Non-local thermal equilibrium spectra of atmospheric molecules for exoplanets

Abstract

<p>Here we present a study of non-local thermodynamic equilibrium (LTE) effects on the exoplanetary spectra of a collection of molecules that are key in the investigation of exoplanet atmospheres: water, methane, carbon monoxide, and titanium oxide. These molecules are chosen as examples of different spectral ranges (infrared and ultraviolet), molecular types (diatomics and polyatomics), and spectral types (electronic and rovibrational); the importance of different vibrational bands in forming distinct non-LTE spectral features is investigated. Most notably, such key spectral signatures for distinguishing between the LTE and non-LTE cases include: for CH<sub>4</sub><span class="Apple-converted-space"> </span>the 3.15 <span class="inline-formula no-formula-id"><span id="MathJax-Element-1-Frame" class="MathJax" tabindex="0" role="presentation" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML">μ"><span id="MathJax-Span-1" class="math"><span id="MathJax-Span-2" class="mrow"><span id="MathJax-Span-3" class="mi">μ</span></span></span></span>m band region; for H<sub>2</sub>O the 2.0 and 2.7 <span id="MathJax-Element-2-Frame" class="MathJax" tabindex="0" role="presentation" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML">μ"><span id="MathJax-Span-4" class="math"><span id="MathJax-Span-5" class="mrow"><span id="MathJax-Span-6" class="mi">μ</span></span></span></span>m band regions; for TiO, a strong variation in intensity in the bands between 0.5 and 0.75 <span id="MathJax-Element-3-Frame" class="MathJax" tabindex="0" role="presentation" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML">μ"><span id="MathJax-Span-7" class="math"><span id="MathJax-Span-8" class="mrow"><span id="MathJax-Span-9" class="mi">μ</span></span></span></span>m; and a sole CO signature between 5 and 6 <span id="MathJax-Element-4-Frame" class="MathJax" tabindex="0" role="presentation" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML">μ"><span id="MathJax-Span-10" class="math"><span id="MathJax-Span-11" class="mrow"><span id="MathJax-Span-12" class="mi">μ</span></span></span></span>m. The analysis is based on the ExoMol cross-sections and takes advantage of the extensive vibrational assignment of these molecular line lists in the ExoMol data base. We examine LTE and non-LTE cross-sections under conditions consistent with those on WASP-12b and WASP-76b using the empirically motivated bi-temperature Treanor model.</span></p> <p>An example variation between the LTE case and Non-LTE case with our approach can be seen here for an important Methane band shown at high resolution and low pressure:</p> <p><span class="inline-formula no-formula-id"><img src="" alt="" width="437" height="316" /></span></p> <p> </p> <p> </p>