## 6. Critique of the basic assumptionsOne of the fundamental assumptions in the present analysis is the
assumption of LTE. However, this assumption may not be valid for
optically thin discs. A crucial feature of the above analysis and
consequences is the fact that the cooling fails to overcome the
heating. From thermodynamics we can generally claim that NLTE cooling
will be less powerful than LTE cooling, as indeed found by DCKL that
the LTE overestimates the actual cooling. Thus, we expect that NLTE
will A similar argument holds for very cool discs when the actual opacity decreases. Again we encounter a situation where the cooling decreases while the heating prevails. We assumed the existence of a thermal balance. Clearly, as soon as the balance is violated the gas starts to expand and the momentum equation in the vertical direction contains two additional terms, namely and . Strictly speaking the 1D approximation is not valid anymore. Thus the entire considerations developed here must be reconfirmed using 2D approach. The actual calculations and figures were carried out for free-free
opacity and naturally the question arise what happens under other
conditions. The situation does not change in an essential way when the
opacity law changes (at least as long as
). The reason is the appearance of
© European Southern Observatory (ESO) 1999 Online publication: March 18, 1999 |