Atmospheric Dynamics and the Variable Transit of KELT-9 b

P. Wilson Cauley, Evgenya L. Shkolnik, Ilya Ilyin, Klaus G. Strassmeier, Seth Redfield, Adam Jensen

Research output: Contribution to journalArticlepeer-review

71 Scopus citations


We present a spectrally and temporally resolved detection of the optical Mg i triplet at 7.8σ in the extended atmosphere of the ultra-hot Jupiter KELT-9 b, adding to the list of detected metal species in the hottest gas giant currently known. Constraints are placed on the density and radial extent of the excited hydrogen envelope using simultaneous observations of Hα and Hβ under the assumption of a spherically symmetric atmosphere. We find that planetary rotational broadening of km s -1 is necessary to reproduce the Balmer line transmission profile shapes, where the model including rotation is strongly preferred over the non-rotating model using a Bayesian information criterion comparison. The time series of both metal line and hydrogen absorption show remarkable structure, suggesting that the atmosphere observed during this transit is dynamic rather than static. We detect a relative emission feature near the end of the transit which exhibits a P-Cygni-like shape, evidence of material moving at ≈50-100 km s -1 away from the planet. We hypothesize that the in-transit variability and subsequent P-Cygni-like profiles are due to a flaring event that caused the atmosphere to expand, resulting in unbound material being accelerated to high speeds by stellar radiation pressure. Further spectroscopic transit observations will help establish the frequency of such events.

Original languageEnglish (US)
Article number69
JournalAstronomical Journal
Issue number2
StatePublished - Feb 2019


  • planets and satellites: atmospheres
  • planets and satellites: composition
  • stars: flare

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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