SILICON Si Z = 14

This element was discovered by J. Berzelius in 1824 in Stockholm, Sweden. The name comes from the Latin word for flint (silici).

Ionization energies
SiI 8.1 eV, SiII 16.3 eV, SiIII 33.5 eV, SiIV 45.1 eV, SiV 166.8eV, SiVI 205 eV, SiVII 246 eV, SiVIII 303 eV, SiIX 351 eV.

Absorption lines of SII

SiI (for instance 5948) appears in A-type stars and is present up to M-type stars, with a flat maximum near G5. A positive luminosity effect exists for supergiants, from F-type onwards.

SiI lines have been identified in the ultraviolet spectrum of one A0V star (Rogerson 1989). The SiI resonance line (UV M.1) is at 2516 Å.

Emission lines of SiI
In T Tau stars (Joy 1945) the line at 3905(3) appears in emission.

In long-period variables, two SiI lines, 3905 and 4103(2), appear in emission around maximum light (Merrill 1952, 1960). The line 3905 is also visible in at least one nova (Joy and Swings 1945).

Absorption lines of SiII

In stars of luminosity class O, W(3856) = 0.755 and 0.404 for A 2 and A 3 respectively.

SiII (for instance the lines at 4128 and 3856) increases from early B-type to F-type and declines toward K-type stars (see the line at 6347). A positive luminosity effect is present.

The SiII resonance line is at 1808 (UV M.1).

SiII has a number of lines from high excitation levels, like 3955, 3992 and 4200, which are prominent in the spectra of the Si subgroup of Bp stars (see the discussion on behavior in non-normal stars).

Emission lines of SiII
Emission lines of SiII were observed by Beals (1951) in P Cyg and by Appenzeller et al. (1980) in T Tau stars. Emissions are also seen from one nova Joy and Swings 1945).

The ultraviolet SiII line 1817(1), if seen in emission in late type stars, is indicative of a stellar chromosphere. The line is visible in late F-type stars, strengthens in G- and K- and disappears in early M-type stars. (See Part Two, section 3.1.)

Absorption lines of SiIII

SiIII (for instance the line at 4552) is present in late O-type and early B-type stars, with a maximum at about B 1. At B 5 the line is no longer visible. A positive luminosity effect is present.

The red line 5740(4) of SiIII is visible around B l(B 0.5 to B 1.5) and has a positive luminosity effect (Walborn 1980). The strong ultraviolet SiIII lines around 1300 (of M.4) are present in types O 3 to O 9, where they disappear (Heck et al. 1984). Attention must be paid to the fact that atter B 0 the 1300 feature is associated with SiII.

The resonance line of SiIII is at 1206 (UV M.2).

Emission lines of SiIII
Emission lines of SiIII were observed by Beals (1951) in P Cyg and by Joy and Swings (1945) in one nova.

The presence of the SiIII line at 1895(1) in the ultraviolet spectra of late type stars is indicative of a transition region. It is seen in G- and K-type stars and disappears in early M-type stars. (See Part Two, section 3.1.)

Absorption lines of SiIV

SiIV (for instance the line at 4089) is present in early O-type stars. It has a maximum at O 9 and disappears at B 2. A positive luminosity effect exists.

The ultraviolet SiIV 1400 feature can be used between O 5 and B 5 stars for classification purposes (Walborn and Panek 1984, Heck et al. 1984). In high-luminosity stars 1400 shows a P-Cygni profile, which is not seen in dwarfs. The P Cyg profile is usually interpreted as indicating mass outflow (Howarth and Prinja 1989). In Be stars the line is visible up B 8V stars (Marlborough 1982).

Emission lines of SiIV
Si IV emissions were seen in one nova by Joy and Swings (1945). If the ultraviolet SiIV feature at 1400 is seen in emission in late type spectra, then it indicates the presence of a stellar transition region. The line is visible in G and K dwarfs up to K2. (See Part Two, section 3.1.)

Behavior in non-normal stars
Si is strong in hot extreme helium stars (Jeffery and Heber 1993).

Si is usually weak in sdO and sdB stars, with an indication that it is much weaker in the former than in the latter (Baschek and Norris 1970, Baschek et al. 1982, Lamontagne et al. 1985).

In Bp stars of the Si subgroup, the SiII lines (for instance that at 3856) are enhanced by factors up to three (Didelon 1986), but there exists a continuous transition between Si stars and normal stars. In certain stars of this subgroup several strong lines appear (4200,3955 and 3992). The attribution of these lines to SiII was made independently by Bidelman (1962b) and Jaschek and Jaschek (1962). Such stars were called formerly ` 4200 stars' and are now called more correctly `Si- 4200 stars'. These stars are the hottest objects of the Bp-Si subgroup.

In Ap stars of the Cr-Eu-Sr subgroup the element tends to be weak (Adelman 1973b). Typically W(4128) = 0.10 (Sadakane 1976). Si is also weak in the Hg-Mn subgroup.

Si is about normal in Am stars (Burkhart and Coupry 1991).

Si lines are weakened in F-type HB stars with regard to normal stars of the same colors by factors up to ten in W(Adelman and Hill 1987).

Si seems to be overabundant by factors of the order of two with respect to iron in metal-weak stars, both in dwarfs and in giants (Gratton and Sneden 1987). Spite (1992) remarks that Si (and S) behave like magnesium in these stars. An overabundance of Si with respect to Fe by a factor of about two is also found in globular cluster stars (Wheeler et al. 1989, Francois 1991) (see Part Two, section 2.1).

SiII and SiIII lines are seen in emission in the ultraviolet spectral region of novae during the `principal spectrum' phase. Sometimes one sees also [SiVII] and [SiIX] lines during the nebular phase (Warner 1989). Si is overabundant by a factor of about 50 in the spectra of the novae of the O-Ne-Mg subgroup (Andreae 1993).

SiII emission lines are also observed in the spectra of supernovae of type Ia (Branch 1990).

A strong absorption blend of SiII at 6355(2) is characteristic of the supernova spectra of type Ia near light maximum. It is accompanied by other strong features of SiII.

Isotopes
Si has three stable isotopes, namely Si 28, 29 and 30, which in the solar system occur with frequencies 92%, 5% and 3% respectively. There also exist five unstable isotopes.

Si isotopes have been examined by Lambert et al. (1987) in four red giants, using the SiO molecular bands at 2495 cm^-1. These authors find for the Si²⁸ / Si²⁹ ratio the solar value (which is about 20) for two stars. For an MS star and one M star they find ratios of 35 and 40. Si³⁰ appears to be underabundant by a factor of two in all four stars. Instead of these bands, one can also use radio transitions of the SiO molecule.

The ratio Sⁱ²⁹ / Si³⁰ has also been derived from observations of the SiS molecular lines of the circumstellar envelope of the C-type object IRC + 10216 in the radio domain. The ratio found does not differ significantly from the solar system ratio.

Origin
All three Si isotopes can be produced by explosive nucleosynthesis. Si²⁸ can also be produced by oxygen burning and Si²⁹ and Si³⁰ by Ne burning.

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