The Goal

I have been designing a planet for quite a while now, and I would like the star to have an [O VII] forbidden line in its atmosphere. There doesn't need to be too much, but I need enough for the star to actively release some soft x-rays, which would excite the neon (15.5% concentration) in my atmosphere and create these "psuedo-aurorae".

The Star

The star is a K2V spectral class and has no another companions (it is a solo star). It is roughly 0.82 Solar Masses, 0.853 Solar Radii, 0.452 Solar Luminosities, a metallicity of 0.2, and has a temperature of about 5127ºK.

The Problem

However, from what I understand, forbidden lines usually only form in hot B and O type stars (and maybe a few A type) and in the SIMBAD database, I believe that I have found only one K star like that (HD 290380). I used the sptypes=k[e] in the criteria query selection.

So is it even possible for a high energy forbidden line to form? Thank you for reading, and please try your best to not handwave.

  • 2
    $\begingroup$ Would you care to provide more details on what an [O VII] forbidden line is? maybe a link? $\endgroup$
    – Monty Wild
    Sep 21 at 14:12
  • 1
    $\begingroup$ @MontyWild This isn't explaining what they are but it talks about them: link. $\endgroup$
    – Neil Iyer
    Sep 21 at 14:40

1 Answer 1


The O VII complex you see in x-ray spectra around 22 Å arises from hot gas, like you said. The emissivity of the line peaks at around $2\times10^6$ K (see the AtomDB database, Foster et al. 2012), dropping off by 1-2 orders of magnitude when you go below $\sim10^6$ K, with a shallower decline moving to higher temperatures. Therefore, our star needs to have a lot of hot gas somewhere.

I used to do x-ray spectroscopy on isolated O and B stars, like Zeta Puppis, some of which show the O VII and O VIII oxygen lines. Even these mid- to early- O stars only have surface of temperatures of 30,000-40,000 K, which much cooler than $2\times10^6$ K! However, the x-ray emission comes not from their surfaces but from embedded wind shocks (EWS) within their powerful, radiation-driven winds. These shocks arise from something called the line deshadowing instability (Owocki et al. 1988). You're not going to see similarly dramatic effects from stars with weak winds, like most K-type stars.

I'm not that familiar with HD 290380, the star you mention, but I notice that it's a T Tauri star. These are young stars that haven't yet reached the main sequence. Interestingly, T Tauri stars often have powerful winds and strong gaseous outflows (arising from different processes than the winds of stars like Zeta Pup). You don't see these winds in main sequence stars of the same spectral type. This gives us a little loophole.

I'm not sure whether T Tauri winds could lead to O VII emission via the line deshadowing instability and don't want to speculate, but there are alternative means of forming wind shocks and x-rays. One option would be colliding wind shocks (CWS; see e.g. Prilutskii & Usov 1976), but you've stated that your star has no companion. Your other alternative would be magnetically channeled wind shocks, sometimes called magnetically confined wind shocks (MCWS, see e.g. ud-Doula & Owocki 2022), which can occur in certain stars cooler than types O or B. The star's magnetic field channels some of the wind from the magnetic poles to the magnetic equator; the collision of the south- and north- moving flows creates shocks and, subsequently, x-ray emission.

So if you have a young, magnetic T Tauri star with a reasonably power, fast-moving wind . . . it's possible that a K-type star could create O VII lines through MCWS. It would certainly pass the sniff test for plausibility. The downside is that once the star moves to the main sequence, the T Tauri wind will die down and you're not likely to see strong x-ray emission. To some extent, things depend on whether you're willing to have the pseudo-aurorae occur just during the early phase of the star's life.

  • 1
    $\begingroup$ Thank you for the very detailed answer and the T-Tauri star phase seems interesting and could work. So, is it even possible to have a main-sequence K-star have it, or is it not plausible? (I wanted to use these pseudo aurorae on my habitable planet that orbits this star, but it looks like that won't happen) $\endgroup$
    – Neil Iyer
    Sep 21 at 18:05
  • $\begingroup$ @NeilIyer Since the T Tauri wind disappears before the star reaches the main sequence, I'd bet against it, yeah. $\endgroup$
    – HDE 226868
    Sep 22 at 3:36
  • $\begingroup$ It seems like it is possible, but very unlikely, but could I create these rare times in very high stellar activity that the star shows some of these forbidden lines? $\endgroup$
    – Neil Iyer
    Sep 22 at 3:42
  • $\begingroup$ @NeilIyer If you're willing to handwave a bit, then maybe? $\endgroup$
    – HDE 226868
    Sep 22 at 13:21
  • 1
    $\begingroup$ What I have done is make the line very weak in elevated activity (at the height of the 18 year stellar cycle) and moderate in extraordinary scenarios (maybe every ten thousand years or so). $\endgroup$
    – Neil Iyer
    Sep 22 at 15:03

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