The Neutral Sodium D(2)LINE in Late-Type Stars

An atlas of high spectral resolution Na I D(,2) and H I alpha spectra is presented for 33 dwarfs, giants, and supergiants covering a range of spectral types from F5 to M4.5. These spectra form the basis of a study in which the effects of a stellar chromosphere on the Na I D(,2) profile are investigated. The analysis of line profile parameters yields the following results: (i) A relationship between the line width and the luminosity for the giants and supergiants does not exist for Na I D(,2) as has been observed for Ca II K core emission (Wilson and Bappu, 1957) and H I alpha (Kraft et al., 1964). This reinforces the concept that the mechanism responsible for the Wilson-Bappu effect does not operate in the high photosphere of these stars. (ii) The correlation between Na I D(,2) line width and luminosity for the F through K dwarfs is explained in terms of differences in effective temperature and surface gravity along with the main sequence. The dMe stars exhibit central reversals in the Na I D lines. (iii) The method of Steinitz et al. (1975) has been used to measure Doppler widths for H I alpha and Na I D(,2) lines under the assumption of a constant properties atmosphere. The H I alpha measurements imply supersonic non-thermal velocities for the giants and supergiants in agreement with Basri (1980) and Zarro and Rodgers (1983). Non -thermal velocities less than 10 km/sec are needed to explain the Na I D(,2) Doppler widths for three dwarfs. For the two stars, EQ Vir (dK5e) and GL616.2 (dM1.5e), a detailed study of the formation of Na I D(,2) is presented applying the non-LTE radiative transfer method of Auer et al. (1972) and using model chromospheres derived by Giampapa et al. (1982). Except for the anomalous central reversal in the observed Na I D(,2) line of GL616.2, these calculations provide a consistency check on the chromospheric models inferred from the Ca II K diagnostics.