In the first version of my answer to Are sunspots vertically displaced from the surrounding photospheric plasma?, I naively used Kramer's law to attempt to show an inverse relationship between opacity $\kappa$ and temperature $T$ due to bound-free or free-free absorption in a region with density $\rho$: $$\kappa\propto\rho T^{-7/2}$$ Rob Jeffries pointed out that this might not be accurate in the photosphere, even though it works well deeper inside most stars (though for high-mass stars, Thomson scattering dominates and $\kappa$ is approximately constant). After some more reading, I concluded that this was right. At lower temperatures, interactions with ionized hydrogen mean that $H^-$ is the major contributor to opacity, and it follows a law $$\kappa\propto\rho^{1/2}T^9$$ I believe the transition of dominance happens around $\sim10^4\text{ K}$, meaning that $H^-$ opacity should be the major contribution in the stellar photosphere.

At even lower temperatures, however ($\sim4000\text{ K}$), molecular scattering can be important (see Tennyson et al. (2007)). While this is not necessarily important at all points in the stellar photosphere, I think that it should be important in sunspots, which can have temperatures in this range.

What is/are the major source(s) of opacity in sunspots? Does molecular scattering dominate, or is $H^-$ still important? If both are important, to varying degrees, how can the mean opacity be calculated?

My knowledge of stellar atmospheres and atmospheric models is limited, so I apologize if this comes down to being a basic question.

  • $\begingroup$ did you see the research papers about different wavelength opacities of sunspots? $\endgroup$ – aliential Nov 11 '16 at 22:23
  • $\begingroup$ @comprehensible I don't know what you're referring to. $\endgroup$ – HDE 226868 Nov 11 '16 at 22:24
  • $\begingroup$ some studies of IR opacity and highly confusing research papers: google.fr/… $\endgroup$ – aliential Nov 11 '16 at 22:34
  • 1
    $\begingroup$ @comprehensible Indeed, I searched Google and read some of those very papers. $\endgroup$ – HDE 226868 Nov 12 '16 at 0:33

In Ignjatovi´c et al. (2014), ion-atom absorption processes are discussed. They suggest that these processes are of equal, if not greater, influence on the observed sunspot opacity as absorption processes in $\mathrm{H}(1s)+\mathrm{H}^+$ and $\mathrm{H}(1s)+\mathrm{X}^+$ collisions (where $\mathrm{X}$ is one of the metal atoms $\mathrm{X} = \mathrm{Na, Ca, Mg, Si, Al}$). The main scattering processes discussed within the paper follow the photo-dissociation of $\mathrm{H}_2^+$ and $\mathrm{HX}^+$ by UV photons in the spectral region ($110$ $\mathrm{nm}$ $\lesssim \lambda \lesssim 230$ $\mathrm{nm}$).

I highly recommend reading through the paper—it is very interesting.

| improve this answer | |
  • $\begingroup$ Awesome, thanks. I'll read through that when I have a solid chunk of free time. $\endgroup$ – HDE 226868 Nov 12 '16 at 0:33

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.