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image Mission concept image Mission concept

Mission concept
To reach our science goals, it is necessary to observe stellar spectral lines and their polarisation in the Visible and UV wavelength domains simultaneously and with a high-cadence continuous monitoring. The Visible domain allows Arago users to characterise the surface of the star: its properties (e.g., temperature, gravity, rotation, magnetic field) and surface features (e.g., spots, chemical enhancements). The UV domain allows the characterisation of the environment of the star: its wind, magnetosphere, chromosphere, etc. Observing both domains simultaneously is the only way to obtain a complete 3D view of the star and its surroundings, and directly link surface features to circumstellar structures, e.g. surface spots to CMEs, or magnetic footpoints to discs.
In addition to the measurement of the stellar spectra (Stokes I), Arago's science goals require the measurement of polarisation (Stokes VQU) in the spectra to detect and quantify the magnetic field and scattering environment of stars. While the intensity (Stokes I) and circular polarisation (Stokes V) are enough to quantify the magnetic field, simultaneously measuring the linear polarisation (Stokes QU) in the spectra allows full characterisation of the 3D configuration of the magnetic field. Moreover, linear polarisation is a diagnostic for scattered light, e.g., from accretion and decretion discs, circumstellar wind structures, and exoplanets. Through their polarisation, scattered photons remember the geometry of the scattering so that the net polarisation from a source encapsulates information about its symmetry properties. Unlike interferometry, the extractability of this information does not depend on Doppler distance (angular size) but on flux only. Linear polarisation is also very important for the Hanle (depolarisation) effect, and to probe photospheric asymmetries. Therefore, Arago should be able to measure all Stokes (IQUV) parameters over most of its wavelength domain.
These requirements determine the design of the Arago mission: a single telescope equiped with a single polarimeter feeding two spectrographs, one working in the UV domain, the other in the Visible domain.