Abundance

Abundance#

sme.abund stores elemental abundances, while sme.monh stores the global metallicity offset. In PySME, these two are combined at synthesis time.

Core rule in PySME#

For elements heavier than He, the abundance used by SMElib is:

effective abundance = pattern abundance + sme.monh

Hydrogen and helium are not shifted by monh.

What this means when you modify abundances#

When you update one element in sme.abund, you are writing the pattern value (before the global metallicity shift).
So if you want a target final abundance A_target (for example in H=12), use:

pattern_value = A_target - sme.monh

then assign that pattern value.

Warning

Note that when printing out abund, monh is applied to the abundance values.

Practical examples#

from pysme.abund import Abund

sme.abund = Abund.solar()
sme.monh = -0.20

1. Set a target absolute abundance (H=12)#

If you want final A(Mg) = 7.40:

target = 7.40
sme.abund["Mg"] = target - sme.monh

Because PySME adds monh later, this produces the intended final Mg abundance.

2. Keep scaled-solar composition, only change metallicity#

sme.monh = -0.50

This shifts all metals (Z > 2) together by -0.50 dex, while keeping abundance ratios fixed.

3. Set an element enhancement at fixed metallicity#

If you want [Mg/Fe] = +0.30 at current monh, add +0.30 to Mg in the pattern relative to your base pattern.

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