2 Diagnostic possibilities in the XUV

Until the launch of the recent XUV satellites, the spectral information available from stars was limited and only provided a few diagnostic possibilities. Some diagnostics of stellar coronae were possible with the Extreme Ultraviolet Explorer (EUVE) spectrometers (SW: 70-190 Å; MW: 140-380 Å; LW: 280-760 Å). EUVE spectra had moderate resolutions ( $\Delta\lambda \approx 0.5$, 1 and 2 Å) but could be used to get estimates of coronal densities from line ratios, and to constrain the DEM at temperatures above a million degrees. Only a few weak lines were available at lower temperatures and for element abundance analyses (cf. Drake et al. 1995). The XMM-Newton and Chandra gratings are now providing a wealth of high-resolution X-ray spectra in the 3-170 Å range. Direct estimates of temperatures, densities and element abundances are now possible using the X-ray spectra, although all the spectroscopic diagnostics are limited to the high-temperature coronal plasma. In fact, only coronal lines emitted at temperatures above a million degrees are observed, with the exception of a few weak transition region C V lines (formed at $\log\, T=5.5$) observed by one of the Chandra gratings.

Information on the TR can be obtained from observations in the FUV spectral region. The IUE mission observed in the FUV with two channels (SWP: 1175-2000 Å; LWR: 2000-3200 Å). Unfortunately, IUE spectra had severely limited diagnostic capabilities, since the available lines do not simultaneously satisfy conditions (a), (b), (d), (e). IUE only observed lines formed in the chromosphere and lower TR, many of which are emitted by neutrals or singly-ionised atoms and are not optically-thin. The exceptions are a few lines of Si IV, C IV, and N V which unfortunately all belong to the class of anomalous behaviour. Another limiting factor was the fact that most lines were heavily blended, since the majority of observations were done with the SWP channel at low resolution ($\simeq$6 Å).

The Goddard High Resolution Spectrograph (GHRS) and Space Telescope Imaging Spectrograph (STIS) instruments on board the NASA/ESA Hubble Space Telescope (HST) have produced very good spectroscopic data in the 117-1700 Å range, with much higher S/N and spectral resolution compared to IUE. This has allowed a large number of diagnostics (in particular density) to be applied to stellar observations. However, the GHRS and STIS instruments cover similar spectral ranges to IUE, and therefore their data share some of the limitations as the IUE spectra. In fact, the lines in the GHRS and STIS spectra do not satisfy conditions (b), (c), (d), (e), (f), as discussed below. Only lines that are formed at temperatures up to log T = 5.4 are observed, with the exception of the Fe XXI 1354 Å coronal line ( $\log\, T =7$). Strictly, the HST spectra on their own cannot directly provide accurate DEM and elemental abundance diagnostics. Even dropping conditions (d) and (f) leaves us with just a few lines.

The Far Ultraviolet Spectroscopic Explorer (FUSE), launched in 1999, covers the 905-1187 Å region with a high resolving power (20000-25000). For cool stars, the FUSE spectral range is more useful than the HST/STIS one, since it is rich in lines from different isoelectronic sequences which cover a large temperature range, from chromospheric to coronal temperatures (Fe XVIII, Fe XIX). FUSE is also the only instrument which observes the O VI doublet, important for bridging the gap between transition region and coronal temperatures. Note that also other instruments, such as the Hopkins Ultraviolet Telescope (HUT) and the Orbiting Retrievable Far and Extreme Ultraviolet Spectrometers (ORFEUS), flown on the Space Shuttle, have provided a few spectra in the FUSE spectral region, although with much lower resolution.