The UV spectrum of the TTS has a weak continuum and several strong emission lines. The continuum is significantly stronger than the observed in main sequence stars of similar spectral types (G to M); this excess represents the short wavelength tail of the veiling continuum detected at optical wavelengths (Herbig, 1962; Hartigan et al 1990). The underlying photosphere is barely detected; only in warm (G-type) wTTS the photospheric absorption spectrum is observed. The UV continuum excess is significantly larger in the cTTS than in the wTTS (see e.g. Imhoff & Appenzeller, 1989). Simple models of hydrogen free-free and free-bound emission added either to black bodies or to the spectra of standard stars reproduce reasonably well the data (Calvet et al 1984; Lago et al 1984; Herbig and Goodrich 1986; Bertout et al 1988; Simon et al 1990). The fits yield electronic temperatures of 1 – 5 × 104 K that are chrosmospheric-like. Two different mechanisms have been proposed to generate this hot plasma. The UV continuum could be originating either in dense chromospheres (Kuhi, 1966; Calvet et al 1984) or in the release of the gravitational binding energy from the infalling material (Bertout et al 1988; Simon et al 1990).
The most prominent lines in the spectrum are those of Mg II at 2800 Å. The surface fluxes are typically 107 - 108 erg cm-2 s-1, approximately 50 times larger than those from the Sun. They are among the highest seen in late-type stars with active chromospheres including those of RS CVn binaries. High resolution profiles of the lines have been obtained only for 17 sources: BP Tau, RY Tau, T Tau, DF Tau, DG Tau, GM Aur, SU Aur, RW Aur, CO Ori, GW Ori, FU Ori, TW Hya, LKHa 332, RU Lup, AK Sco, S CR A and DI Cep (Appenzeller et al 1981,Jordan et al 1982, Penston & Lago 1983, Brown et al 1984, Giampapa & Imhoff 1985, Gómez de Castro & Fernández 1996). They can be generically described as broad, asymmetric emission lines with typical full widths at 10 % intensity of few hundreds km/s. A major narrow absorption feature is detected overimposed to the emission probably of interstellar origin. Redshifted absorption components have been eventually detected in some sources (see e.g. Gómez de Castro & Franqueira 1997). The broad blueward shifted absorption component characteristic of mass-loss has been detected in few sources. The lines are variable and optically thick in most sources.
Fe II lines corresponding to the multiplets: UV 2,3,35,36 (2330-2410Å); UV 32,62,63 (2700-2750 Å); UV 60,78 (2900-3000 Å) are also observed in several sources (Gahm et al 1979, Imhoff & Giampapa 1980, Appenzeller et al 1980, Brown et al 1984, Gómez de Castro & Fernández 1996). The Fe II lines are individually weaker than the Mg II, C II or Si II lines but they are so numerous that altogether become a significant coolant of the PMS stars atmospheres (see e.g. Jordan, 1988). Weaker emission features in the long wavelength range (2000 - 3200 Å) are the C II] and Si II] blend at 2330 Å, the Fe II lines at 2507 Å due to the fluorescence by Ly α and the Al II 2670 Å resonance line.
The short wavelength range (1200 – 2000 Å) is dominated by emission lines as those typically found in the chromospheres and transition regions of cool giants. The strongest lines are those of C IV(UV1), O I(1303 Å) and Si II(UV1). Also lines of He II(1640 Å), Lyα, N V(UV1), Si IV(UV1), Si III](1892 Å), C III](1908 Å), and CII (UV1) as well as of molecular hydrogen have been found (Gahm et al 1979,Appenzeller & Wolf 1979, Appenzeller et al 1980, Imhoff & Giampapa 1980, Brown et al 1981, Penston & Lago, 1982, Brown et al 1984, Lago et al 1985, Simon et al 1990, Lemmens et al 1992, Gómez de Castro & Fernández, 1996, Gómez de Castro & Franqueira, 1997). The surface fluxes of these lines are typically 106-107 erg cm-2 s-1, approximately 3 orders of magnitude larger than the observed in the Sun (Imhoff & Giampapa 1980, Lemmens et al 1992, Gómez de Castro & Fernández, 1996). The TTSs, in general, deviate from active stars in the flux-flux relations. The TTSs have an excess of emission from low ionization with respect to high ionization species when compared with other active stars.
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