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In previous questions, I asked about how a K2V star could develop an [O VII] forbidden line and how those x-rays would create "pseudo-aurorae". Now, I'd like to know if my star could realistically only have one forbidden line, or if it has to have multiple (like if it as an O ᴠɪɪ line, it is very likely or has to have X forbidden line).

Some Real Life Stars with Forbidden Lines

ζ Puppis is a well studied O4Ief star with the O ᴠɪɪ forbidden line and I found this paper that says the star has the lines S xv, Si xɪɪɪ, Mg xɪ, Ne ɪx, and O ᴠɪɪ.

Another star, β Crucis, this time with the spectral type B0.5 III, also has the O ᴠɪɪ line. Using this other paper, I found that the star has many forbidden lines including O ᴠɪɪ, Ne ɪx, Si xɪɪɪ, Mg xɪ, N ᴠɪɪ, and Fe xᴠɪɪ.

I see that both stars share (besides the O ᴠɪɪ line) the Ne ɪx, Si xɪɪɪ, and Mg xɪ lines.

My Hypothetical Star

My star is a K type star, and is much cooler than these extremely hot stars and produces these lines in trace amounts using MCWS (magnetically confined wind shocks). However, my star sometimes (mostly in very high periods of stellar activity) produces these lines in significant quantities.

Here is my star's coronal composition (where most of this stuff takes place):

  • Hydrogen ($H$) - 72.3%
  • Helium ($He$) - 27.4%
  • Oxygen (including the [O VII] line; $O$ and $O^{6+}$) - 0.3%
  • Carbon ($C$) - 0.05%
  • Nitrogen ($N$) - 0.02%
  • Other Elements - 0.02% (mostly neon, magnesium, silicon, and sulfur, and there are almost no metals higher than iron/nickel present in this star)

My question is when my star is excited and produces x-rays, what other lines would most likely be produced (if any) and if I have to or should (because it is very likely to form if I have an O ᴠɪɪ line) include it.

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    $\begingroup$ This is one of those rare questions that would be better asked at Astronomy but would be rejected simply because you're dealing with a hypothetical star. We have some really good astronomers here, so I'm hoping for an answer, but if you don't get one, consider rewriting the question to remove the hypothetical star's details and simply ask, "is it possible for a star to have one, or must it be many?" $\endgroup$
    – JBH
    Oct 3, 2023 at 0:59
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    $\begingroup$ @JBH Yeah, I kind of thought about posting it on Astronomy, and if I don't get an answer, I'll do what you say. $\endgroup$
    – Neil Iyer
    Oct 3, 2023 at 1:11
  • $\begingroup$ In fact, not enough data. O-A class stars should indeed share lines if they have O-VII line in their spectrum, because they are big and hot enough to fuse carbon, making magnesium, silicium and ultimately iron being fused thus being able to get ionized to high levels. Your star is K2V, with "V" part being engineered, so whatever oxygen it has is not its own production (too cold for C-N-O cycle IIRC), but rather the contents of whatever nebula it met on its travel across the galaxy it's in, thus it's pretty possible it just does not have Mg or Si to ionize and produce lines. $\endgroup$
    – Vesper
    Oct 4, 2023 at 8:16
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    $\begingroup$ @Vesper That is interesting! So you are saying that even though other stars have lines of ionized oxygen and ionized other elements, my star might only have ionized oxygen due to what nebula it went through. Or, it could also have magnesium, silicon, and neon, it just depends. $\endgroup$
    – Neil Iyer
    Oct 4, 2023 at 13:39
  • $\begingroup$ I've linked a Wiki article on energy levels for ionization in my answer to your question of pseudo-aurorae, check energies involved, at least anything that's below O-VII level is achievable in your star, provided there are elements in its photosphere to actually emit those lines. $\endgroup$
    – Vesper
    Oct 5, 2023 at 5:17

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Note: Firstly, I would like to thank the people over at astronomy for answering my question there. This is just a summary of the answer there, and also me adding some stuff about the hypothetical K star.

An Overview of Forbidden Lines

Forbidden lines occur when a quantum mechanical effect makes a transition from two energy states unlikely. Below a density threshold, you will see forbidden lines, though unlikely, but above, you will see lines produced from collisions.

These thresholds vary based on the line you are talking about, in this case, O ᴠɪɪ. So, there could be various groups of forbidden lines that occur below similar threshold densities.

I'm guessing that the Ne ɪx, Si xɪɪɪ, and Mg xɪ lines have similar threshold densities as the O ᴠɪɪ line, so that is why they occur together. I'm guessing because I can't find the actual threshold densities for these lines.

The Hypothetical Star

The corona of most stars is similar to their overall composition, so you will be getting lots of hydrogen and helium, with notable amounts of oxygen, carbon, nitrogen, silicon, magnesium, neon, and iron. That means that these elements are present to become ionized and emit forbidden lines.

So, to answer my own question, it depends, but for this star, it is pretty likely that it will have the neon, magnesium, and silicon lines. However, it is possible to get a star with a certain line that has a density threshold that is similar to no other forbidden line, making the star have only 1 forbidden line.

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