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Spatially Resolved Stellar Disk Spectra at Hyper-high Resolution: Toward Earth-like Exoplanet Detection

Published onJun 01, 2020
Spatially Resolved Stellar Disk Spectra at Hyper-high Resolution: Toward Earth-like Exoplanet Detection

High-precision spectroscopy might find ‘truly’ Earth-like exoplanets. Instrumental precisions are close to being achieved but limitations arise in the complexities of spectral-line formation. Spectral lines become somewhat asymmetric by being formed in dynamic gas flows. Radial-velocity signatures differ between different types of lines, change between stars, vary across stellar disks, and are modulated by magnetic activity. Spectroscopy across spatially resolved stellar disks has become possible by using transiting exoplanets as occulting spatial probes, permitting to test center-to-limb atmospheric hydrodynamics in stars also other than the Sun. Additional suitable target stars will likely be found in exoplanet surveys, and simulated observations are in progress to identify strategies for their near-future observations. From a grid of 3-D hydrodynamic CO5BOLD model atmospheres for solar-type stars, synthetic spectra have been computed at hyper-high spectral resolution (R greater than 1 million), for several center-to-limb locations across stellar disks. (The term ‘hyper-high’ is used since ‘ultra-high’ is already taken for lower-resolution data.) Such resolutions are required to fully resolve intrinsic line asymmetries. To segregate those from such arising due to blends, and also to obtain absolute wavelength shifts irrespective of errors in laboratory wavelengths, 3-D spectra are matched against similar data from 1-D models. There, unblended lines appear symmetric at their laboratory wavelength positions, and differences to 3-D profiles isolate effects arising in the dynamic photospheres. Synthetic spectra are surveyed for unblended lines with different strengths, excitation potentials, and ionization levels, each of which contribute characteristic signatures of line asymmetries and apparent Doppler shifts. The hyper-high resolution data are degraded to common spectrometer values to appreciate what signatures may realistically be observed. An adequate understanding of both line formation and of spectrometer performance should enable to disentangle effects from variable stellar atmospheres from those induced by even small Earth-like exoplanets.

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