A method to estimate the effect of line blanketing in NLTE radiative transfer calculations

We present a method to estimate the contribution of line opacity to the total opacity as a function of wavelength. The estimated line-opacity function can then be used to simulate line-blanketing in NLTE radiative transfer calculations. Given a reference flux distribution (either observed or theoretical), our method allows to obtain a good estimate of the spectrum without the need for considering in detail all the millions of lines contributing to line blanketing. We applied the method to the spectra computed from a sample of photospheric models with effective temperatures , 5200 and 6200 K, , 4.5, 5.0 and [A/H] , -1.0, -2.0, taken from the NextGen database (Allard & Hauschildt [CITE]). The computed flux distributions agree quite well with the corresponding LTE line-blanketed NextGenfluxes when we introduce the estimated line-opacity contribution as a multiplicative factor of the continuum opacity in the radiative transfer calculations. In particular we discuss the importance of a correct estimate of the continuum flux, mainly in the UV, in the NLTE formation of the Caii H & K, the Caii InfraRed Triplet (IRT: 8498, 8542, 8662), Nai D, Lii and Ki resonance lines.