On the far-ultraviolet radiation of the T Tauri-type stars

Abstract

It is shown that the far-ultraviolet radiation (shorter than 2000 Å) discovered by ANS observations in the few T Tauri-type stars does not have any relation to the two-photon emission of hydrogen, as suggested by some authors. This is obtained from the observational data of the numerical values of the ratioQ ^*(2q)/Q ^*(β) for these stars, whereQ ^*(2q) is the complete number of the observed 2q-photons andQ ^*(β) is the number of observedHβ-photons. The observational values ofQ ^*(2q)/Q ^*(β) for four T Tauri-type stars turned out to be in the region of 20–90, while the theoretical value of this relation is 6. Hence, the observed fluxes in the region λ<2000 Å are 3–15 times larger than the theoretically expected values.

The emission discovered in the region λ<2000 Å is of non-thermal origin, and can be identified with high probability with thetransition radiation. The latter originates as a result of the electromagnetic interaction of so-called fast electrons (E∼1.5 MeV) with dust particles in the gas-dust clouds surrounding these stars. The theoretical spectral curves of the transition radiation, for a few values of the plasma frequency ω₀ for the dust particles, are calculated taking into account also the self-absorption effect of the radiation in the cloud and the absorption in the interstellar medium. Qualitatively, these curves (Figures 2, 3 and 4) are in good accord with the observed spectral distribution curves for the T Tauri-type stars (Figure 1). In particular, in both cases a minimum of radiation flux occurs near to 2200 Å, and a maximum near 1800 Å.

The starting point of our analysis has been the concept of the identity of the processes, non-thermal and non-stationary in character, taking place at the time of the flare phenomenon of UV Cet-type stars in one case, and at the generation of continuous emission and the excitation of the emission lines in T Tauri-type stars on the other. In the latter case, the T Tauri-type stars can be regarded aspermanently flaring stars, with a very high frequency of flare events.