Menu
▾
▴
Re: [mesa-users] main sequence problem
|
From: David A. <wda...@gm...> - 2013-02-25 21:49:10
|
Hi Arlette, I am trying to make sense of the simulations and various algorithms used in stellar evolution, with the goal of having the most plausible boundary conditions in MESA. Do you have the reference for the treatment of Henyey that you mention? Dave On Fri, Feb 15, 2013 at 2:50 AM, Arlette Grotsch <Arl...@ul...> wrote: > Hi David, > > Thanks for your interesting comment. I agree with you that historically Schwarzschild was used in order to increase the mass of the convective core with no other physical reason behind. However the fact that Ledoux supposedly leads to a smaller convective core is highly questionable. In the earlier versions of stellar evolution codes, Henyey for instance, the implementation of the Ledoux criterion was correctly made and convective cores were about the same whatever criterion was used. By that I mean that the limit was diagnosed from the "inside" of the convective region. If the layer just above the convective core is to be engulfed in the convective core, its chemical composition becomes the same as that of the convective core itself. On the contrary if the layer adjacent to the convective core is treated as independent of a possible extension of the core, Ledoux will prevent it from becoming convective and convective cores are smaller. The easiest and more coherent way to define the limit is to extrapolate the grad-rad from the "inside" of the core and then to add a mesh point (double preferentially if one wants to keep trace of the discontinuity in chemical composition and density). This is true for H burning and for He burning. Some He burning models have a discontinuity in grad-rad at the convective boundary with a grad-rag higher than grad-ad. This is of course not correct since this situation is unstable. Now for a layer which is not adjacent to a convective region, it can only become convective if Ledoux criterion is violated since as you point out, Schwarzschild criterion has no chemical composition dependence. > > The simulations are a great challenge and we are all looking forward to seeing the progress made in this field! > > Cheers, > Arlette > >> I wish to add a comment to the point made by Arlette. >> >> Actually, the convection region is not static, but fluctuates >> vigorously (high Reynolds number). Thus the boundary is determined by >> the accumulated effect of these fluctuations (in velocity, pressure, >> etc.). I think that, in addition to the suggestion of Arlette, there >> will probably be important modifications due to this. We certainly see >> such behavior in simulations. >> >> Note that many astro simulations are single fluid, so that the effect >> of abundance is ignored (eg., the Stagger code and ASH, to name two >> illustrious examples). Properly doing multi-fluid flow, to describe >> composition differences, is still a challenge, but we seem to be >> making progress. >> >> I think Schwarzschild was used historically because it gave more >> mixing than Ledoux, and agreed with the larger cores needed for >> fitting observational data on massive stars. Ledoux with entrainment >> would also give larger cores than without entrainment. The >> observations do not yet tell us which (if either) is what nature does. >> >> Experiment suggests Ledoux plus entrainment (basically a Richardson >> criterion). Because Schwarzschild has no composition dependence, it >> cannot be correct in general. Composition can affect stability, as >> Arlette emphasizes. >> >> ------------------------------------------------------------------------------ >> Free Next-Gen Firewall Hardware Offer >> Buy your Sophos next-gen firewall before the end March 2013 >> and get the hardware for free! Learn more. >> http://p.sf.net/sfu/sophos-d2d-feb >> _______________________________________________ >> mesa-users mailing list >> mes...@li... >> https://lists.sourceforge.net/lists/listinfo/mesa-users > -- David Arnett Regents Professor Steward Observatory University of Arizona Facts are stubborn, but statistics are more pliable. Mark Twain Facts do not cease to exist because they are ignored. Aldous Huxley |
