What Is The Expected Ionization Fraction Of Magnesium And Iron In The Upper Atmospheres Of A 2-3 Earth-mass Mini-Neptune Orbiting A K-type Main-sequence Star, Given A Stellar XUV Flux 10 Times That Of The Sun, And How Might This Impact The Atmospheric Escape Rates And Observed Transmission Spectra?

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The expected ionization fraction of magnesium and iron in the upper atmospheres of a 2-3 Earth-mass mini-Neptune orbiting a K-type star with a stellar XUV flux 10 times that of the Sun is significantly higher than in similar planets around Sun-like stars. This increased ionization is due to the higher energy flux, which ionizes these elements more efficiently. Magnesium and iron, with ionization potentials of approximately 7.6 eV and 7.87 eV respectively, are more susceptible to ionization under such conditions.

Impact on Atmospheric Escape:

  • The higher ionization fraction makes these metals more susceptible to being stripped away by the stellar wind, potentially leading to higher atmospheric escape rates. Ionized particles are more easily accelerated by stellar winds and magnetic fields, contributing to the loss of the planet's atmosphere over time.

Impact on Transmission Spectra:

  • In transmission spectra, the characteristic absorption lines of neutral magnesium and iron may weaken or disappear because ionized atoms (ions) have different spectral features. This could result in the atmosphere appearing less metallic-rich in observations. Additionally, the heating effect of the XUV flux may cause the atmosphere to expand, further affecting the spectral features.

Additional Considerations:

  • The planet's magnetic field strength could mitigate atmospheric loss by protecting against the stellar wind. Without a strong magnetic field, the atmosphere may be stripped more efficiently.
  • The exact ionization fraction depends on factors like temperature, density, and the detailed XUV spectrum, but the trend of increased ionization and its effects on escape and spectra is clear.

In conclusion, the mini-Neptune's atmosphere likely experiences significant ionization of magnesium and iron, enhancing atmospheric escape and altering transmission spectra by reducing metal absorption lines.