Radiative cooling of shocked gas in stellar atmospheres I. Contribution from spectral lines

The radiative cooling of a shock wake under typical conditions of stellar atmospheres is considered. A simplified model of the radiative relaxation zone is used, and the transfer equation is solved for the line radiation across the shock. Using this solution, the contribution of spectral lines to the radiative cooling is studied, including the elements H, He, C, N, O, Na, Mg, Si, S, Ca and Fe. The line cooling rate is found to be always high in a zone of optical depth unity behind the front. We show that in this zone the relative contribution from each line to the total energy loss mainly depends on the local conditions (density, temperature, postshock gas velocity) and on the line wavelength, but is independent of the line strength. Consequently, the cooling by elements having many line transitions can be very effective. A preliminary analysis shows that the cooling due to Fe lines considerably reduces the spatial extent of the radiative shock wake with respect to the one obtained with H− and H continua. A more sophisticated analysis is needed to determine the exact thermal structure of the wake, which will be the subject of a forthcoming paper.