Analyses of light-ion spectra in stellar atmospheres.
An analysis of the N iii emission lines in 0 stars has been carried out on the basis of a detailed solution of the coupled statistical-equilibrium and transfer equations for a multiline, multilevel, multi-ion ensemble. Our calculations, using static, plane-parallel models reproduce successfully the observed emission at AA4634, 4640, 4641 (3p 2P -3d 2D) and absorption at AA4097, 4103 (3s 2S-3p 2P ). The multiplet is found to come into emission at the observed temperature for both main-sequence and low-gravity objects. The equivalent widths of the emission lines agree very well with those measured for the class of Of stars thought to have compact atmospheres, i.e., those classified as O((f)) by Walborn. In these stars the basic physical mechanism responsible for this phenomenon is the overpopulation of 3d by means of dielectronic recombinations from the low-lying 2s2p(1P )3d autoionizing states with cascades 3d 3p. The 3p state is drained by the "two-electron jumps" coupling 3p to the 2s2p2 (25, 2p, 2D) states, thus preventing emission in the 3s-3p lines. The possible importance of the Swings mechanism to the fully developed Of stars (in Walborn's sense) is pointed out, and the irrelevance of the Bowen mechanism to all Of stars is firmly demonstrated. The fact that the N iii emission lines can be produced in static nonextended atmospheres in radiative equilibrium has the far-reaching significance that the presence of emission lines in a spectrum is not in itself sufficient evidence for the existence of a stellar chromosphere (i.e., an extended, nonradiatively heated region). Subject headings: atmospheres, stellar - emission-line stars line formation Of-type stars
The Astrophysical Journal
- Pub Date:
- February 1973