LINE EMISSION IN STELLAR ENVELOPES

We examine the problem of what could be called a 'compact planetary ne bula' in studying the radiative equilibrium of a spherical envelope wi th inner radius comparable or equal to the radius of the illuminating core. The envelope is composed of hydrogen atoms in statistical equili brium and photoionized by a central radiation of relatively high tempe rature T > 15 000 K. We consistently solve the equations of radiative transfer in spherical geometry for all lines and continua, including the Lyman transitions, together with the equations of statistical equi librium. Yet we ignore the energy balance equation and consider the el ectronic temperature as a given parameter. We show that Balmer and oth er subordinate lines may appear in emission, even at low temperature, provided that (i) the density is low enough for the radiative terms to partly control the populations of the levels and (ii) the geometrical extension of the H II region is significant. Although the present mod el is only intended to isolate and illustrate a specific physical mech anism, we suggest that the emission features detected in the spectrum of cool giant or supergiant stars could result from purely radiative p rocesses, at least in some cases.