Figure 2. X-ray luminosity along H/R diagram.
During the phase in which the cloud core collapses and the star is still deeply embedded, the UV radiation density in the gas is drastically reduced, and the dominant ionizing source is X-rays. Indeed, unless a low-density region allows UV radiation to escape from the innermost region of the envelope, its influence on the chemistry is restricted to a small volume surrounding the star. Almost all stars are X-ray emmiters, see Fig. 2.
Figure 3. X-ray flux luminosity as a function of the age.
In addition, in young stars, X-rays may dominate the high-energy emission. In the case of a solar-type star of 100 Myr old, the X-ray flux is larger than the extreme UV, and it remains within a factor of two up to 1 Gyr old (Fig. 3). Such a copious hard emission must affect the chemistry of interstellar material.