[mesa-users] modus videndi with very thin shell burning

[Theodore A. S. <ta...@MI...>]

dear mesa-users:

The entrainment of the henyey algorithm to progressively smaller time steps
is a general consequence of the accentuation of shell thinness in low mass stars
ascending the RGB, worsening inversely with mass and metallicity. Sub-stellar
mass Pop. III stars thus exhibit the worst manifestation of this problem.

However, in the star's quiescent climb up the RGB, you should graphically
observe that, in the expanding shell front's frame of reference, the shell's
profile appears almost structurally static. One physically sound resolution is
to artificially propagate, or shift, the composition profile's outward using a
larger time-step, interspersing this with smaller henyey chosen time steps.

Harm and Scwarzschild adumbrated this "shell shifting" method during the late
dark age of stellar modeling ( see sec. (d) the hydrogen burning shell,
attached .pdf for RED GIANTS OF POPULATION II. IV )

Perhaps a more sophisticated version with an attendant "error" control could be
incorporated into MESA. I would be interested to assist on this, especially on
ascertaining the divergence between the brute force evolution and that
incorporating a subset of shell shifting steps expediting the RGB ascent. My
intuition is that, with the exception of detailed calculations of precise core
mass and luminosity at the RGB tip, the error introduced via a judicious
admixture of shifting steps could be negligible. I could be wrong, but, if not,
the computational burden could be substantially alleviated, esp. if a large
array of runs are required.

Theodore Arthur Sande

MIT Department of Physics
ta...@mi...