Hydrogen- and helium-like atoms only for Z > 6 and 10?

[jwreep]

The definition of hydrogen-like requires Z > 6:
return (self.atomic_number - self.charge_state == 1) and (self.atomic_number >= 6)
and helium-like Z > 10:
return (self.atomic_number - self.charge_state == 2) and (self.atomic_number >= 10)

Why is this? Is He II not hydrogen-like? Or is there some logic or functionality that only applies to elements with higher atomic numbers?

[wtbarnes]

That is a good question...I think I created these properties in order to use them in the ionization cross-section calculation here:

fiasco/fiasco/ions.py

Line 943 in 282b819

if self.hydrogenic or self.helium_like:

and here:

fiasco/fiasco/ions.py

Line 975 in 282b819

B = 1 if self.hydrogenic else 2

. I was following the logic both from the Fontes cross-section paper as well as what is in the ioniz_cross.pro routine: https://sohoftp.nascom.nasa.gov/solarsoft/packages/chianti/idl/ionrec/ioniz_cross.pro

The conditions on $Z$ are likely just specific to that calculation so that condition should be dropped from these properties. I'm actually still a bit confused by the $Z\ge6$ condition. In the Fontes cross-section paper, it explicitly states that these cross-section calculations are for $10\le Z\le 92$ so the $Z\ge10$ condition makes sense, but I don't understand the $Z\ge6$ condition.

[jwreep]

Perhaps it would be worth dropping this in the definitions, and putting the requirements on atomic number into the cross-section calculation directly? I could handle that, if so.

[wtbarnes]

Yes, that's definitely the right move. Thanks for spotting this.

As a side note, I spoke with Peter Young about this issue of the range of Z for the cross-section calculation. He pointed out that in the Dere et al. 2007 (http://adsabs.harvard.edu/abs/2007A%26A...466..771D) paper, there's a note about the Fontes cross-sections being used for H-like ions of Carbon and above and using the FAC or BT scaling calculations for others. We should probably add a comment somewhere in that function that summarizes the justification in that paper.