SODIUM Na Z = 11

This element was discovered by H. Davy in 1807 in London, England. The name comes from the English soda, in Latin natrium.

Ionization energies
NaI 5.1 eV, NaII 47.3 eV, NaIII 71.6 eV, NaIV 98.9 eV, NaV 138.4 eV, NaVI 172.1 eV.

Absorption lines of NaI

The blend of the lines 5889 and 5895 is oLten called `D line' (following a designation given by Fraunhofer); in particular, 5889 = D1 and 5895 = D2.

NaI (for instance the line at 5889) appears in B-type stars and increases monotonically toward later types. Before G-type, a positive luminosity effect is seen. After G-type, the luminosity effect reverses and is negative. (This was already known to Luyten (1923).) Zhou Xu (1991) found a similar behavior of the 8183-8194 doublet. This anomaly is not present in the line at 5688.

The intensity of the NaI 5889 and 5895 lines passes through a minimum at M5, because of the superposed absorption of TiO. A similar phenomenon happens for C stars at C 5 to C 6, although in this case the veiling absorption isdue to CN and C₂. In S-type stars there is also a band of ZrO superimposed, but since ZrO is less opaque than TiO, the NaI lines are stronger in S-type than in M-type stars (Keenan 1957).

Attention should be paid to the fact that NaI 5889 and 5895 lines (M.1) often have interstellar component(s).

NaI emission lines
One observes the 2.206 - 2.209 µm doublet in emission in some high-luminosity stars (McGregor et al. 1988) and in F-type supergiants (Lambert et al. 1981). The 5889 and 5895 lines are seen in emission in at least one B[e] star (Merrill 1951a).

The NaI D lines are in emission in dMe stars (Pettersen 1989).

Na I lines are frequently in emission in T Tau stars, exhibiting complex profiles. Usually the emission is strong in stars that also exhibit CaII emissions. These lines have been used extensively to study the physical state of the envelopes (Mundt 1984).

Behavior in non-normal stars
In lambda Boo stars, NaI lines are weaker than in normal stars of the same temperature. Typically W(5889) = 0.080 for A 0V (Venn and Lambert 1990).

Na is normal in Am stars (Smith 1973, 1974).

Na seems to be overabundant by a factor of three in four giants of the Hyades open cluster (Arimoto and Cayrel de Strobel 1988) and similar overabundances were found in metal-rich stars. Since the lines used are strong ones, the results are to be taken with caution.

Na is probably underabundant with respect to Fe in the most metal-weak stars, Fe / H = - 4dex (Molaro and Bonifacio 1990). In moderately metal-weak stars Na may be normal with respect to Fe (Wheeler et al. 1989). In globular cluster stars it probably behaves in a manner parallel to that of iron. However, Gonzalez and Wallerstein (1992) find Na to be enhanced in one globular cluster supergiant. Smith and Wirth (1991) find an erratic behavior of Na in globular cluster stars having different CN strengths, and Francois (1991) finds a general overabundance of Na. It is evident that it is best to consider the Na abundance as being an open problem.

NaI (5889) is very strong in the spectra of C stars (Fujita et al. 1963), which probably implies an overabundance of Na. In some very cool variables Keenan (1957) finds W(5859+5895) up to 50 Å.

Na lines are very weak in the spectra of CH stars (Yamashita 1967). [NaV] and [NaVI] lines are sometimes seen in the spectra of novae during the nebular stage Joy and Swings 1945, Warner 1989).

Strong emission lines of Na I are visible in the spectra of supernovae of types Ib and II (Branch 1990).

Isotopes
The only stable isotope is Na²³. There also exist six short-lived isotopes.

Origin
Na can be produced by carbon burning, neon burning or explosive nucleosynthesis.

Published in "The Behavior of Chemical Elements in Stars", Carlos Jaschek and Mercedes Jaschek, 1995, Cambridge University Press.