[Mesa-users] ZAMS models: a bug or feature?

[Natasha Ivanova]

Hello all,

By chance, I have found that the use of pre-calculated models
(create_pre_main_sequence_model = .false.) in MESA version 23.05.1 leads to
a huge discrepancy with the use of the models that are obtained as pre-MS
(create_pre_main_sequence_model = .true.).

A simple inlist to reproduce is attached.

Specifically, the problem seems to be in that the stored low mass ZAMS
model(s) have a significantly depleted central hydrogen -- see the table
below, where I provided ages at the end of the MS, obtained with and
without pre-MS calculations (and so MS starts with X=0.7), and also the H
content in the center with which the calculations start in the stored zams
model (ZAMS X_0).

The minimum central H content seems to be for 1.25 Msun - it is as small at
the zero age as 0.578248. This also leads to a more massive star having a
longer MS lifetime.

For me, it seems that the trend in deleted central H concentration is due
to only one stellar model with a very low central H, and the rest of the
models are interpolated; I also believe that that specific stored model is
wrong. So my question is, is that a bug (an erroneous stored model) or a
feature, and if that is the physical feature, then why?

My question is motivated by the fact that I used to use the ZAMS models
when teaching my stellar astro class, in the first introductional
computational lab (and then later on). But if one can no longer believe in
pre-calculated ZAMS models, I need to update my lab(s) - unless of course I
am missing something and that is, for some reason, how it should be.


M/Msun Age with preMS Age from "ZAMS"      ZAMS X_0
1.0 8.7599E+09     8.7154E+09          0.699498
1.1 5.6064E+09     5.0309E+09      0.666448
1.2   4.2840E+09     3.4859E+09        0.614212
1.23 3.9819E+09 3.1354E+09      0.587010
1.25 3.7561E+09 2.9156E+09      0.578248
1.27 3.6174E+09 2.7330E+09      0.583083
1.28 3.4796E+09 2.6863E+09      0.596698
1.29 3.4140E+09 2.6717E+09      0.605975
1.3 3.3239E+09 2.6728E+09      0.613373
1.4 2.6296E+09 2.2952E+09      0.650336
1.5 2.1518E+09 1.9827E+09      0.672308

Best,
Natasha
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