Exoplanets & Exomoons

Accurate Modeling Of Lyman-alpha Profiles And Their Impact On Photolysis Of Terrestrial Planet Atmospheres

By Keith Cowing
June 12, 2022
Filed under
Accurate Modeling Of Lyman-alpha Profiles And Their Impact On Photolysis Of Terrestrial Planet Atmospheres
HST STIS spectra of Ross 825 (K3; left), Ross 1044 (M0; middle) and Kapteyn’s Star (M1; right) plotted as thick black lines reproduced from Schneider et al. (2019) and Youngblood et al. (2022). An altered version of the HST COS spectrum of Kapteyn’s Star from Guinan et al. (2016) is plotted as the grey curve in the third panel. This line profile was produced by mirroring the right-hand side of the stellar component of the Lyα observation. The large RVs of Ross 1044 (-169.6 km s−1 ), Ross 825 (-340.2 km s−1 ), and Kapteyn’s Star (+245.2 km s−1 ) allow their stellar Lyα emission lines to be well separated from the contaminating geocoronal airglow emission, with little Lyα flux scattered by the ISM. Model ISM transmittance curves are plotted as black dotted lines (Schneider et al. 2019; Youngblood et al. 2022). The blue profiles are the intrinsic PHOENIX model profiles, which when multiplied by the ISM transmittance curves yield the red profiles. These red profiles should reproduce the solid black curves, but severely underestimate the core flux.

Accurately measuring and modeling the Lyman-α (Lyα; λ1215.67 Å) emission line from low mass stars is vital for our ability to build predictive high energy stellar spectra, yet interstellar medium (ISM) absorption of this line typically prevents model-measurement comparisons.

Lyα also controls the photodissociation of important molecules, like water and methane, in exoplanet atmospheres such that any photochemical models assessing potential biosignatures or atmospheric abundances require accurate Lyα host star flux estimates. Recent observations of three early M and K stars (K3, M0, M1) with exceptionally high radial velocities (>100 km s−1) reveal the intrinsic profiles of these types of stars as most of their Lyα flux is shifted away from the geocoronal line core and contamination from the ISM.

These observations indicate that previous stellar spectra computed with the PHOENIX atmosphere code have underpredicted the core of Lyα in these types of stars. With these observations, we have been able to better understand the microphysics in the upper atmosphere and improve the predictive capabilities of the PHOENIX atmosphere code. Since these wavelengths drive the photolysis of key molecular species, we also present results analyzing the impact of the resulting changes to the synthetic stellar spectra on observable chemistry in terrestrial planet atmospheres.

Sarah Peacock, Travis S. Barman, Adam C. Schneider, Michaela Leung, Edward W. Schwieterman, Evgenya L. Shkolnik, R. O. Parke Loyd

Comments: 17 pages, 12 figures
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2206.05147 [astro-ph.SR] (or arXiv:2206.05147v1 [astro-ph.SR] for this version)
Submission history
From: Sarah Peacock
[v1] Fri, 10 Jun 2022 14:43:02 UTC (1,752 KB)

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