High resolution emission FT spectra of sodium in a microwave discharge: Intensity variation of the D1/D2 lines in exoplanetary atmospheres. (August 2021)

• Record Type: Journal Article
• Authors: Civiš, Svatopluk; Heays, Alan N.; Knížek, Antonín; Ferus, Martin
• Is Part Of: Journal of quantitative spectroscopy & radiative transfer. Volume 270(2021)
• Publication Date: 2021-08
• Subjects: FT Spectrosopy -- Sodium -- Exoplanet atmospheres -- Laboratory astrophysics; 95.75.Fg -- 82.80.Gk; Spectrum analysis -- Periodicals; Radiation -- Periodicals; Analyse spectrale -- Périodiques; Rayonnement -- Périodiques; Radiation; Spectrum analysis; Periodicals; 543.0858
• DOI: 10.1016/j.jqsrt.2021.107689
• Languages: English
• ISSNs: 0022-4073

Abstract

Highlights: Observed sodium D 1 and D 2 lines in exoplanetary atmospheres show irregular ratios. One of the outcomes of this work is the proposal that this effect may be caused by self-absorption (absorption combined with emission). In contemporary astronomical observation of exoplanetary atmospheres, this effect has not been resolved yet. To observe this effect, at least 0.01 c m − 1 - 0.05 c m − 1 resolution (resolving power 1 600 000) is required. We have measured the Na D 1 /D 2 ratio in laboratory conditions using the microwave discharge technique and observed an apparent change of the D 1 /D 2 ratio explained by self-absorption occurring under some laboratory conditions. Graphical abstract: Abstract: The intensity ratio of the D 1 and D 2 sodium-doublet emission lines varies both across spectra of astronomical sources and in laboratory plasmala. This article probes the behaviour of this ratio under controlled laboratory conditions. In order to study this variation, we have recorded microwave discharge spectra of sodium vapour in range of 15000 to 18000 cm − 1 . Emission bands of the molecular nitrogen buffer gas and kinetic broadening of nitrogen and sodium lines have been used in a characterisation of the discharge temperature. We show that the relative intensity of doublet lines is determined by self-absorption and depends on the buffer-gas pressure and discharge power. The ratio and line shapes can be explained by a two-temperature model of sodium emission and foreground absorption. Line ratios estimated for spectra with various strength of the self-absorption of electric discharges can be used for characterising their temperatures and densities. In contemporary astronomical observation of exoplanetary atmospheres, this effect has not been resolved yet. To observe this effect, at least 0.01 - 0.05 cm − 1 resolution (resolving power 1 600 000) is required. We make a connection to stellar Na emission and possible absorption processes for future observations of exoplanetary atmospheres.