Degradation interpretation of O3 spectral features in exoplanet atmosphere observations due to interstellar UV uncertainties: a 3D case study with TRAPPIST-1e

Transmission spectrum atmospheric effective height plotted against wavelength between 0.2 µm and 11 µm for P19 PI (orange), P19 10% PAL (brown), P19 1% PAL (yellow) and P19 0.1% PAL (red) simulations. and W21 PI (light blue), W21 10% PAL (blue), W21 1% PAL (pink), and W21 0.1% PAL (gray) simulations. The non-tidally locked cases are excluded for clarity, but show small differences compared to the equivalent tidally locked case. Transit depth, relative to the star, is indicated in parts per million (ppm) on the right vertical axis. The spectrum is coupled to a spectral resolving power of R = 250. Spectral features are represented in gray. The wavelength range of the proposed Habitable World Observatory (HWO; blue shaded area) and the JWST NIRSpec instrument (yellow shaded area) are shown. They overlap in the green shaded area. The wavelength range of the JWST MIRI instrument is indicated in the magenta shaded area. The wavelength interval between 0.2 – 1 µm is shifted between 1 – 11 µm to clearly show the UV and visible regions. The black bar indicates the uncertainty in the measurement that reaches the noise floor of the instrument, where the noise floor is specified as 5 ppm. Note that this error bar is an estimate of telescope performance and does not represent a measurement. – astro-ph.EP

TRAPPIST-1e is a potential terrestrial exoplanet orbiting an ultra-cool M Dwarf star and is a prime target for observation by the James Webb Space Telescope (JWST). A one-dimensional photochemical model of terrestrial planetary atmospheres shows the importance of the incoming stellar UV flux in modulating the concentration of chemical species such as O3 and H2O.

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Additionally, three-dimensional (3D) modeling has demonstrated anisotropy in chemical abundances due to transport in tidally locked exoplanet simulations. TRAPPIST-1e (We use the Whole Atmosphere Community Climate Model version 6 (WACCM6), a 3D Earth system model, to examine how uncertainty in the UV flux phenomenon is affected by assuming an early Earth-like atmospheric composition, coupled with transport. )

We use two semi-empirical stellar spectra for TRAPPIST-1 from the literature. The UV flux ratio between them is a factor of 5000 in some wavelength bins. As a result, photochemically prepared bulk O3 columns differed by a factor of 26. The spectral features of O3 in both transmission and emission spectra differ between these simulations (e.g. 19 km differences in transmission spectral efficiency for O3 at 0.6 μm).

This leads to potential ambiguities when interpreting observations, including overlap with scenarios considering alternative O2 concentrations. Therefore, the characterization of the UV spectra of their host stars is crucial to achieve robust interpretations of terrestrial exoplanetary spectra. In the absence of such stellar measurements, the atmospheric environment can be derived from other spectral features (e.g. H2O) or compared by combining direct imaging and transmission spectra.

Gregory Cook (1, 2), Dan Marsh (1), Catherine Walsh (1), Alison Youngblood (3), ((1) School of Physics and Astronomy, University of Leeds, UK, (2) Institute of Astronomy, University of Cambridge, UK , (3) NASA Goddard Space Flight Center, Solar System Exploration Division, USA).

Comments: 20 pages, 11 figures, accepted in ApJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Citation: arXiv:2309.15239 [astro-ph.EP] (or arXiv:2309.15239v1 [astro-ph.EP] for this version)
Submission history
Posted by: Gregory Cook
[v1] Tue, 26 Sep 2023 20:05:40 UTC (7,260 KB)

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