Friday 10 August 2012, by cneiner
From an astrophysical point of view, the ultraviolet (UV) region (90-320 nm) is particularly rich in atomic and molecular transitions, and covers the region in which the intrinsic spectral distribution of hot stars, starburst galaxies and active galactic nuclei peak. In particular, electronic transitions from ground and low-lying metastable levels of astrophysically abundant elements and ions, and the important molecular species of CO and H2, appear, with only very few exceptions, in the UV spectrum. These are lines which are least influenced by non-LTE effects in stellar photospheres and are thus most useful for quantitative abundance determinations. The lower levels of these lines are least likely to depopulate in low density environments such as chromospheres, circumstellar shells, stellar winds, nebulae and the interstellar medium, and so remain the only useful diagnostics in most of these environments. Another advantage of observing in the UV is the extreme sensitivity of the Planck function to the presence of small amounts of hot gas in dominantly cool environments. This allows the detection and monitoring of various phenomena that would otherwise be difficult to observe: accretion continua in young (T Tauri) stars, magnetic activity, chromospheric heating, and starspots on cool stars, and intrinsically faint, but hot, companions of cool stars.
The Earth’s atmosphere is opaque to most of the electromagnetic spectrum, with the exception of narrow windows in the visible (330 to 950 nm), the near infrared (950 to 5 micrometers) and radio wavebands. Gamma rays, X rays and the UV regime, emitted by the hottest and most energetic objects in the Universe, are therefore only accessible from space. Since the late 1960s, astronomers all over the world have had access to ultraviolet telescopes: Copernicus (1972—1981), the International Ultraviolet Explorer (IUE; 1978—1996), the Hubble Space Telescope (1990—2013?) or the Far Ultraviolet Spectroscopic Explorer (FUSE;1999—2007) to name just a few. In the coming years a few operational space mission with UV spectroscopic capability will become available, especially WSO, as the importance of UV astronomy has been recognized by many astronomical national institutions. However nothing is planned after 2023.
The success of the IUE in its later years showed that a need exists for time series high resolution UV spectroscopy, both for bright and faint stars. Such time series document phenomena on stars that can be impulsive (flares, infall), periodic (pulsations, rotational migration of spots, corotating clouds), quasi-periodic (evolution of blobs from hot winds), and gradual (evolution of spots). The study in the UV is particularly important to record responses of lines formed in various ions (e.g., HeII vs. HeI, CIV vs. CIII) and over a range of excitations, as well as the of "Fe curtain" features that respond to changes in local irradiation. In such cases conditions on the surface can result in different responses with stellar latitude, toward the disk limbs, or as a result of nearby high energy activity.
In the concept described here, two spectral resolution modes are proposed to meet the scientific objectives of the mission: a high resolution setting (R=100000) aimed at the brightest, isolated stars and a low resolution setting (R=1000-5000) suitable for the observation of faintest targets.