image Additional science image Additional science image Additional science

Additional science

In addition to its main science goals (Sect. I.2), Arago will also be able to tackle additional science topics. In particular, Arago will provide useful information about atomic and molecular physics. For example, abundant elements (such as Al, P and S), Cu, Zn, and Ga [153, 129, 1] have only strong and easily detectable lines at solar abundances in the UV; elements with low abundances in stars such as the Rare Earths (lanthanides) from Cerium to Lutetium (Z=58 up to 71) and the very heavy elements from Osmium up to Lead (Z=76 to 82) also have their strongest transitions in the UV, mostly in the range 180-220 nm; most of the C, N, and O and the iron-peak elements have their strongest resonance lines in the UV, and will benefit from the combined UV+Visible wavelength range; many molecules are also available in Arago's spectral range, e.g., CO, H 2 , O 2 , O 3 , H 2 O. Therefore, obtaining Arago spectra will allow the investigation of many largely unstudied chemical elements. Another important reason to model UV lines is that, for most elements, these lines have much more accurate theoretical atomic data (energy levels, wavelengths, and oscillator strengths) than their Visible counterparts. These atomic data are critically evaluated in the NIST database with estimates of their errors ( Measurements of high resolution profiles of resonance lines of heavy and cosmically rare elements should allow us to confirm/rule out the presence of the predicted hyperfine transitions of their isotopes. This will be feasible in stars with low apparent rotational velocities, and will yield primary information of the relative abundances of various isotopes of rare elements in stars. Finally, high resolution spectra of WDs can be used to constrain the atomic fine structure constant a, a fundamental atomic constant that characterises the strength of the electromagnetic interaction. The high gravitational field of WDs causes detectable wavelength shifts, which are large for atoms with high Z, such as Fe and Ni. The measurement of these wavelength shifts will allow us to constrain/tighten the value of a below the current value of a few 10 -5 (relative uncertainty). A precise determination of a in the local universe will also be an important reference for other projects (e.g. ESPRESSO or E-ELT) that aim at constraining differences elsewhere.
In addition, Arago will be useful for the study of solar system objects, especially for the giant planets. The previous IUE and HST UV telescopes allowed important discoveries in this field, and Arago will be able to contribute as well. In particular, Arago will be able to study the solar light reflected onto the planets, which will allow us to study their middle atmospheres, and the fluorescence related to the upper atmosphere and the auroral processes. In this framework, UV and Visible spectroscopy represent a wide source of information on atmospheric and auroral processes, especially to constraint the energetic input in the upper atmosphere of planets. For example, the hydrogen Lya line profile is an important parameter to constrain the atmospheres. In the same way, H2 emission recently allowed a better constraint on Uranus' auroral energetic input. In addition, polarisation is often underused in solar system studies, and UV polarisation has never been explored in the solar system. Indeed, no UV instrument has measured the polarisation of solar scattered light or airglow/auroral emissions. However, auroral emissions can be polarised, as mentioned by [96] for the terrestrial red line and [8] for Jupiter. [10] simulated the polarisation of the scattered Lya line in the jovian magnetic field and showed a sensitivity of this polarisation to the magnetic field orientation and strength. Moreover, light diffusion by small particles is often polarised. Polarisation studies will provide inputs on dust and aerosols in the atmosphere of the solar system planets or in comet comas and tails. For example, the jovian polar haze, which can be seen using polarisation, is still poorly understood. Titan aerosols will also likely give specific polarisation signal. Surface light reflection is also polarised. For the faint atmospheres of icy moons, it will allow measurements of the surface state.