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Arxiv: JWST Resolves CIA Features in White Dwarfs Published: 9/24/2024 7:38:07 PM Updated: 10/11/2024 11:13:54 PM

Paper abstract: Infrared-faint white dwarfs are cool white dwarfs exhibiting significantinfrared flux deficits, most often attributed to collision-induced absorption(CIA) from H_2-He in mixed hydrogen-helium atmospheres. We present James WebbSpace Telescope (JWST) near- and mid-infrared spectra of three such objectsusing NIRSpec (0.6-5.3 \mum) and MIRI (5-14 \mum): LHS 3250, WD J1922+0233,and LHS 1126. Surprisingly, for LHS 3250, we detect no H_2-He CIA absorptionat 2.4 \mum, instead observing an unexpected small flux bump at thiswavelength. WD J1922+0233 exhibits the anticipated strong absorption featurecentered at 2.4 \mum, but with an unexpected narrow emission-like featureinside this absorption band. LHS 1126 shows no CIA features and follows a\lambda^{-2} power law in the mid-infrared. LHS 1126's lack of CIA featuressuggests a very low hydrogen abundance, with its infrared flux depletion likelycaused by He-He-He CIA. For LHS 3250 and WD J1922+0233, the absence of a 1.2\mum CIA feature in both stars argues against ultracool temperatures,supporting recent suggestions that infrared-faint white dwarfs are warmer andmore massive than previously thought. This conclusion is further solidified byKeck near-infrared spectroscopy of seven additional objects. We explorepossible explanations for the unexpected emission-like features in both stars,and temperature inversions above the photosphere emerge as a promisinghypothesis. Such inversions may be common among the infrared-faint population,and since they significantly affect the infrared spectral energy distribution,this would impact their photometric fits. Further JWST observations are neededto confirm the prevalence of this phenomenon and guide the development ofimproved atmospheric models.