, so it’s hopefully negligible if it does cool. I would expect the surface to heat up since compressional losses from the accretion will release more energy.
  2.  Since accretion impacts the thermal structure of the WD’s outer layers, yes, the physical values (broadly defined) must change. This is true regardless of whether or not you use a relaxation process.
  3.  Amber summed this up nicely. The “relaxation” phase is really an engineering, rather than physical process. Many physical process like nuclear burning and convection are shut off during a relaxation phase. The point is to get form one state to another with as little needing to change as possible, regardless of whether or not that process is really physical.

In general, try it out. What features do you think would make your model believable or not? If you can’t answer that for determining the effects of a relaxation process, you probably wouldn’t know if the model made sense if you just turned accretion on full blast.

As to Amber’s observation about maximum accretion rates: I’m not surprised. Depending on what material you are accreting, you may run into a thermonuclear shell flash. If the accretion rate is higher than the rate that the material can be processed by the burning, it will pile up and create an extended structure that typically causes numerical issues in the lower density regions. Since the maximum shell-burning luminosity increases with the mass of the core, I’m not surprised that the maximum accretion rate increases similarly. I very much doubt that has changed much, as extended WD envelopes in outburst are still a numerically challenging situation.

Regards,

Bill

From: Amber Lauer-Coles <amberlauer at gmail.com>
Date: Tuesday, August 17, 2021 at 12:20 PM
To: harish sorrout <harishsorrout at gmail.com>
Cc: Wolf, William M <WOLFWM at uwec.edu>, Mesa-users <mesa-users at lists.mesastar.org>
Subject: Re: [Mesa-users] taking very small steps during accretion
Hi Harish, "relax" is a namelist setting that refers to the act of changing the model by only adjusting one quantity while turning the rest of evolution off. So this would not strictly be a realistic astrophysical scenario, but more of a "stellar engineering" step. With the stellar modelling, there are often limitations between what we believe happens in real life vs what we can model, and we have to apply numerical fixes to get as close as we can. This is one such scenario.

Bill, when last I tried to do high accretion rates, I found that for WD masses, I couldn't get higher than 1d-4 and still have a stable model. I did find that the highest tolerable rate would increase with the starting model mass. Has this situation improved since then? Thanks.
============================================================
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Dr. Amber Lauer-Coles
Research Associate
Nuclear Science & Technology dept.
National Nuclear Data Center
Brookhaven National Laboratory
acoles at bnl.gov<mailto:acoles at bnl.gov>

On Tue, Aug 17, 2021 at 1:47 AM harish sorrout via Mesa-users <mesa-users at lists.mesastar.org<mailto:mesa-users at lists.mesastar.org>> wrote:
Thankyou sir,

I have some query regarding the relax mass_ change .

1. When I will do relax mass_change over 100 yrs. Does white dwarf cool during this years?
2. Does physical values of WD change or not?
3. What is the purpose of relax mass_change.

 I request you  Please clarify.

With regards

Harish Kumar

On Tue, 17 Aug, 2021, 07:56 Wolf, William M, <WOLFWM at uwec.edu<mailto:WOLFWM at uwec.edu>> wrote:
You can still do that. You just need to relax the value of mass_change over many time steps (which can be short) rather than turning it on instantaneously. Once the new value of mass_change is achieved, it can remain at the high value indefinitely. It’s the process of getting from one value of mass_change to another that requires the relaxation process.

For instance, you might relax the value of mass_change slowly over 100 years of evolution, and then leave it on at the new constant high rate for 100,000 more years of evolution.

Regards,

Bill Wolf
________________________________
From: harish sorrout <harishsorrout at gmail.com<mailto:harishsorrout at gmail.com>>
Sent: Monday, August 16, 2021 8:11:01 PM
To: Wolf, William M <WOLFWM at uwec.edu<mailto:WOLFWM at uwec.edu>>
Cc: Mesa-users <mesa-users at lists.mesastar.org<mailto:mesa-users at lists.mesastar.org>>
Subject: Re: [Mesa-users] taking very small steps during accretion

Thanks for the tip!
But sir I wanted to study long term evolution of steady buring of helium during accretion on to WD i.e accretion with fixed rate. Also some people have done this with such high accretion rates.

On Tue, 17 Aug, 2021, 02:37 Wolf, William M, <WOLFWM at uwec.edu<mailto:WOLFWM at uwec.edu>> wrote:

Hi Harish,

It looks like in your inlist, you have mass_change = 1.5d6. I advise that instead of just turning on the mass change full blast, you ramp it over time. In particular, I’d look at the relax_mass_change controls, which let you change the value of mass_change over time.

However, with such a high accretion rate, you might have to do this in steps, as this control increases the accretion rate linearly. That is, if you ramp things up from 0 to 1.5d-6 over 100 timesteps, the very first timestep will abruptly turn on accretion at 1.5d-8 Msun/yr, which may also be too fast. Perhaps first adjust it to 1d-8, save a model, and then adjust it again to your final accretion rate. If the model you are loading was already accreting matter, you may be able to be more aggressive and do this in one relaxation.

Good luck!

Regards,

Bill Wolf

From: Mesa-users <mesa-users-bounces at lists.mesastar.org<mailto:mesa-users-bounces at lists.mesastar.org>> on behalf of harish sorrout via Mesa-users <mesa-users at lists.mesastar.org<mailto:mesa-users at lists.mesastar.org>>
Date: Monday, August 16, 2021 at 6:09 AM
To: Mesa-users <mesa-users at lists.mesastar.org<mailto:mesa-users at lists.mesastar.org>>
Subject: [Mesa-users] taking very small steps during accretion

Respected sir,

I am performing a simulation of long term evolution of WD accreting matter with accretion rate 1d-6 on my laptop with 4 threads and 16 GB ram. It is taking very small time steps and to increase time steps I have done many things like change mesh_coeff, var_control but I have failed to increase the time steps and I have run it more than 3-4 days. I wanted to achieve the Chandrasekhar limit of WD but it looks very different with this time step . because it will take 20 to 25 days to achieve ch mass limit. Here I am sharing an inlist file and TRHO plot.

kindly help me where am i wrong. and how can i achieve the target.

Thanking you

with regards

Harish kumar

research scholar
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